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ISSN 1030-2662 11 November 1996  1 9 771030 266001 R AUSTRALIA’S BEST AUTO TECH MAGAZINE It’s a great mag... but could you be disappointed? If you’re looking for a magazine just filled with lots of beautiful cars, you could be disappointed. Sure, ZOOM has plenty of outstanding pictorials of superb cars, but it’s much more than that. If you’re looking for a magazine just filled with “how to” features, you could be disappointed. Sure, ZOOM has probably more “how to” features than any other car magazine, but it’s much more than that. If you’re looking for a magazine just filled with technical descriptions in layman’s language, you could be disappointed. Sure, ZOOM tells it in language you can understand . . . but it’s much more than that. If you’re looking for a magazine just filled with no-punches-pulled product comparisons, you could be disappointed . Sure, ZOOM has Australia’s best car-related comparisons . . . but it’s much more than that If you’re looking for a magazine just filled with car sound that you can afford, you could be disappointed. Sure, ZOOM has car hifi that will make your hair stand on end for low $$$$ . . . but it’s much more than that. If you’re looking for a magazine just filled with great products, ideas and sources for bits and pieces you’d only dreamed about, you could be disappointed. Sure, ZOOM has all these . . . but it’s much more than that. But if you’re looking for one magazine that has all this and much, much more crammed between the covers every issue, there is no way you’re going to be disappointed with ZOOM. Look for the June/July 1998 issue in your newsagent From the publishers of “SILICON CHIP” Contents Vol.9, No.11; November 1996 FEATURES 4 Striking A Blow Against Lightning Lightning is by far the single biggest cause of power interruptions. Now there’s new technology that can track lightning strikes with impressive accuracy, so its effects can be minimised – by Ross Tester 82 Adding An Extra Parallel Port To Your Computer Add an extra parallel port to your PC and forget switching printer cables. We show you how to upgrade the serial ports as well – by Greg Swain BUILD AN 8-CHANNEL STEREO MIXER – PAGE 20 PROJECTS TO BUILD 20 Build An 8-Channel Stereo Mixer; Pt.1 Easy-to-build unit has eight main inputs, LED bargraph level meters, effects send and comprehensive headphone monitoring facilities – by John Clarke 30 Low-Cost Fluorescent Light Inverter Low-current unit is suitable for solar power installations and can drive both fluorescent and neon tubes – by Branco Justic 42 How To Repair Domestic Light Dimmers Don’t toss that crook light dimmer. Repair it by fitting a more rugged Triac and it should be fixed for good – by Leo Simpson LOW-COST FLUORESCENT LIGHT INVERTER – PAGE 30 59 Build A Multimedia Sound System; Pt.2 We describe two speaker systems designed specifically to go with the amplifier card described last month – by Rick Walters 66 600W DC-DC Converter For Car Hifi Systems; Pt.2 Second article has all the constructional details. Build it and fit high-power amplifiers to your car – by John Clarke SPECIAL COLUMNS 38 Serviceman’s Log Of ships and shoes and sealing wax – by the TV Serviceman HOW TO REPAIR DOMESTIC LIGHT DIMMERS – PAGE 66 54 Radio Control AM vs FM: the real facts in the argument – by Bob Young 88 Vintage Radio A pair of Astor valve radios – by John Hill DEPARTMENTS 2 16 18 53 79 Publisher’s Letter Circuit Notebook Bookshelf Mailbag Product Showcase 87 92 95 94 96 Order Form Ask Silicon Chip Market Centre Notes & Errata Advertising Index ADDING AN EXTRA PARALLEL PORT TO YOUR PC – PAGE 82 November 1996  1 Publisher & Editor-in-Chief Leo Simpson, B.Bus., FAICD Editor Greg Swain, B.Sc.(Hons.) Technical Staff John Clarke, B.E.(Elec.) Robert Flynn Rick Walters Reader Services Ann Jenkinson Advertising Manager Christopher Wilson Phone (02) 9979 5644 Mobile 0419 23 9375 Regular Contributors Brendan Akhurst Garry Cratt, VK2YBX Julian Edgar, Dip.T.(Sec.), B.Ed John Hill Mike Sheriff, B.Sc, VK2YFK Philip Watson, MIREE, VK2ZPW Bob Young SILICON CHIP is published 12 times a year by Silicon Chip Publications Pty Ltd. A.C.N. 003 205 490. All material copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Printing: Macquarie Print, Dubbo, NSW. Distribution: Network Distribution Company. Subscription rates: $54 per year in Australia. For overseas rates, see the subscription page in this issue. Editorial & advertising offices: Unit 34, 1-3 Jubilee Avenue, Warrie­ wood, NSW 2102. Postal address: PO Box 139, Collaroy Beach, NSW 2097. Phone (02) 9979 5644. Fax (02) 9979 6503. PUBLISHER'S LETTER On the track of lightning Every year at about this time thunderstorms and lightning become an important part of the weather. This applies no matter where you live in Australia although in some parts the storms are often a great deal more severe. In the tropical north, thunder­storms during the “Wet” are an almost daily occurrence while in the more temperate southern regions, thunderstorms invariably sweep in from a particular direction and can cause a great deal of damage. Many people, those who would rather watch television than go outside to see what the weather is like, are blissfully una­ware of the forces unleashed in a large thunderstorm. Others, much fewer in number, are greatly in awe of the almost mind-numbing energy being released and the ways in which it happens. Think about it for a while. A big band of thunderstorm cells can originate as far north as the Kimberleys and then will move southwest in a period of 24 hours or so. It will move into northern NSW, pass over Dubbo or thereabouts and continue on to lash Sydney or the Gosford region. A day or so later, its rem­nants can be seen in the evening, far out in the Tasman, still active with lightning, thunder and lots of rain. To try and comprehend how much energy has been released during such a period of thunderstorms is impossible. Not only may millions of tonnes of rainfall have been dumped over several states with widespread flooding, but fierce winds may have caused further damage, unroofing buildings and downing trees. Lastly, there is perhaps the most fearsome aspect, the lightning. This may have run at several thousand strikes an hour over a front which may be a hundred kilometres wide or more. The total energy release may easily be equivalent to several 100 megaton bombs. These thoughts have been triggered by our feature article this month on the subject of tracking thunderstorms, beginning on page 4. Even while this article was being written, we experienced several severe thunderstorms, a timely reminder that summer is with us again. So read the article and enjoy it. And next time a thunder­storm passes over your area think of the forces involved. Think about protecting all your electrical and electronic gear too - the only sure way to so is to disconnect it while ever the storm is in progress. And if the time between lightning strike and thunderclap is three seconds or less, the lightning is directly overhead! Lastly, if a thunderstorm is in progress, stay off the telephone – the next victim could be you! Leo Simpson ISSN 1030-2662 WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable. 2  Silicon Chip SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Macservice Pty Ltd They say there are two things that are inevitable: death and taxes. We would add a third: lightning! We cannot control it, we cannot even make use of it. But we can be ready for it and plan to at least minimise the effects of its incredibly destructive power. By ROSS TESTER LPATS: Striking a Blow Against Lightning 4  Silicon Chip Lightning photo by MICHAEL BATH C ONSIDER THIS SCENARIO: you are in charge of an electricity distribution network and the weather forecast is not good. “Thunderstorms”, it says. Now thunder is no great problem – ear muffs can stop the noise. But it is the immense power behind the thunder that has you worried – lightning. You know that lightning is by far the number one cause of electricity supply failure. The problem is that you don’t know how bad the lightning will be. Or where it will strike. Or when it will strike. Do you put your maintenance crews on standby – just in case? Or do you cross your fingers and hope this storm will miss your area altogether? And then the lights go out... Now consider this: same person, same situation. But instead of casting an anxious eye to the heavens, you are looking instead at your computer screen. What you see doesn’t look good: stroke after stroke of lightning, advancing at an alarming rate in your direction. You get on the 2-way to the maintenance crew chief: get the crew ready to move as soon as you give the all-clear. They’ll be needed at suchand-such grid coordinates because that’s where the lightning will strike in the next 15 minutes. Sure, the lights still go out. But you smile as they come on again after a minimal delay. Move that same storm to a busy airport. Everyone knows that planes don’t land or take off during lightning. Lightning damage, either by direct hit or by a struck tree bringing down lines is by far the most significant cause of power supply faults. Reducing the costs associated with lightning damage is therefore of major importance to electricity supply authorities. (Photo courtesy Integral Energy). But when does the air traffic controller say “stop” and “go”? When he can see the lightning? It could be 50km away – more than enough time to get many flights in or away. What about the all-clear? Again, the controller looks at his screen. He can see exactly where the lightning is striking, in real time. He can see when the storm is going to hit, or even if it is going to hit, and when it has passed. Another example: a bush fire control centre. At least 30% of bush fires are caused by lightning strikes. If only you knew exactly where the lightning was a problem, you could have fire fighters there before the small blaze became a conflagration. The screen tells you exactly where they have to go! The same scenario could be repeat- ed over and over across the country. Sports arenas, building sites, mines, oil rigs, the military, shows and exhibitions, ports and so on – all benefit from having accurate data on the direction, speed, severity and likely duration of storms containing lightning. Critical processes in industry, radio & TV stations, hospitals and the like could have their emergency generation equipment up and running before Somewhere under this kaleidoscope lies the Sydney metropolitan area being belted by a violent storm during the night of February 8, 1996. The LPATS screen dramatically shows one hour's worth of a storm front crossing the Northern Suburbs. Every dot is a lightning stroke: the grey dots occurred in the last 10 minutes, ranging back to one hour previously. The time graph (bottom left) shows a massive build in intensity as time passes. This screen, which stretches from roughly Palm Beach in the north to Botany Bay in the south, could be zoomed i­n much closer if required. If you think the southern suburbs were spared, the same LPATS file some two hours earlier shows another two massive storm fronts, one passing between Sydney city and Sutherland and the other hitting the greater Wollongong area with even greater fury! November 1996  5 HYPERBOLAS Fig 1: time-of-arrival lightning stroke positioning depends on gaining a very accurate time "fix" from three or more special receivers, widely spaced. This gives a single, unambiguous position accurate to within a few hundred metres. the storms hit: proactive instead of reactive. How it is done Back in September 1991, SILICON CHIP readers were told of an exciting new method of tracking thunderstorms by detecting the intense electromagnetic (e-m) field generated when lightning occurs. Readers would be aware of the static they hear on ordinary AM radio receivers when a thunderstorm is even some distance away. That static is the direct result of that e-m field and basically lightning tracking systems are “listening” for that “static”. The e-m field is generated over a very, very wide frequency range – almost “from DC to daylight”, as amat­eur operators put it. However, by tailoring the frequency response of the receiver, the system can be made dramatically more sensitive to lightning only. In 1991, two methods were under Fig 2: how lightning is located by time of arrival: (a) The signal will be detected at each receiver at a different time relative to the stroke, depending on the distance from the stroke. (b) Time is measured at each site with a resolution of 100 nanoseconds (±50 nanoseconds). (c) Each receiver has a 10MHz timebase which is typically synchronised 20 times each second from the precise time signals of the Global Positioning System satellites. (d) A minimum of three receivers is required for a solution. Achievable accuracy is 1 microsecond and within 200 metres, depending on the distance from the lightning stroke to the receivers. 6  Silicon Chip investigation – direction finding and time-of-arrival. As its name implies, the direction finding method uses trad­ itio­nal radio direction finding methods and is reasonably accurate if enough data is available. What has really captured the imagination, however, is the other meth­od reported at the time, although then in its infancy (and not then available in Australia). Now things have changed: time-of-arrival detection is not only VISIT OUR WEB SITE OUR COMPLETE CATALOGUE IS ON OUR SITE. 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Power output is varied using a trimpot., operates from 10 to 15V, current is 5-600mA ...IR LASER DIODE KIT: Barely visible 780nM/5mW (Sharp LT026) laser diode plus constant current driver kit plus collimator lens plus housing plus a suitable detector Pin diode, for medical use, perimeter protection, data transmission, experimentation: $32 ...WIRELESS IR EXTENDER: Converts the output from any IR remote control into a UHF transmission, Tx is self contained and attaches with Velcro strap under the IR transmitter, receiver has 2 IR Led’s and is place near the appliance being controlled, kit includes two PCB’s all components, two plastic boxes, Velcro strap, 9V transmitter battery is not supplied: $35, suitable plugpack for the receiver: $10 ...NEW - LOW COST 2 CHANNEL UHF REMOTE CONTROL: Two channel encoded UHF remote control has a small keyring style assembled transmitter, kit receiver has 5A relay contact output, can be arranged for toggle or momentary operation: $35 for one Tx and one Rx, additional Tx’s $12 Ea. OATLEY ELECTRONICS PO Box 89 Oatley NSW 2223 Phone (02) 9584 3563 Fax (02) 9584 3561 orders by e-mail: branko<at>oatleyelectronics.com major cards with phone and fax orders, P&P typically $6. November 1996  7 TRACKING A STORM WITH It formed over the Channel Country in the early evening. By 9.37 LPATS had record­ ed 174 strokes in the past hour. here but is proving its worth continuously. If you missed the earlier article, a brief recap is in order. Basically, a number of sensitive radio receivers pick up the extremely strong electromagnetic field generated by the lightning discharge. The exact time of arrival (to 100ns) is compared to the extremely accurate time signals from the Global Positioning Satellite (GPS). If two radio receivers separated by some distance detect the emf of a lightning strike at precisely the same moment, it stands to reason that the strike was somewhere along a straight line between those two receivers – see Fig.1. But if one receiver detects the strike at a slightly different time than the other, the differences between the times can be used to work out two hyperbolas about the receivers on which the strike could have occurred. These hyperbolas will intersect in two places; one of these two places is the location of the lightning strike. Add a third receiver to the system and by using the time differences between each of the three pairs, three hyperbolas can be drawn. However, there will only be one point where all three hyperbolas intersect: this is the location of the lighting strike. This point can be located with quite impressive accuracy: within 200 metres of the actual strike location within the baseband of the receivers, and within 500 metres outside (and 8  Silicon Chip Half an hour later further cells had developed and more than 500 strokes had been recorded in the past 50 minutes. By midnight it had moved southeast but had reduced in intensity – under 300 strokes in the hour. Was it dying out? remember, the actual location can be thousands of kilometres from the receivers). The accuracy of the GPS “commercial” signal is only ±100 metres, so the fix is very close indeed. Fig.2 shows the system graphically. As we said, a minimum of three receivers is necessary to calculate an accurate position. Add a fourth and subsequent receivers and the accuracy can be further increased. Under the acronym of LPATS, the Lightning Positioning and Tracking System is provided in Australia by Kattron, a company based on the central coast of NSW. As an aside, whether by luck (bad!) or design, Kattron’s head office just happens to be located in one of most active storm belts on the East Coast. “It is incredible”, said Kattron’s Ken Tice­hurst, “to see the number of storms which come through this area and then affect Sydney.” Ken is not just speaking from anecdotal evidence: Kattron now has five years of historical data to demonstrate the effectiveness of the LPATS method of lightning tracking. Not only does the data correlate perfectly with weather bureau data, it actually surpasses it in many respects. In fact, the Bureau of Meteorology has been using Kattron data since April 1992 for general forecasting as well as upper air reports for commercial aircraft flight paths. LPATS operation As mentioned, it takes three LPATS receivers to obtain an accurate “fix”. At present, there are six receivers in place, ranging from Rockhampton in central Queensland to East Sale in Victoria. Other receivers are located at Moree, Cobar, Coffs Harbour and Power Network Faults Faulty Type Percentage Lightning 58.98 Other Weather 6.49 Trees 2.12 Personnel Error 2.95 Equipment Failure 6.35 Other Misc. Causes 5.56 Unknown Causes 17.56 This table, from the records of Minnesota Power in the USA, clearly shows the overwhelming proportion of problems to the power network caused by lightning. LPATS helps to minimise the effects and the costs. LPATS The night of September 24, 1992 was not one to be outdoors. A huge storm made its way from southwest Queensland down through northern NSW, finally crossing the NSW central coast. These Australia-wide "screen grabs" (which could in fact be much smaller areas) track its path in real time by recording lightning strokes. The grey strokes are the most recent (previous 10 minutes) ranging back to one hour before. It was just fooling everyone. By 2am it was recording a massive 2000 strokes per hour. No one slept over half the state! By 4am it was crossing the coast between Newcastle & Sydney, still recording 1000+ strokes per hour. That's some storm . . . As dawn broke it was moving out into the Tasman and people over a 2000km path were counting the cost. Nowra. This gives more than enough receivers to ensure the three-receiver fix but also gives a very high level of built-in redundancy. As more and more users come on line, so more receivers will be added to the LPATS network. The receivers themselves use a simple whip antenna to receive the lightning signal and a helix antenna to receive the GPS satellite timing signals. The receivers monitor the 2-450kHz radio band; ie, the spectrum below the AM broadcast band. AM detection is used. When a lightning stroke is detected, the receivers digitise 100 microseconds of the stroke information and store it in memory. At the very first peak of the received signal a very accurate time stamp is used to measure rise time and to provide the essential time-of-arrival reference, which is derived from the Global Positioning Satellite and accurate to 100 nanoseconds. Embedded in the digitised information is the polarity (positive or negative) and the peak stroke current which determines the size (and therefore the damage capability) of the stroke. This information is then sent to a “Central Analyser” computer via a modem and continuous data link. Various algorithms are used to not only reject false strokes but also determine the exact location of the stroke. The central computer also generates the lightning stroke data to be both disseminated to system users and also stored for later evaluation and use. With the location of the six LPATS receivers, lighting can be detected across a very wide area – virtually the whole of Australia. For Perth and Darwin, strokes with an amplitude of 50kA and greater can be detected. To demonstrate the effectiveness of the system, LPATS regularly records lightning strokes in Japan, Indonesia and way out into the Pacific Ocean. Indeed, New Zealand can be more-than-adequately covered using the current setup, though accuracy would be increased with an LPATS receiver or two in the Shaky Isles. Distributing the information It’s fine for Kattron to know about lightning approaching but how do customers find out about it? Many larger organisations go “on line” to Kattron’s Central Analyser computer and obtain their lightning If you believe, as do many people, that lightning strikes occur mostly at night, look again: these graphs from LPATS data record the number of strikes per hour in central western NSW over each of three months: November, December 1995 and January 1996 (coincidentally, the peak lighting period in NSW). November had most strikes around midday, December was all over the place while January peaked very much in the early evening, with very little at other times. November 1996  9 The one that got away . . . or that we got away from! Two much more recent screens (from September 19/20 last) demonstrate the fickle nature of lightning. The first screen taken at 7.30pm on September 19, shows a truly massive line of thunderstorms virtually unbroken from central Queensland to the Victoria/NSW border. More than 1400 strokes had been recorded in the previous hour. The second screen, showing the same storm at 6am next morning and "zoomed in" on the central NSW coast, shows just a few isolated strokes in the Hunter Valley and the mountains northwest of Sydney, with just 153 strokes recorded in the hour. data in as much detail as they want it, any time of the day or night. Organisations such as electricity supply authorities and similar “must know” bodies have become major customers. The software enables customers to utilise the data in a variety of ways to suit their particular needs. Most users are of course interested in their local area(s) and this information is available constantly. Sometimes, however, the “broader picture” is required and information is also available over a larger area by zooming out – even to the whole of Australia. It’s fascinating watching the buildup of a storm near Indonesia and eventually seeing the lightning strike Sydney! But Kattron has a much wider distribution (and lower cost) network available to anyone who can receive a television picture from any station in the Seven network (including Prime and other affiliates). If your local TV 7 affiliate station transmits Teletext, it also transmits Datacast. Like Teletext, Datacast is transmitted during the vertical blanking interval (VBI) – the black lines you see on a TV screen when the picture rolls. Through the use of a suitable decoder, various LPATS data can be displayed on any personal computer. When this service commenced in January 1993, it was a world first for Australia – no other country had 10  Silicon Chip lightning data available via Datacast. The data is also available in report form for such bodies as insurance companies and assessors. With the accuracy of the lightning data now beyond question, Kattron has been called on many times to verify (or alternatively to dispute) insurance claims. With bogus claims costing the industry many, many millions of dollars a year, insurance companies are glad to pay $150 for a report. For example, take the claimant who insisted his freezer was damaged by lightning between a certain Friday night and Sunday night when he was away from home. He said that all the frozen food was of course spoiled and therefore the claim was quite significant. Unfortunately for the claimant, the insurance company purchased a report from Kattron which proved that there was no strike within 50km of his house that weekend, nor even a few days either side. Faced with the black and white data, the claim was withdrawn. Conversely, individuals having a fight with their insurance companies can also purchase a report to back their claim. There have been many instances where claims have been accepted with lightning data after they were initially rejected. But by far the biggest users of the lightning data are power supply au- thorities, telecommunication companies, oil companies and airports. Figures produced for one of the major power distributors showed savings of more than $50,000 per annum in maintenance costs alone, simply because the controllers knew exactly where the trouble spots were. Add to that the dramatically quicker restoration of power from lightning damage – and its almost incalculable savings to the community – and it’s not hard to see why authorities are so enthusiastic about LPATS. Power outages have become something of a political football of late. Anything that helps get the power back on sooner is sure to be a winner! Contact: Ken Ticehurst Kattron Phone/Fax (043) 89 2024 Footnote: Michael Bath, the photographer who captured the lightning strike on page 4, is also the editor of the severe weather newsletter “Storm News”. For more information, contact Michael on (02) 9625 9700 (ah) or visit his web sites at: http://www.geocitites.com/capecaneral/1801/ (lightning photos).­ http:/atmos.es.mq.edu.au/AMOS/weather­ watch/photos.htm (storm photos). http:/www.ozemail.com.au/~jimmyd/news.htm (storm news). What do you know about lightning? Everyone has experienced static electricity, caused when two insulated objects rub across each other. Lightning is simply the most violent manifestation of a static electricity charge which has become too high to be maintained. The amount of static electricity generated between objects depends on several factors, not the least of which are the amount of movement creating friction and the insulation between the objects. In a storm cloud enormous amounts of unstable air are constantly on the move. This movement picks up ice crystals within the cloud, forcing them upwards until they are too heavy and fall back down again. Back near the base of the cloud the crystals may again be forced upwards, the cycle repeating over and over. This movement creates friction and hence static charge. Eventually, the charge builds to such a high level that the insulation of the air is insufficient to prevent some electrons “jumping the gap” to a point of a lower potential. That may be another point on the same cloud or another cloud altogether (cloud to cloud or C-C strokes). Or it can be between a cloud and ground or earth (C-G strokes). The latter is the type of major interest to humans, as C-G strokes have the most potential – no pun intended – to cause damage, injury and death. In a C-G stroke, as the insulation begins to break down a stepped leader begins to zig-zag from the cloud, ionising the air in its way and thus creating a very low impedance path. When the electron path is about 200 metres from the ground it “searches” for a point or points which form the easiest path to ground: a mountain, a tall building, an electricity tower, a tree, a person standing on a golf course... When a suitable point is found a massive return stroke occurs from the ground up. The electrons blast towards the cloud at half the speed of light. The circuit is completed and the huge amount of energy stored in the cloud is then virtually “short circuited” to ground, resulting in a rapid and spectacular electron flow from cloud to ground – the phenomenon we know as lightning. And all this in a few millionths of a second! The amount of energy involved boggles the imagination: all lightning strokes have peak currents of thousands of amperes while the very largest strokes can easily exceed a quarter of a million amps! The potential difference between the cloud and ground can be millions of volts. The huge discharge also results in a very large electromagnetic field being generated (which we can hear as static on an AM radio, even from a storm hundreds of kilometres away). Lightning may be either positively or negatively charged – or, more accurately, the cloud which contains the energy may be either positively or negatively charged. In fact, in a large, anvil-shaped thunderstorm cell, there will be areas of positive charge and areas of negative charge (which is why C-C strokes occur). Recorded lightning data suggests that both the leading and trailing edges of the cloud are usually positively charged, resulting in positive lightning strokes. To some degree, these can be used to pinpoint the start and finish of the storm cell. The centre of the storm cell is more likely to result in negative strokes. There is a difference between a lightning flash and lightning stroke. A flash will typically contain more than one stroke – on average two to three, but up to 20. Each stroke will normally last only 20-50µs and strokes will be about 20-50ms apart. The area covered by all the strokes can be quite large: a 10km radius is not unknown. Because we humans cannot differentiate such small periods (and also because of persistence of vision), we tend to see this multiplicity of strokes as one flash, lasting up to say half a second. So where does the thunder come from? As the air is ionised by the electrons (creating the flash of light) it is violently heated to around thirty thousand degrees. This massive influx of energy causes the air to expand extremely rapidly, creating a shock wave which we hear as thunder. The closer you are to the lightning stroke, the shorter and sharper the shock wave. If you are very close, all you will hear is one mighty “K-ER-A-C-K!” – and if you’re still alive afterwards, you might think to yourself “goodness gracious me . . .”, or words to SC that effect! November 1996  11 SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: dicksmith.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: dicksmith.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: dicksmith.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: dicksmith.com.au CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates. Swimming pool lap counter While intended for the backyard pool, this simple lap counter could be used anywhere some form of lap or event counter is required. In a swimming pool application, the unit is waterproofed and placed at either end of the pool on the edge. When a lap is completed, the swimmer touch­ es the button to advance the display by 1. An effective 2km distance in a 10m backyard pool is achiev­ed with a 2digit display. The circuit is straightforward. The regulator provides a 5V rail for the ICs while the two transistors form a debounce circuit for the “count” pushbutton switch. IC1, IC3 and DISP1 make up the counter for the units digit. IC1 is a decade counter with BCD outputs, while IC3 is a BCD to 7- segment decoder/driver. Similarly, IC2, IC4 and DISP2 count and display the tens digit. The reset button ensures the display resets to “00”. If necessary, a hundreds counter could be added, taking its input from the tens counter (IC2). The circuit can run from a 6-12V battery supply; mains power is not an option for safety reasons. As shown above, it is powered from a 9V supply, however a “216” type 9V battery will have limited life. Two 6V lantern batteries connected in series to give 12V are recommended due to their high capacity (the regulator needs in excess of 7V to function properly). Care should be taken with waterproofing, especially the count and reset switches. The count switch ideally could be some form of touch pad almost immersed in the pool. S. Isreb Traralgon, Vic. ($40) Obtaining balanced & isolated 9V supply rails While there are a number of circuits to generate a negative 9V rail from a positive supply, if you want a pair of balanced 9V rails which are isolated from the input DC rails, there is only one practical approach; a transformer must be used. This circuit employs a 555 timer operating at 25kHz to drive a transformer based on a small ferrite toroid. The primary is wound with 15 turns while the secondary has 36 turns (18 turns either side of the centre-tap). It should be capable of delivering up to 20mA but the output is not regulated. The output voltage can be varied by varying the mark space ratio of the driving waveform from IC1; 16  Silicon Chip ie, by varying trimpot VR1 (10kΩ). Note: this circuit has not been tested. SILICON CHIP Thermostatic fan controller This circuit is based on the Low Fuel Indicator published in the March 1993 issue of SILICON CHIP. By fitting a relay in place of the warning lamp and connecting the sensor lead to an existing temperature sender, the unit was converted to operate a thermostatic fan. The relay is a PC type and was mounted along with the other extra 9V nicad battery saver/regulator This circuit provides a 5V regulated output from a 9V nicad battery supply and switches off before the battery is over-discharged, to avoid permanent damage to the cells. Q1 is a 5V regulator while IC1 monitors the battery and disconnects the regulator at a preset input voltage. In operation, IC1 functions as a non-inverting Schmitt trigger, with trimpot VR1 setting the trip voltage parts on a small piece of Veroboard which fitted into the moulded slots of the existing case. A hole was drilled through the main circuit board and the outer case so that a small screwdriver can access trimpot VR1 for adjustment. The LED isn’t really necessary but was fitted for testing and was cheap enough to leave in place. The 10-second delay is adequate to avoid “hunting” at switch on, but as the unit was also hunting when near switching off, a 4.7µF capacitor was fitted across the relay coil to pro­vide a short time delay before switch off (around 2 seconds). Because the unit is in a plastic case, it has to be mounted in the coolest practicable position. Note: the controller will come on for a short time when the engine is started. Ed Fudala, Forest Lake, Qld. ($30) and VR2 set­ting the degree of hysteresis. In some applications, a fair amount of hysteresis is needed to prevent the Schmitt trigger tripping on and off due to fluctuations in load current. When the voltage at pin 3 is below the reference voltage at pin 2, IC1’s output will be low and LED1 is illuminated, indicat­ing that the battery voltage is low and that the output is disabled. Diode D1 will now be reverse-biased, thereby preventing current flow to the base of transistor Q1. This turns Q1 off, thereby reducing the output voltage to 0V. When the output of IC1 is high, Q1 is turned on and then Q1, Z2 and D2 operate as a regulator, providing a nominal output voltage of +5V. LED 2 lights to indicate that the battery is OK and that the circuit is operating. The optional input circuit incorporating Q2 is used to further conserve battery power. It switches power to the main circuit only while the button (S1) is pressed. S. Carroll, Timmsvale, NSW. ($35) November 1996  17 BOOKSHELF EDN Designer’s Companion EDN Designer’s Companion, Hickman/Travis, published August 1994 by Butterworth-Heinemann, Australia. Hard covers, 250 x 195mm, 254 pages. ISBN 0 7506 1721 7, Price $69.00. Electronic Design News is a long established US publication with a worldwide readership and its technical articles, some written by the staff and others submitted by a broad spectrum of the electronics industry, are usually of a very high standard. Following on the success of the authors’ previous work, Electron­ ic Circuits Systems & Standards released in 1990, this new book contains a selection of articles published in EDN since that time. The book is divided into six major headings: Digital Tech­niques; Analog Technology; Analog/Digital Conversion; Oscillators, Generators & Clocks; Power Sources & Motor Control; and Test, Measurement & Standards. The main topic in the first section is a comprehensive discussion on data compression, with particular emphasis on im­ ages. When you calculate the amount of storage necessary to display images on the screen, you quickly realise why so much effort is being concentrated in this field. A full colour VGA screen needs nearly 1Mb, while an SVGA image needs 2.3Mb! Once we get to moving images the data rate as well as the volume becomes significant. The aim is to do as much compression as possible without losing significant detail. The characteristics of our vision allow some information to be discarded. The eye is relatively insensitive to small changes in intensity and even less sensitive to colour detail. 18  Silicon Chip Information on the JPEG (Joint Photographics Expert Group) compression algorithm for stills, the MPEG (Moving Pictures Expert Group) for moving pictures and the P*64 standard for video telephony are described and compared. Other newer systems are examined, including a two-chip set which is capable of hardware compression of 2.5:1. Compression algorithms The next chapter deals with compression algorithms for black & white images, such as technical drawings or faxes. Some areas of rapid data change can actually have a negative compres­sion; ie, the compressed code is actually bigger than the original. To overcome this problem, some schemes use bytes of literal, unencoded pixels amid the compressed code. This chapter concludes with details of the CCITT group 3 and group 4 image-data compression algorithms. The final two chapters in the first section describe cir­ cuits using DSP (Digital Signal Processing) chips to implement a random number generator and a random noise generator. The second section, Analog Technology, is the largest in the book and covers the selection of single (5V) supply op amps, external compensation, improving CMRR (Common Mode Rejection Ratio), current feedback amplifiers, filters and phase compensa­ tion for photodiodes. With the proliferation of 5V and 3.3V microprocessors and integrated circuits, the analog interfaces to these products are becoming more critical. Methods of obtaining stable Vcc/2 voltages are covered, along with low dropout regulated supplies. Circuits are shown for current loops, DACs and tempera­ ture sensors operating from these low voltages. Improving the CMRR of an instrumentation amplifier by using common mode driven supplies is an interesting application note. By keeping the supply voltage to the input amplifiers at the common mode voltage ± a low supply voltage (typically 5V), the CMRR can be improved by 40dB. In addition, by running the op amps at lower than the usual ±15V, the quiescent power dissipation and its associated temperature rise is reduced by at least 300%. This provides a commensurate decrease in thermally induced er­rors. Doug Smith of Burr-Brown Corporation contributes the next article on the use of current feedback amplifiers in high frequency filter design. Until recently, the design of active filters with cutoffs above 1MHz was difficult because voltage feedback ampli­fiers with sufficient gain-bandwidth products and short propaga­tion delays were too expensive. With the availability of current feedback or transimpedance amplifiers this situation has changed. Details of circuit designs using the OPA063 in a fifth order Cauer, a bandpass and a twin-T filter are given. Chapter 15 is titled Unusual Oscillators and Filters. It shows a method of compensating for the rather large capacitance of a ceramic filter to obtain a more symmetrical response and better out-of-band attenuation. Another circuit shows a FET stabilising a 20MHz oscillator. Also included are a 1MHz to 10MHz Wien bridge oscillator and a continuously variable function generator covering the range from 1Hz to 1MHz for a 0-10V input. The analog section finishes with a discussion on phantom circuits, whereby N pairs of wires can carry 2N - 1 signals and an article on methods of increasing the bandwidth of photo­ diode amplifiers. A/D conversion The third section, covering analog to digital conversion, begins with a method for sampling a narrow percentage bandwidth signal, such as an IF signal, at a sub-harmonic of the IF. This is followed by a prim­er, “sense and nonsense about sampled systems”. The author points out that while the Nyquist theorem promises frequency recovery without aliasing, it can produce peak amplitude errors of 30% for an 11kHz sinewave sam­ pled at 44kHz. The next chapter is devoted to oversampling data convert­ers. While these are slower they offer greater resolution (20-22 bits) and linearity. The newer 1-bit DACs are also covered. The section concludes with a method of using two 8-bit DACs to obtain 14-bit resolution. Chapter four covers oscillators of various types. One interesting circuit shows how to generate very low frequency triangular waveforms using two high frequency oscillators. Anoth­er explains how to extend the control range of a CMOS 4046 phase locked loop from the usual 10:1 range to 1000:1 by using a cur­rent sink. An unusual application of SAW filters as the reference in 140MHz and higher VCOs (voltage con­trolled oscillators), plus a servo loop amplifier to control the amplitude of a 10MHz crystal oscillator, are among several other circuits in this section. Power Sources and Motor Control are covered in chapter five. The topics covered here begin with inductor selection in DC/DC converters and continue with methods of using spe- cial ICs to control DC motors without using Hall sensors. By placing a linear regulator in the servo loop of a switching regulator, the next article describes how you get the precision of a linear regulator with the efficiency of a switcher. A discrete low dropout (130mV) linear regulator with 3V output from a 3.6V nicad using an LM358 and a FET is a good example of battery regulator design. The next item is a switching regulator operating from 8-40V which, by using two inductors, gives a dual stabilised output. The last reprint in this section shows how to create a 1A diode with a forward drop of only 0.04V using an LM393 to control a FET. The final section of this book consists of a collection of articles on testing, measuring and standards. Testing covers high speed ADCs and high speed digital circuits. Measurement of fast transients by building your own single shot recorder, techniques for measuring the distortion of advanced op-amps, measuring transient voltages between separate ground points and non-contact measurements are covered in the second group. The implementation of an ISO 9000 quality management system by the technical editor of EDN covers the benefits and pitfalls involved in seeking this qualification. There seems to be little difference between the US situation and that existing here. Electromagnetic compatibility The last article is an in-depth treatment of the current situation regarding EMC (Electromagnetic Compatibility) in the EEC. From January 1996 all electronic equipment sold must conform with the relevant standards and bear the “CE” mark of compliance. To quote one authority “if an electron flows in your product, the standards are applicable to it”. This means that any Australian exporters of electronic goods to the EEC will have to comply. The certification can be “in house” but for legal protection it will probably be safer to have the equipment approved by an independ­ent testing authority. This is an interesting collection of reprints from EDN, although it is unlikely that all items will be of interest to every reader. It is an ideal reference to browse through when you need SC inspiration. (R.J.W.) Are you frustrated using DOS or non-compliant Windows software? If so then you may be interested in the following schematic design software trade-in offer from OrCAD. Here are 7 good reasons to trade-in your old schematic software tool to OrCAD Capture for Windows… ❶ De-facto standard schematic capture software. OrCAD is the best-selling package with over 180,000 licensed users worldwide. ❷ Easy to use and learn. Capture has an online tutorial and hypertext ‘Help’. ❸ Works on Windows 3.x, Windows 95 and Windows NT. Support for all platforms provided in one box. ❹ True 32-bit application. Faster processing on 32-bit platforms. ❺ Cut, copy and paste between Capture and other Windows compliant software. Developed to comply with Microsoft Foundation Class. ❻ Supports hierarchical designs. Create complex designs in modular form. ❼ Only $799 (Trade-in offer to all registered owners of Protel schematics and selected other schematic capture software tools. Normally $2195). ✄ Please send me more information on OrCAD Capture for Windows. My details are: Name: Company: Address: Phone: Fax: I am using the following brands of software: Schematic Entry: Simulation: PCB Design: (Fax this form to EDA Solutions on 02-9413 4622 or ring and ask for Richard on 02-9413 4611) SC11/96 Level 3, South Tower 1-5 Railway Street CHATSWOOD NSW 2067 Australia Ph: +61-2-9413 4611 fax: +61-2-9413 4622 email: info<at>eda.com.au Offer for a limited time only. November 1996  19 Build an 8-cha stereo mixer; At last there’s a comprehensive mixer that’s easy to build. This unit features eight main input channels, an auxiliary input channel, LED bargraph level meters, effects send and comprehensive headphone monitoring facilities. 20  Silicon Chip annel Pt.1 By JOHN CLARKE W HEN IT COMES to mixers, every­- one has their own ideas about how the signal should be directed from the input to the output. And of course there are numerous options to be decided on in the design process. In this design, we have produced a practical arrangement which should be suitable for most mixer users. There are of course the usual level controls for each of the inputs, plus separate output pots for the left and right channels. Stereo effects are provided by separate Pan pots for each input channel. These allow tailored mixing into either the left or right channel bus, or a mixture in both. Each of the eight Main inputs includes tone control facili­ties. The monitor signal, however, is not affected by the tone controls and is a mono output only. This output can be fed to a foldback amplifier and speakers, so that musicians can hear themselves on the stage. An Effects signal output is also provided on each channel, immediately following the tone controls. It is intended for use with reverberation or other effects boxes for added sound en­ hancement. The resulting signal from the effects box can then be applied to the Auxiliary input of the mixer and subsequently level controlled and panned to the left and right outputs. If effects are not required, the Effects bus can be used as a second monitor output. Note that both the Monitor and Effects outputs for each channel have individual level controls. These are situated in two rows along the top of the front panel. All signal monitoring is provided via a headphone output which is situated near the top righthand corner of the front panel. An adjacent 12-way switch allows any of the eight main inputs to be monitored (after the tone controls), or the Monitor Bus, Effects Bus, Left Main Bus or Right Main Bus can be moni­tored. Two 10-step LED bargraph displays are used to indicate the left and right channel output levels. These cover signal levels from -24dB to +3dB and allow the operator to see what’s going on at a glance. Design considerations With all those facilities, the new 8-Channel Mixer is quite large. It fits into a 125mm deep metal case and this carries a front panel that measures 485mm wide x 310mm high. These dimen­ sions comply with a 7-unit rack sizing, which means that the unit can be mounted vertically in a rack frame. Alternatively, it can be used as a “standalone” unit which sits either horizontally or vertically. In addition, at least one retailer has indicated that they intend producing a complete kit of parts for this design and that their case will have a sloping front panel. Despite the amount of circuitry involved, the unit is easy to build since virtually all the parts go on a single large PC board. Even the input and output plugs and sockets mount on the PC board. This leaves only a small amount of wiring to be run for the power supply. By contrast, many other mixer designs use a separate PC board for each input channel plus several others for the output controls. The amount of wiring between these PC boards is consid­erable. Our new circuit is also much simpler than previous designs with similar facilities. This has been made possible by using a spe­cial purpose balanced input amplifier IC which provides a low noise signal for the following stages. Let’s now take a look at how it all works. Block diagram Fig.1 shows the general signal path arrangement of our new mixer. There are eight Main inputs plus a single Auxiliary input. Note, however, that only one Main input channel (IC1-IC3) is shown here in order to simplify the diagram. The other seven input channels are identical. Each main input can accept either an XLR plug or a 6.35mm stereo jack. For unbalanced inputs, you can ground one of the input pins (2 or 3) on the plug. This is standard practice and is done to avoid hum pickup when an unbalanced lead is connected to a balanced input. IC1 is the input amplifier and it can be switched to pro­vide either +30dB or +10dB of gain. The +30dB setting (LOW) is suitable for microphone levels and provides the mixer with an overall sensitivity of 4mV. The +10dB (HIGH) setting is suitable for higher input levels, such as from electric guitars and key­boards. In the latter mode, the overall sensitivity is reduced to 40mV November 1996  21 Fig.1: the general signal path arrangement of the new mixer. There are eight Main inputs (although only one is shown here) plus a single Auxiliary input and an Effects input. The outputs are metered using LED bargraph displays. and clipping occurs at 9V RMS. Note that the input does not provide phantom power for electret microphones. If you want to use electrets, they will either have to be battery-operated or powered from some other source. Following IC1, the signal is split two ways and drives level control pots VR1 (Main) and VR2 (Monitor). VR2 in turn feeds op amp stage IC2 (+12dB) which then drives the monitor bus via a mixing resistor. VR1, on the other hand, feeds IC2a which also provides +12dB of gain. Its output then drives a tone control stage con­ sisting of IC3 and potent­ iometers VR3 (bass) and VR4 (treble). 22  Silicon Chip The bass control provides a nominal 10dB boost or cut at 100Hz, while the treble control gives a 12dB boost or cut at 10kHz. Note that the bass control is usually only used to remove any “boominess” from instruments, while the treble control can help with sibilant (S) sounds by curtailing high frequencies in voice signals. The output of the tone control stage drives Effects level control VR5 and Pan control VR6. It also provides the Channel 1 headphone signal via an isolating resistor. The Effects control sets the signal level to be applied to the Effects bus (again via a mixing resistor), while the Pan control sets the signal levels fed to the left and right buses. If the signal is intended for the right channel only, then the Pan pot is fully rotated to shunt the left channel signal to ground. Conversely, if the signal if for the left channel only, the Pan pot is fully rotated in the opposite direction to shunt the right channel signal to ground. If the Pan pot is centred, then equal amounts of signal are applied to both the left and right buses. By contrast with the Main channels, the Auxiliary channel provides an unbalanced input only. This input is buffered by IC10a and this then drives pan control pot VR11 via level control VR10. The resulting left and right channel signals are then mixed onto the left and right main buses. From there, the left bus signal is fed to IC4a, while the right bus signal goes to IC7a. For the left channel, IC4a provides 11dB of gain and drives IC4b via output level control VR7. IC4b provides an extra 12dB of gain. Its output directly drives pin 2 of an XLR output socket and pin 3 of this same socket via inverting amplifier IC5a. As a result, two out-of-phase signals appear on pins 2 and 3 to provide the balanced output. Alternatively, an unbalanced output can be obtained by connecting between pin 2 and ground. In addition, IC4b drives a 10-LED bargraph display which indicates the signal level in 3dB steps. This display has a range from -24dB to +3dB. IC7a, IC7b, IC5b and the LED bar­ graph process the right channel bus signals in exactly the same manner. The Effects bus and Monitor bus output stages employ iden­ t ical circuitry. For the Effects bus, IC9a amplifies the mixed signal and drives IC9b via level control VR9. IC9b then drives the Effects Send socket to provide an unbalanced output signal. IC11a, VR12 and IC11b do exactly the same job for the mixed Monitor bus signal. Finally, the headphone amplifier is connected as an invert­ing stage (mixing) so that it can monitor the selected bus via switch S9. You can listen to all the input channel signals and each of the buses as shown. The amplifier is mono only, which means that the left and right bus signals are monitored separate­ly rather than in stereo. Circuit details Refer now to Fig.2 for the circuit details. To simplify matters, this shows only one of the eight input channels (channel 1) and only one of the two main output stages. IC1 is an Analog Devices SSM2017 Self-Contained Audio Preamplifier. This is a balanced input amplifier with a typical common mode rejection of 74dB at a gain of 10. Its total harmonic distortion and noise figures are also very low. IC1’s gain is determined by the resistance value between pins 1 and 8. In this case, the gain can be switched between +10dB and +30dB by S1 which selects either a 4.7kΩ resistor or a 330Ω resistor respectively. The 10kΩ resistors at the pin 2 and pin 3 inputs of IC1 ensure that it oper- Features • • • • Eight Main inputs plus Auxiliary input. • • • Bass and treble controls on eight Main inputs. • • • Special purpose low noise input amplifier. • Headphone monitoring for eight Main channels, plus Monitor, Effects, Right main and Left main buses. • Easy to build – single PC board construction eliminates all external wiring except for power supply. • Case conforms to 7-unit rack sizing; suitable for vertical or horizontal use. Stereo outputs. Effects and monitor for all eight Main inputs. Panning between left and right channels for all eight Main inputs and Auxiliary. High and low input signal selection for eight Main inputs. Balanced inputs for eight Main channels using XLR sockets or 6.35mm sockets. Balanced left and right main XLR outputs. Signal level metering for Left and Right output channels using dual LED bargraphs. Specifications Signal-to-Noise Ratio at Left and Right Main outputs 80dB unweighted <at> 1V out and 100mV input (all channel inputs unloaded and set at maximum) Bass and Treble controls ±10dB at 100Hz and ±12dB at 10kHz Sensitivity for 1-8 Channel inputs 4mV RMS for 1V output on LOW setting 40mV RMS for 1V output on HIGH setting Sensitivity for Monitor and Effects outputs 2mV RMS for 1V output on LOW setting; 20mV RMS for 1V output on HIGH setting Sensitivity for Auxiliary input 120mV RMS for 1V output Maximum input levels before clipping 2.9V RMS on LOW setting; 9V RMS on HIGH setting Frequency response -3dB at 20Hz and 32kHz (Main, Monitor and Effects) Total Harmonic Distortion 0.008% at 1kHz (100mV in and 1V out); 0.02% at 10kHz (100mV in and 1V out) ates within its correct common mode range when no DC connection is made to the input. If a balanced microphone is connected, its low 600Ω impedance will reduce the input load resistance, with a subsequent reduction in noise. The 270pF capacitor shunts any high frequency signals to improve common mode rejection at high frequencies and reduces the possibility of RF pickup. Note that the input to IC1 is not AC-coupled via a capaci­ tor. That’s because microphone and guitar signals November 1996  23 24  Silicon Chip Fig.2 (left): this circuit diagram shows only one of the eight Main input channels and only one of the two output stages. Note that all the Main channels have balanced inputs and feature tone control circuitry. The headphone monitoring circuit uses op amp IC12 to drive complementary output pair Q1 & Q2. are from a balanced or unbalanced transformer or inductive pickup and hence carry no DC voltage. What’s more, any small DC offset from say a line signal or keyboard will not cause problems since the amplifier can handle high DC offsets before any superimposed AC signal will be clipped. The output from IC1 is AC-coupled to prevent any DC flow in the following Main and Monitor pots (VR1 & VR2). This is neces­sary since any DC flow in these potentiometers will cause noise in the signal as they are adjusted. The output from VR2 is AC-coupled to the input of IC2b (pin 5) via a 2.2µF non-polarised (bipolar) capacitor. A 22kΩ resistor to ground sets the input bias, while the 10Ω resistor in series with the input reduces RF pickup. The gain is set to four by the 6.8kΩ and 2.2kΩ feedback resistors, while a 270pF feedback ca­pacitor rolls off the high frequency response from about 87kHz to prevent high-frequency instability. The amplified output from IC2b appears at pin 7 and is mixed onto the Monitor bus via a 10kΩ resistor. IC2a in the main signal path functions in exactly the same manner as IC2b. Besides providing gain and a high impedance load for level control VR1, IC2a also acts as a low impedance source for the following tone control stage based on IC3. This stage has the tone control pots (VR3 & VR4) connected in the negative feedback network. When the bass and treble controls are centred, the gain of the stage is unity, up to at least 50kHz. Winding the bass or treble controls toward the input side of IC3 increases the gain for frequencies above 2kHz for the treble control and 300Hz for the bass control. Conversely, when the tone controls are rotated in the opposite direction (to apply bass or treble cut), the gain is reduced above 2kHz and below 300Hz. This is because the negative feedback has been November 1996  25 AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 23 SEP 96 10:10:41 10.000 5.0000 0.0 -5.000 This stage operates with a gain of 4.09 and has a 100Ω resistor in series with its output to prevent instability when driving ca­pacitive loads. Following IC4b, the signal is fed to pin 2 of the XLR output socket via a 47µF capacitor. It also goes to pin 2 of inverting buffer stage IC5a, which then drives pin 3 to provide the other side of the balanced output signal. As with IC4b, IC5a has a 100Ω resistor in series with its output to prevent in­stability. Note that the outputs of IC4b and IC5a are both AC-coupled to the output XLR socket. This is to prevent any DC in the out­put. LED level indicator -10.00 -15.00 20 100 1k 10k 20k Fig.3: this graph shows the frequency response of the tone controls at their maximum boost and cut settings and at the flat setting. Note that the amount of boost and cut is set to ±12dB maximum in both instances. increased, giving a reduction in gain at these frequencies. The maximum bass boost and cut is limited to about ±12dB by the 22kΩ resistors on either side of the bass pot, VR3. Simi­larly, the amount of treble boost and cut provided by VR4 is limited to ±12dB by the 4.7kΩ resistors on either side of the treble pot, VR4. Fig.3 shows the action of the tone controls at their maximum boost and cut settings and also at the flat setting. Note that OP27GP op amps have been specified in the tone control circuitry. The reason for this is that the DC across VR3 must be as low as possible to limit noise when adjusting the bass con­trol. The input offset voltage for the OP27 is typically just 30µV while the input offset current is only 12nA and so the resulting DC in VR3 will be negligible. The output from IC3 appears at pin 6 and drives Effects pot VR5, the 10kΩ headphone signal resistor and the left and right channel Pan control circuitry. As mentioned previously, the Pan control operates by shunting signal in the unwanted channel to ground. When the wiper of VR6 is towards the left main bus side, the left channel signal is shunted to ground. Similarly, when the wiper of VR6 is towards the right bus side, the right channel signal 26  Silicon Chip is shunted. Finally, when the pot is centred the left and right signals are attenuated and so are equally mixed into the left and right channels. The seven other inputs are identical to this first input circuit but with different IC and pot numbering. Op amps IC10a and IC10b are used to process the unbalanced auxiliary input signal. IC10a is wired as a unity gain buffer stage, with its pin 3 (non-inverting) input biased to 0V by a 22kΩ resistor. The output signal appears on pin 1 and is AC-coupled to the Auxiliary level control (VR10) via a 2.2µF capaci­tor. Following VR10, the signal is fed to IC10b which operates with a gain of 4.09. IC10b in turn drives the Auxiliary pan control circuitry which mixes the signal onto the left and right main buses. Output stages The mixed left main bus signal is fed to the pin 2 input of IC4a via a 2.2kΩ resistor. This op amp (an LM833) is wired as an inverting stage and amplifies the left bus signal by a factor of 3.4. The 27pF capacitor across the 68kΩ resistor provides high frequency rolloff above about 87kHz. IC4a’s output at pin 1 feeds the Left Main level control (VR7), after which the signal is coupled to gain stage IC4b. IC4b also drives the LED level indicator circuit. This circuit is based on IC6 which is a logarithmic LED display driver wired to operate in bargraph mode. The signal from IC4b is ap­plied to pin 5 via a 100Ω de­coupling resistor. Inside the IC, the negative-going signal excursions are clamped via a diode while positive signal excursions are fed to comparator circuits which then drive the individual LEDs. The meter circuit responds instantaneously to the waveform and thus shows the peak voltage of a sinewave. Note, however, that the peak LED does not light on very short transients and so the meter can be considered to be an averaging display. The meter calibration is set by the voltage on pins 6 & 7. This voltage is determined by first applying the 1.2V internal reference that appears between pins 6 & 8 to a 330Ω resistor. The resulting 3.6mA current then flows to ground via a 68Ω resistor, which thus has 0.25V across it. As a result, pins 6 & 7 of IC6 sit at 1.45V (1.2V + 0.25V). This means that the LED bargraph reaches full-scale (equivalent to +3B) when the applied signal level reaches 1.45V, corresponding to a nominal 1V RMS sinewave. The 0dB level (LED 9) occurs at 0.7V RMS. The 270Ω resistor in series with the LED anodes limits the dissipation in IC6, while the associated 100µF capacitor decou­ples the LED supply rails. A 10µF capacitor decouples the supply rail to the IC. The right channel output stage is identical to the left channel circuitry. In this case, the devices and their Despite the amount of circuitry involved, the assembly is really very easy since virtually all the parts are on this single large PC board. Note that this photo shows an early prototype board. corre­sponding pin numbers are indicated in brackets. Monitor & effects stages IC11a is the Monitor bus summing amplifier and this stage operates with a gain of about 5.7. Its output is fed to level control VR12 and from there to IC11b which provides a gain 2. IC11b’s output is then fed via a 100Ω resistor (for stability) and a 47µF capacitor to the Monitor output socket. The Effects summing and output amplifier stages (IC9a and IC9b, respectively) operate in exactly the same manner as the Monitor stages. Headphone amplifier The headphone amplifier is based on op amp IC12 and this operates in combination with transistors Q1 and Q2 which form a fairly conventional push-pull output stage. The transistors are there to boost the output current cap­ability of the TL071 op amp. Note that they are slightly forward-biased (to minimise cross­ over distortion) by connecting diodes D1 and D2 in series between their bases. The distortion produced by the output transistors is also minimised by incorpo­rating them inside the feedback loop of the op amp. The 33Ω emitter resistors have been included to maintain the bias stability. Together with the 68Ω output series resistor, they also provide short circuit protection and protect the head­phones against damage in the unlikely event of an amplifier failure. Power supply Power for the circuit is derived from a toroidal transform­er which delivers a 30V centre-tapped AC output. The primary of the transformer is fused for safety, while a .001µF 2kV capaci­tor is connected across the power switch to minimise switch-off transients. The secondary voltage is applied to bridge rectifier D3-D6 and the resulting DC rails filtered using two 1000µF capacitors to obtain ±21VDC. These rails are then fed to 3-terminal regu­ lators REG1 and REG2 which provide stable ±15V rails for the mixer circuit. Note that the output of each regulator is decoupled with a 10µF capacitor to maintain stability. In addition, there are many other 10µF capacitors scattered around the circuit which decouple the supply rails to the ICs. These are important to ensure stability in the op amps. Power indication is provided by LED21 which is connected in series with a 4.7kΩ resistor between ground and the -15V rail. By the way, the toroidal type has been specified to mini­ m ise hum induction in the mixer circuit. Do not use a standard E-core type since the signal-to-noise ratio will suffer greatly and hum will be heard in the mixer output. That’s all we have space for this month. In Pt.2, we shall present the parts list and give the full construction SC details. November 1996  27 Silicon Chip Back Issues December 1990: The CD Green Pen Controversy; 100W DC-DC Converter For Car Amplifiers; Wiper Pulser For Rear Windows; 4-Digit Combination Lock; 5W Power Amplifier For The 6-Metre Amateur Transmitter; Index To Volume 3. January 1991: Fast Charger For Nicad Batteries, Pt.1; Have Fun With The Fruit Machine; Two-Tone Alarm Module; LCD Readout For The Capacitance Meter; How Quartz Crystals Work; The Dangers of Servicing Microwave Ovens. September 1988: Hands-Free Speakerphone; Electronic Fish Bite Detector; High Performance AC Millivoltmeter, Pt.2; Build The Vader Voice. Quality Audio Oscillator, Pt.2; The Incredible Hot Canaries; Random Wire Antenna Tuner For 6 Metres; Phone Patch For Radio Amateurs, Pt.2. April 1989: Auxiliary Brake Light Flasher; What You Need to Know About Capacitors; 32-Band Graphic Equaliser, Pt.2; The Story Of Amtrak Passenger Services. March 1990: Delay Unit For Automatic Antennas; Workout Timer For Aerobics Classes; 16-Channel Mixing Desk, Pt.2; Using The UC3906 SLA Battery Charger IC; The Australian VFT Project. May 1989: Build A Synthesised Tom-Tom; Biofeedback Monitor For Your PC; Simple Stub Filter For Suppressing TV Interference; The Burlington Northern Railroad. April 1990: Dual Tracking +/-50V Power Supply; Voice-Operated Switch (VOX) With Delayed Audio; 16-Channel Mixing Desk, Pt.3; Active CW Filter; Servicing Your Microwave Oven. July 1989: Exhaust Gas Monitor; Experimental Mains Hum Sniffers; Compact Ultrasonic Car Alarm; The NSW 86 Class Electrics. June 1990: Multi-Sector Home Burglar Alarm; Low-Noise Universal Stereo Preamplifier; Load Protector For Power Supplies; Speed Alarm For Your Car; Fitting A Fax Card To A Computer. September 1989: 2-Chip Portable AM Stereo Radio (Uses MC13024 and TX7376P) Pt.1; High Or Low Fluid Level Detector; Studio Series 20-Band Stereo Equaliser, Pt.2. October 1989: FM Radio Intercom For Motorbikes Pt.1; GaAsFet Preamplifier For Amateur TV; 2-Chip Portable AM Stereo Radio, Pt.2; A Look At Australian Monorails. November 1989: Radfax Decoder For Your PC (Displays Fax, RTTY & Morse); FM Radio Intercom For Motorbikes, Pt.2; 2-Chip Portable AM Stereo Radio, Pt.3; Floppy Disc Drive Formats & Options; The Pilbara Iron Ore Railways. December 1989: Digital Voice Board; UHF Remote Switch; Balanced Input & Output Stages; Operating an R/C transmitter; Index to Volume 2. January 1990: High Quality Sine/Square Oscillator; Service Tips For Your VCR; Phone Patch For Radio Amateurs; Active Antenna Kit; Designing UHF Transmitter Stages; A Look At Very Fast Trains. February 1990: A 16-Channel Mixing Desk; Build A High July 1990: Digital Sine/Square Generator, Pt.1 (Covers 0-500kHz); Burglar Alarm Keypad & Combination Lock; Simple Electronic Die; Low-Cost Dual Power Supply; Inside A Coal Burning Power Station. February 1991: Synthesised Stereo AM Tuner, Pt.1; Three Inverters For Fluorescent Lights; Low-Cost Sinewave Oscillator; Fast Charger For Nicad Batteries, Pt.2; How To Design Amplifier Output Stages. March 1991: Remote Controller For Garage Doors, Pt.1; Transistor Beta Tester Mk.2; A Synthesised AM Stereo Tuner, Pt.2; Multi-Purpose I/O Board For PC-Compatibles; Universal Wideband RF Preamplifier For Amateur Radio & TV. April 1991: Steam Sound Simulator For Model Railroads; Remote Controller For Garage Doors, Pt.2; Simple 12/24V Light Chaser; Synthesised AM Stereo Tuner, Pt.3; A Practical Approach To Amplifier Design, Pt.2. May 1991: 13.5V 25A Power Supply For Transceivers; Stereo Audio Expander; Fluorescent Light Simulator For Model Railways; How To Install Multiple TV Outlets, Pt.1. June 1991: A Corner Reflector Antenna For UHF TV; 4-Channel Lighting Desk, Pt.1; 13.5V 25A Power Supply For Transceivers, Pt.2; Active Filter For CW Reception; Tuning In To Satellite TV. August 1990: High Stability UHF Remote Transmitter; Universal Safety Timer For Mains Appliances (9 Minutes); Horace The Electronic Cricket; Digital Sine/Square Generator, Pt.2. July 1991: Loudspeaker Protector For Stereo Amplifiers; 4-Channel Lighting Desk, Pt.2; How To Install Multiple TV Outlets, Pt.2; Tuning In To Satellite TV, Pt.2; The Snowy Mountains Hydro Scheme. September 1990: Low-Cost 3-Digit Counter Module; Simple Shortwave Converter For The 2-Metre Band; the Bose Lifestyle Music System. August 1991: Build A Digital Tachometer; Masthead Amplifier For TV & FM; PC Voice Recorder; Tuning In To Satellite TV, Pt.3; Step-By-Step Vintage Radio Repairs. October 1990: The Dangers of PCBs; Low-Cost Siren For Burglar Alarms; Dimming Controls For The Discolight; Surfsound Simulator; DC Offset For DMMs; NE602 Converter Circuits. September 1991: Digital Altimeter For Gliders & Ultralights; Ultrasonic Switch For Mains Appliances; The Basics Of A/D & D/A Conversion; Plotting The Course Of Thunderstorms. November 1990: How To Connect Two TV Sets To One VCR; Egg Timer; Low-Cost Model Train Controller; 1.5V To 9V DC Converter; Introduction To Digital Electronics; Simple 6-Metre Amateur Transmitter. October 1991: Build A Talking Voltmeter For Your PC, Pt.1; SteamSound Simulator Mk.II; Magnetic Field Strength Meter; Digital Altimeter For Gliders, Pt.2; Military Applications Of R/C Aircraft. 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Or call (02) 9979 5644 & quote your credit card details or fax the details to (02) 9979 6503. ✂ Card No. November 1991: Build A Colour TV Pattern Generator, Pt.1; Junkbox 2-valve receiver; Flashing Alarm Light For Cars; Digital Altimeter For Gliders, Pt.3; A Talking Voltmeter For Your PC, Pt.2. December 1993: Remote Controller For Garage Doors; LED Stroboscope; 25W Amplifier Module; 1-Chip Melody Generator; Engine Management, Pt.3; Index To Volume 6. December 1991: TV Transmitter For VCRs With UHF Modulators; Infrared Light Beam Relay; Colour TV Pattern Generator, Pt.2; Index To Volume 4. January 1994: 3A 40V Adjustable Power Supply; Switching Regulator For Solar Panels; Printer Status Indicator; Mini Drill Speed Controller; Stepper Motor Controller; Active Filter Design; Engine Management, Pt.4. January 1992: 4-Channel Guitar Mixer; Adjustable 0-45V 8A Power Supply, Pt.1; Baby Room Monitor/FM Transmitter; Experiments For Your Games Card. March 1992: TV Transmitter For VHF VCRs; Thermostatic Switch For Car Radiator Fans; Telephone Call Timer; Coping With Damaged Computer Directories; Valve Substitution In Vintage Radios. April 1992: IR Remote Control For Model Railroads; Differential Input Buffer For CROs; Understanding Computer Memory; Aligning Vintage Radio Receivers, Pt.1. May 1992: Build A Telephone Intercom; Electronic Doorbell; Battery Eliminator For Personal Players; Infrared Remote Control For Model Railroads, Pt.2; Aligning Vintage Radio Receivers, Pt.2. June 1992: Multi-Station Headset Intercom, Pt.1; Video Switcher For Camcorders & VCRs; IR Remote Control For Model Railroads, Pt.3; 15-Watt 12-240V Inverter; A Look At Hard Disc Drives. July 1992: Build A Nicad Battery Discharger; 8-Station Automatic Sprinkler Timer; Portable 12V SLA Battery Charger; Multi-Station Headset Intercom, Pt.2. August 1995: Fuel Injector Monitor For Cars; Gain Controlled Microphone Preamp; Audio Lab PC Controlled Test Instrument, Pt.1; Mighty-Mite Powered Loudspeaker; How To Identify IDE Hard Disc Drive Parameters. March 1994: Intelligent IR Remote Controller; 50W (LM3876) Audio Amplifier Module; Level Crossing Detector For Model Railways; Voice Activated Switch For FM Microphones; Simple LED Chaser; Engine Management, Pt.6. September 1995: Keypad Combination Lock; The Incredible Vader Voice; Railpower Mk.2 Walkaround Throttle For Model Railways, Pt.1; Jacob’s Ladder Display; The Audio Lab PC Controlled Test Instrument, Pt.2. April 1994: Sound & Lights For Model Railway Level Crossings; Discrete Dual Supply Voltage Regulator; Universal Stereo Preamplifier; Digital Water Tank Gauge; Engine Management, Pt.7. October 1995: Geiger Counter; 3-Way Bass Reflex Loudspeaker System; Railpower Mk.2 Walkaround Throttle For Model Railways, Pt.2; Fast Charger For Nicad Batteries; Digital Speedometer & Fuel Gauge For Cars, Pt.1. May 1994: Fast Charger For Nicad Batteries; Induction Balance Metal Locator; Multi-Channel Infrared Remote Control; Dual Electronic Dice; Simple Servo Driver Circuits; Engine Management, Pt.8; Passive Rebroadcasting For TV Signals. June 1994: 200W/350W Mosfet Amplifier Module; A Coolant Level Alarm For Your Car; 80-Metre AM/CW Transmitter For Amateurs; Converting Phono Inputs To Line Inputs; PC-Based Nicad Battery Monitor; Engine Management, Pt.9. July 1994: Build A 4-Bay Bow-Tie UHF Antenna; PreChamp 2-Transistor Preamplifier; Steam Train Whistle & Diesel Horn Simulator; Portable 6V SLA Battery Charger; Electronic Engine Management, Pt.10. September 1992: Multi-Sector Home Burglar Alarm; Heavy-Duty 5A Drill speed Controller (see errata Nov. 1992); General-Purpose 3-1/2-Digit LCD Panel Meter; Track Tester For Model Railroads; Build A Relative Field Strength Meter. August 1994: High-Power Dimmer For Incandescent Lights; Microprocessor-Controlled Morse Keyer; Dual Diversity Tuner For FM Microphones, Pt.1; Build a Nicad Zapper; Engine Management, Pt.11. October 1992: 2kW 24VDC - 240VAC Sinewave Inverter; Multi-Sector Home Burglar Alarm, Pt.2; Mini Amplifier For Personal Stereos; A Regulated Lead-Acid Battery Charger. September 1994: Automatic Discharger For Nicad Battery Packs; MiniVox Voice Operated Relay; Image Intensified Night Viewer; AM Radio For Weather Beacons; Dual Diversity Tuner For FM Microphones, Pt.2; Engine Management, Pt.12. February 1993: Three Projects For Model Railroads; Low Fuel Indicator For Cars; Audio Level/VU Meter (LED Readout); An Electronic Cockroach; 2kW 24VDC To 240VAC Sinewave Inverter, Pt.5. March 1993: Solar Charger For 12V Batteries; Alarm-Triggered Security Camera; Reaction Trainer; Audio Mixer for Camcorders; A 24-Hour Sidereal Clock For Astronomers. April 1993: Solar-Powered Electric Fence; Audio Power Meter; Three-Function Home Weather Station; 12VDC To 70VDC Converter; Digital Clock With Battery Back-Up. May 1993: Nicad Cell Discharger; Build The Woofer Stopper; Alphanumeric LCD Demonstration Board; The Microsoft Windows Sound System; The Story of Aluminium. June 1993: AM Radio Trainer, Pt.1; Remote Control For The Woofer Stopper; Digital Voltmeter For Cars; Windows-based Logic Analyser. July 1993: Single Chip Message Recorder; Light Beam Relay Extender; AM Radio Trainer, Pt.2; Quiz Game Adjudicator; Windows-based Logic Analyser, Pt.2; Antenna Tuners – Why They Are Useful. August 1993: Low-Cost Colour Video Fader; 60-LED Brake Light Array; Microprocessor-Based Sidereal Clock; Southern Cross Z80-Based Computer; A Look At Satellites & Their Orbits. September 1993: Automatic Nicad Battery Charger/Discharger; Stereo Preamplifier With IR Remote Control, Pt.1; In-Circuit Transistor Tester; A +5V to ±15V DC Converter; Remote-Controlled Cockroach. October 1993: Courtesy Light Switch-Off Timer For Cars; Wireless Microphone For Musicians; Stereo Preamplifier With IR Remote Control, Pt.2; Electronic Engine Management, Pt.1. November 1993: Jumbo Digital Clock; High Efficiency Inverter For Fluorescent Tubes; Stereo Preamplifier With IR Remote Control, Pt.3; Siren Sound Generator; Engine Management, Pt.2; Experiments For Games Cards. July 1995: Electric Fence Controller; How To Run Two Trains On A Single Track (Plus Level Crossing Lights & Sound Effects); Setting Up A Satellite TV Ground Station; Door Minder; Adding RAM To A Computer. February 1994: 90-Second Message Recorder; 12-240VAC 200W Inverter; 0.5W Audio Amplifier; 3A 40V Adjustable Power Supply; Engine Management, Pt.5; Airbags - How They Work. August 1992: An Automatic SLA Battery Charger; Miniature 1.5V To 9V DC Converter; 1kW Dummy Load Box For Audio Amplifiers; Troubleshooting Vintage Radio Receivers; MIDI Explained. January 1993: Flea-Power AM Radio Transmitter; High Intensity LED Flasher For Bicycles; 2kW 24VDC To 240VAC Sinewave Inverter, Pt.4; Speed Controller For Electric Models, Pt.3. Security System; Multi-Channel Radio Control Transmitter For Models, Pt.1; Build A $30 Digital Multimeter. October 1994: Dolby Surround Sound - How It Works; Dual Rail Variable Power Supply; Talking Headlight Reminder; Electronic Ballast For Fluorescent Lights; Temperature Controlled Soldering Station; Engine Management, Pt.13. November 1994: Dry Cell Battery Rejuvenator; Novel Alphanumeric Clock; 80-Metre DSB Amateur Transmitter; Twin-Cell Nicad Discharger (See May 1993); Anti-Lock Braking Systems; How To Plot Patterns Direct To PC Boards. December 1994: Dolby Pro-Logic Surround Sound Decoder, Pt.1; Easy-To-Build Car Burglar Alarm; Three-Spot Low Distortion Sinewave Oscillator; Clifford - A Pesky Electronic Cricket; Cruise Control - How It Works; Remote Control System for Models, Pt.1; Index to Vol.7. January 1995: Sun Tracker For Solar Panels; Battery Saver For Torches; Dolby Pro-Logic Surround Sound Decoder, Pt.2; Dual Channel UHF Remote Control; Stereo Microphone Preamplifier;The Latest Trends In Car Sound; Pt.1. February 1995: 50-Watt/Channel Stereo Amplifier Module; Digital Effects Unit For Musicians; 6-Channel Thermometer With LCD Readout; Wide Range Electrostatic Loudspeakers, Pt.1; Oil Change Timer For Cars; The Latest Trends In Car Sound; Pt.2; Remote Control System For Models, Pt.2. March 1995: 50W/Channel Stereo Amplifier, Pt.1; Subcarrier Decoder For FM Receivers; Wide Range Electrostatic Loudspeakers, Pt.2; IR Illuminator For CCD Cameras; Remote Control System For Models, Pt.3; Simple CW Filter. April 1995: Build An FM Radio Trainer, Pt.1; A Photographic Timer For Darkrooms; Balanced Microphone Preamplifier & Line Filter; 50-Watt Per Channel Stereo Amplifier, Pt.2; Wide Range Electrostatic Loudspeakers, Pt.3; 8-Channel Decoder For Radio Remote Control. May 1995: What To Do When the Battery On Your PC’s Mother­ board Goes Flat; Build A Guitar Headphone Amplifier; FM Radio Trainer, Pt.2; Transistor/Mosfet Tester For DMMs; 16-Channel Decoder For Radio Remote Control; Introduction to Satellite TV. June 1995: Build A Satellite TV Receiver; Train Detector For Model Railways; 1W Audio Amplifier Trainer; Low-Cost Video November 1995: Mixture Display For Fuel Injected Cars; CB Transverter For The 80M Amateur Band, Pt.1; PIR Movement Detector; Dolby Pro Logic Surround Sound Decoder Mk.2, Pt.1; Digital Speedometer & Fuel Gauge For Cars, Pt.2. December 1995: Engine Immobiliser; 5-Band Equaliser; CB Transverter For The 80M Amateur Band, Pt.2; Subwoofer Controller; Dolby Pro Logic Surround Sound Decoder Mk.2, Pt.2; Knock Sensing In Cars; RAM Doubler Reviewed; Index To Volume 8. January 1996: Surround Sound Mixer & Decoder, Pt.1; Magnetic Card Reader; Build An Automatic Sprinkler Controller; IR Remote Control For The Railpower Mk.2; Recharging Nicad Batteries For Long Life. February 1996: Three Remote Controls To Build; Woofer Stopper Mk.2; 10-Minute Kill Switch For Smoke Detectors; Basic Logic Trainer; Surround Sound Mixer & Decoder, Pt.2; Use your PC as a Reaction Timer. March 1996: Programmable Electronic Ignition System For Cars; Zener Tester For DMMs; Automatic Level Control For PA Systems; 20ms Delay For Surround Sound Decoders; Multi-Channel Radio Control Transmitter; Pt.2; Cathode Ray Oscilloscopes, Pt.1. April 1996: Cheap Battery Refills For Mobile Telephones; 125W Power Amplifier Module; Knock Indicator For Leaded Petrol Engines; Multi-Channel Radio Control Transmitter; Pt.3; Cathode Ray Oscilloscopes, Pt.2. May 1996: Upgrading The CPU In Your PC; High Voltage Insulation Tester; Knightrider Bi-Directional LED Chaser; Duplex Intercom Using Fibre Optic Cable; Cathode Ray Oscilloscopes, Pt.3. June 1996: BassBox CAD Loudspeaker Software Reviewed; Stereo Simulator (uses delay chip); Rope Light Chaser; Low Ohms Tester For Your DMM; Automatic 10A Battery Charger. July 1996: Installing a Dual Boot Windows System On Your PC; Build A VGA Digital Oscilloscope, Pt.1; Remote Control Extender For VCRs; 2A SLA Battery Charger; 3-Band Parametric Equaliser; Single Channel 8-bit Data Logger. August 1996: Electronics on the Internet; Customising the Windows Desktop; Introduction to IGBTs; Electronic Starter For Fluores­cent Lamps; VGA Oscilloscope, Pt.2; 350W Amplifier Module; Masthead Amplifier For TV & FM; September 1996: Making Prototypes By Laser; VGA Oscilloscope, Pt.3; Infrared Stereo Headphone Link, Pt.1; High Quality PA Loudspeaker; 3-Band HF Amateur Radio Receiver; Feedback On Pro­grammable Ignition (see March 1996). October 1996: Send Video Signals Over Twisted Pair Cable; Power Control With A Light Dimmer; 600W DC-DC Converter For Car Hifi Systems, Pt.1; Infrared Stereo Headphone Link, Pt.2; Build A Multi-Media Sound System, Pt.1; Multi-Channel Radio Control Transmitter, Pt.8. PLEASE NOTE: November 1987 to August 1988, October 1988 to March 1989, June 1989, August 1989, May 1990, February 1992, November 1992 and December 1992 are now sold out. All other issues are presently in stock. For readers wanting articles from sold-out issues, we can supply photostat copies (or tear sheets) at $7.00 per article (includes p&p). When supplying photostat articles or back copies, we automatically supply any relevant notes & errata at no extra charge. A complete index to all articles published to date is available on floppy disc at $10 including packing & postage. November 1996  29 This photo shows two versions of the PC module, one with filament over-windings fitted to the transformer. The module can drive virtually any gas-filled discharge tube. A low-cost f luorescent light inverter This low cost DC-AC converter will drive just about any fluorescent tube and also neon tubes up to about 1.5m long. It is suitable for solar power installations and has low current drain. It is easy to build and comes with a prewound transformer. Design by BRANCO JUSTIC This project is a bit of a novelty since it will drive just about any type of gas-filled tube, whether fluorescent or not. It has a high output voltage to fire these tubes but a relatively high output impedance as well, so it cannot deliver a lot of current. Because it can’t deliver a lot of current, it is inherently self-protecting and is quite economical to run. The downside is that it will not drive typical fluorescent tubes to their full brilliance – their output will be no30  Silicon Chip ticeably reduced compared to normal operation from the 240VAC 50Hz supply. However, there are plenty of applications where this type of operation is all that is required. A prime example of this is for emergency lighting. Most commercial, office and industrial buildings have emergency light­ ing based on modified fluorescent light fittings. A typical fitting contains a standard 240VAC ballast and starter for 50Hz mains operation but also contains a battery pack, DC-AC inverter and charger. If there is a blackout, such fittings will normally operate for several hours under battery power but their light output is considerably less than normal. The above remarks concerning reduced output apply mainly to 18W and 36W fluoro tubes. Smaller tubes can deliver almost their full output, depending on how the circuit is set up. Just to increase the novelty, the circuit can be made to provide continuous light from the tube or flashed operation. PC module The PC module itself is quite compact, measuring 85 x 40mm. It has a fairly substantial high frequency inverter transformer mounted on it, along with one CMOS 4093 hex Schmitt trigger IC, one Mosfet and a handful of other bits. The circuit Fig.1: the circuit is based on a 4093 Schmitt trigger IC. IC1b is the main oscillator and runs at over 50kHz. diagram is shown in Fig.1. IC1b is the heart of the circuit. It is connected as a free-running oscillator with a frequency of about 50kHz, as set by the 330pF capacitor and resistors R2 & R3. The actual frequen­cy will depend on the hysteresis of the particular 4093 IC used. The output pulse waveform has its duty cycle determined by the ratio of resistors R2 and R3. These two resistors vary the charge and discharge times of the 330pF capacitor. When the capacitor is charging up, it is fed by D2 and R3 and when it is being discharged, the current path is via D1 and R2. If R2 & R3 have the same value, the output at pin 3 will be a square wave; ie, the duty cycle will be 50%. We’ll discuss the need for vary­ing the duty cycle a little later on. The pulse waveform at pin 3 is buffered by IC1c and IC1d which are connected in parallel. These square up the waveform and then drive the gate of Q1, a P222 Mosfet. It drives the step-up transformer T1. This has 9 turns on the primary and 800 turns on the secondary. The pulsed DC applied by Q1 to the primary is about 19V peak-to-peak and this is stepped up in the transformer secondary to deliver a high frequency AC waveform of a thousand volts or more, depending on the loading. When a fluorescent tube fires (which can require a peak voltage of 800V or more), the output voltage is then loaded down to the tube maintaining voltage, typically 120V peak. The output from the transformer is AC coupled via two 22pF 3kV ceramic capacitors. Such small capacitors are adequate to feed the current to the tube because of the relatively high operating frequency of around 50kHz. The scope waveforms of Fig.2 show the pulse waveform at pin 3 of IC1b (Channel 1 signal) and switching waveform at the drain of Q1 (Channel 2 signal). Note that while the pulse waveform from pin 3 of IC1b is quite clean and has a long positive duty cycle of about 86%, the corresponding waveform at the drain of Q1 has been “dirtied up” by the transformer loading and also is rounded off to a degree by the 0.12µF capacitor, C6, connected across the transformer primary. PARTS LIST 1 PC board, 85 x 40mm 1 prewound transformer (T1) 1 4093 hex Schmitt trigger (IC1) 1 P222 Mosfet (Q1) 2 1N4148 diodes (D1, D2) Capacitors 1 100µF 16VW electrolytic 1 10µF 16VW electrolytic 1 0.12µF metallised polyester (greencap) 1 330pF ceramic 2 22pF 3kV ceramic Resistors (0.25W, 1% or 5%) 1 220kΩ 1 15kΩ (R2 - see text) 1 100kΩ 1 22Ω Varying the duty cycle Depending on the size of tube driven by this circuit, the light output may be increased by varying the duty cycle of the oscillator, IC1b. This is done by selecting the value of R2 and this can be varied between 15kΩ and 100kΩ. With R3 at 15kΩ the current drain will be minimum and any increase in value will result in increased current drain as well as more light output from the tube. Adding filament windings So far we have described just the basic inverter and this will drive virtually any gas-filled tube, as mentioned above. However, if fluorescent tubes are started without having their filaments heated, the tube ends will tend to blacken in a short time. This does not reduce the tube’s light output to any extent but it is unsightly. The degree of blackening depends on the tube current. The blackening is minimal when the tubes are used at low currents (with R2 = 15kΩ). In this case, useful light output can be ob­tained from 36W tubes with approximately 1.2W input (100mA drawn from a 12V battery). However, if the light output is increased by select­ing a higher value of R2, the tube ends will quickly blacken. This blackening is cathode material that has been splattered off the filaments. Ultimately, this damage will mean that all the emissive material from the filaments will have been re-deposited inside the tube ends. When this happens, no more electrons can be emitted from the filaments and the tube will be impossible to start. The way around this problem is November 1996  31 Fig.2: scope waveforms for the circuit, taken at pin 3 of IC1b (top) and the drain of Q1 (bottom). to provide a small voltage to both filaments. This is done by adding two filament windings to the transformer. These are shown dotted on the circuit diagram of Fig.1. Adding the filament windings does increase the current but also improves the life of the tube. Flashing operation Finally, we need to talk about the option of flashing. Schmitt trigger IC1a is connected as a low frequency square wave oscillator operating at about 0.5Hz. Its output at pin 11 is connected via a link between points C and A, to gate oscillator IC1b on and off for flashed operation. This is not really an option for use with fluorescent tubes but could be an interesting addition if the circuit was used for driving neon tubes, as in under-car installations. Assembly The good news about this project is that you don’t have to wind and assemble the transformer. This is just as well because it would be a tricky little beast. Not only does it have a lot of turns on the secondary but it has insulation between each winding layer, to cope with the high voltage operation. So all you need to do is to assemble all the active and passive components on the board and solder the transformer into place. Make sure that the IC and the Mosfet are correctly orient­ed and the same comment applies to the electrolytic capacitors. Note that the 0.12µF capacitor is wired onto the underside of the PC board, as shown in Fig.3. Also shown in Fig.3 are the linking options. Connect link A-B for normal operation, or a piece of hookup wire between points A & C for flashing operation. One of these links must be installed. If you wish to add the filament windings, they can be wound with light duty hookup wire. Do not attempt to disassemble the transformer to do this job – just wind them on over the exist­ing windings. Five turns are required for each winding and they are terminated to four pads on the PC board. Note that for some smaller tubes, if the filaments can be seen to be glowing orange, then the filament turns should be reduced. When you have finished assembling the PC board, check your work carefully against the diagram of Fig.3. Then connect a fluorescent tube and apply 12V from a power supply or battery. The tube should start immediately. Note that you can vary the brilliance by varying the value for R2 but for 18W and 36W tubes, the light output will be less than normal, as SC noted above. Kit Availability Fig.3: the component overlays for the two versions of the PC board. The version at top with filament windings on the transformer is recommended for driving fluorescent tubes. 32  Silicon Chip This project was designed by Oatley Electronics who own the copyright. They can supply the complete kit, including the pre­ wound transformer and a small fluorescent tube, for $18 plus $4 for postage and packing. Their address is PO Box 89, Oatley NSW 2223. Phone (02) 9579 4985; fax (02) 9570 7910. SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Rod Irving Electronics Pty Ltd SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: Rod Irving Electronics Pty Ltd SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Rod Irving Electronics Pty Ltd SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: Rod Irving Electronics Pty Ltd SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Rod Irving Electronics Pty Ltd SERVICEMAN'S LOG Of ships & shoes & sealing wax Of ships & shoes & ..? Well, something like that. More exactly, this months column is a collection of items which have accumulated over the past few months, typically letters from readers. Unfortunately, good intentions don’t always work out in practice. For one reason or another, there always seems to be a space problem and so, finally, they simply had to be dealt with in one hit. So here goes – and my apologies to all those con­cerned for the inevitable delays. My first item is from one of my regular contributors of the past, J. L. of Tasmania. We haven’t heard much from J. L. in recent months and I had gained the impression that he has scaled his activities down somewhat. 38  Silicon Chip Anyway, here is his latest story. I had a job last week that was totally wasted on an old bloke like me. The manager of the local gymnasium had asked me to have a look at their cassette deck, the one used by the aerobics in­structors to play the music for their students to puff by. I was told that the deck would only play half a tape and would then slow to a stop, just when the exercises were getting up to full speed! What I wasn’t told was that I would have to do the job on the job, during a class, because their spare deck was not up to continuous use either! They had to swap from one ma­chine to the other to get through a session. I don’t know if you have ever tried to work in a room with 50 young women, each scantily clad in lycra tights and bouncing about all over the place to the thumping beat of loud rock. As I said at the top, the job was totally wasted on me. The deck turned out to be a Tascam Ministudio, a 4-track 4-channel recorder designed primarily as a semipro deck for rock bands and the like. It is quite a rugged unit, which was why it had been chosen for play-only use in the gymnasium. Unfort­unate­ ly, it had “play-onlyed” just a little too often and was now due for some serious maintenance. Flying blind There was no way I could do a proper test of the deck in situ. It was accessible enough, on top of a small wooden cabinet, but there just wasn’t enough space around it for tools and test gear. Quite obviously I was going to have to fly blind with this one. Fortunately, I have had quite a lot of experience with audio tape decks, going back to long before VCRs became common. I also have a useful collection of specialised tools, including a torque meter which proved to be the saviour in the present job. First up, I loaded a 1kHz test tape into the machine and set it playing. With a pair of headphones plugged into the unit’s “phones” socket, I was just able to hear the test tone and con­ firm that it was running at about the correct speed. As I stood watching the frenetic activity all around me, I became aware that the tape was definitely slowing. This was after only about five minutes, so I could just picture all the exercis­ing slowly grinding to a stop. My next move was to replace the test tape with the torque meter. A tape deck in good condition should give a take-up torque reading of around 4050 gram/centimetres. This one was hard pressed to reach 5g/cm, which explained why the tape was so loosely spooled in the cassette that I could see daylight through the layers. I can’t remember when I first learned the usual cause of this problem, so I can’t tell our younger readers just how I first found it. I do know that I have since found hundreds of stretched or hardened main drive belts in both audio and video cassette decks. And so it was in this case. I opened the machine and exposed the bottom of the mechan­ism. I used a finger to rotate the main flywheel, Well, that’s J. L.’s story and it is good to hear from him again. But I can’t help wondering whether he is perhaps coming on a little too strong in his protestions – that he is promising to not enjoy himself just a little too emphatically! The Marantz VCR which should have caused the motor pulley to spin. It didn’t move! When I powered the mechanism, the wedging action of the motor pulley managed to drive the belt but the flywheel barely moved. That was enough – I removed the old belt which had become quite hard and highly polished. I selected a replacement belt about 1cm shorter than the original and fitted it into place. When the machine was reassembled, the torque figure had gone up to 55g/cm – a whisker high but one that will provide a margin for the future. By this time the noise and activity around me were begin­ning to wear me down. So rather than hang around to test the deck, I set it running and made my departure. When I checked next day, I was told that it was working perfectly, so my diagnosis must have been correct. I promised to go back later to complete the cleanup and general maintenance of the unit but only if they could assure me an hour free from the distractions of young women and loud rock! When I related this story to one of my younger colleagues he couldn’t believe that I was more interested in repairing a tape deck than in watching the girls do their aerobics exercises. Apparently, at his gymnasium, they have segregated classes and he would have given his left leg to get into the girls’ class. He tried to convince me that he was only interested in “...all that stretchy Lycra” but somehow I don’t believe him! Next up, is a letter from A. M. of North Turramurra, NSW, concerning a problem with a Marantz 740A VCR. It is similar to the situation described in the Serviceman’s Notes for December 1995. Anyway, here’s his story. Like many readers of SILICON CHIP, I usually read your column first when the magazine arrives. The December issue was, however, a double treat because it contained the solution to a problem that has consumed many hours of head scratching and frustra­ tion. I refer to the power supply fault in the Marantz 740A. The problem first showed itself as a curious sort of “rotating echo” effect in the sound. After some fiddling, I found that if the Audio Play switch which had been set on Mix was set to Hi-Fi or Linear, the effect disappeared. As a result, I dis­missed the episode as an intended gimmick. Some months later, I noticed that the picture was somewhat degraded when the machine was first switched on but that this cleared after a few minutes. This slowly became worse, until it took about 15 minutes for the picture to clear. At this time, I also found that with the Audio Play switch in the HiFi position there was an intolerable frame buzz, while the picture was degrad­ed. The fault was obviously heat affected and the service manual showed that the Video Head Amp PWB-Y was the only common signal path for hifi audio and video. I heated and froze every component on the board but without any result. Incidently, the circuit diagram for PWB-Y and others have been helpfully (?) marked to show the signal paths on playback and record. I assume this was originally in colour but the manu­als as supplied are black and white copies, so that all the path markings succeed in doing is to obliterate the details of the circuit! On the grounds that most faults are mechanical but without any clear idea as to where the thermal effect would arise, I carefully cleaned the heads, November 1996  39 Serviceman’s Log – continued aligned the tape path, and replaced the pinch roller – all of which effected a slight improvement but did not correct the real fault. It was clear that there was no problem with the record function but, when we could no longer stand the playback delay, we bought a Sanyo VHR-310. It was a nice luxury to have two recorders, even if one could record only. You can no doubt imagine my joy on reading your December 1995 column and I wasted little time in diving into the monster again. Your comments about access are spot on but I’ve delved into this device so many times that the operation didn’t take long. I was glad of your encouraging comments about bending the support bracket out of the way. This is something I would normal­ly be reluctant to do but which almost cannot be avoided in this design. Of course I was rewarded with immediate and 40  Silicon Chip complete success. It is not at all obvious why the fault manifests itself in this way, or why the ±9V regulator is designed in a way which is bound to make it more fault prone. Thank you once again for the solution to a most obscure problem and for an always entertaining and informative column. And thank you, A. M. for your interesting report of this exercise. I have no doubt someone else will benefit from your time and effort. The GC181 colour TV set My next letter comes from N. B. of Epping, NSW and concerns a story in the Serviceman’s Notes for January 1989. The set on the Serviceman’s bench was a GC181 colour TV set and it had suffered an imploded picture tube and sundry other damage but without any clear reason for the fail- ure. So here is N. B.’s story about his GC181 TV set. The set was bought in 1977, together with a small trolley that held it about half a metre off the floor. Around five years later, my wife and I were snoozing on a Sunday morning to the usual restful background of our three sons squabbling over which cartoon show they would watch. Suddenly there was a thump, fol­ lowed by absolute silence. After some minutes, our eldest son put his head around the bedroom door and announced that the TV set was dead. Feeling that it would do them no harm to miss the cartoons, I told him to switch it off at the mains outlet. When I later went to investigate, I found that the set had been pushed backwards off the trolley and was lying backwards at about 45 degrees. The middle of its base was against the back edge of the trolley and the extension of the back cover was resting against the wall. The soft old plaster of the Glebe terrace house had been dented by the case but had not come away from the wall. Now, I know almost nothing about how TV sets work but I had been building audio gear since about 1960. I figured that the damage was probably physical and that I might be able to repair it. Taking the back off, I found that the neck board had broken in half. I had on hand some single-strand copper bell wire, so I carefully cleaned the tracks each side of the break and soldered a short length of bell wire across each broken track. The neck board is so small and light that the soldered joint provided enough mechanical strength. However, this did not fix the set. On investigating further, I found that (as in the set you described) the main board was cracked from front to back about 50mm from its lefthand side. I carefully removed the various plugs and boards from the main board, noting where each went, and considered the problem. Obviously, wire links would not provide enough mechanical strength for this board, so I used Araldite first to repair the break. When this had set, I soldered bell wire links across each of the broken cracks and then reassembled the set. Much to my surprise, it worked perfectly. The point is that a fairly minor fall backwards snapped both the neck and main board. If the set had fallen all the way to the ground, it is quite possible that it would have suffered the same damage as your customer’s set. I wonder how much dif­ference there is between an impact that snaps a neck board and one that breaks the neck off the tube. Incidentally, about five years ago the set again fell off the trolley. That time it fell forwards and landed flat on its screen. There was no damage. It still works, although now as a second set. Its only problem is that it really needs new volume and colour pots – the sliders have to be wedged in place with bits of cardboard so that the contacts bear on a relatively unworn area of the track. Not bad for a TV set that is almost 20 years old! Servicing at sea Finally, there is a most unusual story from A. D. of Whan­garei, New Zealand. And this is another of those letters which has languished too long in the “too hard” file, while trying to fit everything in. Here is his story. Intermittent faults are the most annoying things, especially when it’s your own equipment and is currently in use. To set the scene of this story I should explain that I live on a yacht and I picked up a TV set while cruising in Australia. It is a 12.5cm Trakka 15 colour set made by Philips (KA212). I have another TV set, a 25cm model, so the 12.5cm model was stored in a box and didn’t get much use over the winter after I bought it. It was a very wet winter and very damp on the boat. When the set was eventually pressed into service so that I could watch TV in bed, it wasn’t long before it developed a couple of faults. When first turned on, a hissing sound could be heard from the EHT supply but this usually stopped after about five minutes, so I wasn’t too concerned about it. The more serious fault would cut out the picture and sound completely. The channel selector indicators would remain lit, but I couldn’t change channels. Then the picture and sound would return but the set would always revert to channel 1. I thought a few hours in the sun might dry the set out, as my suspicions were that the EHT system was being loaded and that this set might have an overload trip that was resetting itself. But after sunning the set the fault became more frequent and annoying. And it always seemed to fail right on the punch line of the show I was watching. I found that I could sometimes get the picture back by rocking the monitor/TV switch on the back of the set and, since I had stowed the set resting on its back, I suspected that I had damaged the switch. So, finally, the cover came off. This set has two large circuit boards with quite a few discrete components, plus a sub-board with the monitor/TV switch on it. The switch was going to be impossible to remove so I simply linked the respective switch points, as the set was never going to be used as a monitor any­way. I thought that would be it and sat back to watch Fawlty Towers but, five minutes later, it was in fault status again – just as Basil was annoying some German diners. I removed the cover again and this time did some probing with a meter. The sub-board has a number of cables to it: video/audio, in/out and power. I did consider trying to remove the whole sub-board but settled for leaving the meter connected to the 9V supply rail and waited. It read 8.92V when the set was normal but dropped to 4.2V when the fault occurred. And so the tracking down began. I had to find the start of the 9V stabilised rail, which involved checking other supply rails carrying higher voltages. I finally came to a TO220 package with 12V on one pin and around 9V on the other two. So out came the magnifying glass and, sure enough, I dis­covered a dry joint. I can only assume that flicking the switch may have applied extra load to the circuit and temporarily remade the contact. I cleaned the EHT connections, applied silicone grease to the EHT cap and put the cover back on for the final time. I am not a TV technician but an experienced general techni­cian. Well, that’s A. D.’s story of his life on the ocean wave. And I don’t know about not being a TV technician, A. D. I reckon you’ll make a pretty good substitute in the meantime. And that’s the roundup for this month. Which isn’t a bad effort considering that we have been from Tasmania to New Zealand and points in between, and covered everything from aerobics SC to marine electronics. November 1996  41 Got a dud dimmer? The fault is bound to be a blown Triac. Fix it for good with a higher-rated Triac. How to repair light dimmers Do you have light dimmers in your home? Has one or more of them failed? Are you cheesed off with the thought of buying another one? Well, don’t. Repair it with a more rugged Triac and it should be fixed for good. By LEO SIMPSON Light dimmers are a great accessory in lounge and dining rooms, to subdue the lights and set the mood. The same comment goes for lights in bedrooms. They are handy too when there are young children in the household. You can set the dimmer low to check on them without disturbing their sleep and they can also help a child go to sleep if he or she becomes anxious in the dark. But while they have their good points, dimmers can fail. Usually they 42  Silicon Chip fail when the lamp filament blows and this is par­ticularly the case if the lamp fitting has the bulb upright. What often happens is that when the filament blows, a loose section of it flails around and makes contact with one of the filament supports. The resulting arc blows the Triac and from then on all you have is a light switch – it’s either on or off. In our article entitled “Power Control With A Light Dimmer” in last month’s issue, we mentioned that it was generally possi­ble to repair a failed light dimmer by replacing the Triac with an SC141D. That is true but for long term reliability it is better to take the approach outlined in this article. This whole subject was brought into focus once again when one of the SILICON CHIP staff recently had a dimmer blow in his home. We decided to bite the bullet and see how difficult it was to repair. Removing the dimmer The first step is to have the dimmer removed and temporari­ ly replaced with a standard light switch on the same size switch plate. Don’t even think about working on the dimmer while it is still connected. Having had the dimmer removed, you can inspect the small rectangular module itself. This will have an end panel which is generally secured with integral clips and plastic tape. Peel off the tape and then pop out the end panel by carefully pushing on the integral clips – it is more difficult to describe than actu­ally do it. Now remove the knob – it just pulls off. Make a drawing of how the two wires from the dimmer module connect to the switch. Disconnect the two wires and then it is simply a matter of push­ing on the knob shaft to remove the small PC board assembly from the plastic housing. The Triac is mounted at one end of the board and usually has a small aluminium heatsink. On some dimmers this heatsink is pop-riveted to the metal tab on the Triac but on the one shown in the photos in this article it was merely placed in contact with the Triac tab by the pressure from the end plate of the plastic module. This is hardly an effective method but it does make it easy to replace the Triac. Now why did the Triac fail? We have already described the mechanism of failure but since it is such a common hazard you wonder why the dimmer manufacturers don’t simply use more rugged Triacs. In the dimmer shown in the photos, the original Triac fitted was a Philips BT137 series. This is rated at 8A which ostensibly is more than adequate considering that the dimmer was rated for a maximum load of only 300 watts. The problem is that the BT137 series Table 1: Triac Ratings Type BT137/500 BT138/500 BT139/500 SC141D SC146D SC151D MAC320A8FP BTA10-600B BTB16-400B Current Rating 8A 12A 16A 6A 10A 15A 20A 10A 16A Triacs have a non-repetitive peak surge current (ITSM) rating of only 55A. This is insufficient to cope with the arc currents mentioned above. The obvious solution is to replace the Triac with a higher-rated unit and there are quite a few to choose from, all costing $6 or less. Table 1 shows a list of Triacs which are widely available from electronic parts retailers. Looking at Table 1, you will see that there are several devices with at least double the surge current ratings of the BT137. All have similar packages and the same pinouts so they are drop-in replacements for the BT137. However, it is good practice to go for one with the same or a higher voltage rating as well. Therefore, if you can get the alternatives, you can reject the 400V SC146D, SC151D and the BTB16-400B Voltage Rating 500V 500V 500V 400V 400V 400V 600V 600V 400V Surge Rating 55A <at> 50Hz 90A <at> 50Hz 140A <at> 50Hz 74A <at> 50Hz 110A <at> 50Hz 110A <at> 50Hz 150A <at> 50Hz 100A <at> 50Hz 170A <at> 50Hz although the last-named device does have a massive surge rating of 170A. Because we had them on hand, we plunked for the Motorola MAC­320­ A8FP, a 20A device with a surge rating of 150A (at 60Hz) and an insulated tab. While the insulated tab is a good feature in some equipment it is of no advantage in domestic dimmers and there is a slight drawback in that the insulated tab is a little taller than the metal tab on TO-220 packages. The next step is to use your soldering iron to remove the failed Triac from the PC board. Take care not to overheat the copper tracks otherwise there is a risk that they may lift off the board substrate. If the heatsink has been pop-riveted on, you will need to drill out the rivet. Clean off any burrs or metal swarf Popping out the end panel on the dimmer module reveals the PC board assembly and the Triac’s aluminium heatsink. Before the PC assembly can be removed, you must remove the knob from the front panel. November 1996  43 Replacing the Triac is merely a matter of unsoldering the dud one and soldering in the new. The heatsink should be attached to the Triac’s tab using a screw, lockwasher and nut. Make sure that the finished PC assembly will still slide easily into its plastic case. around the hole when you have finished drilling. Next, solder in the new Triac, making sure that you don’t have any solder bridges between the Triac pads. The Triac must be oriented the same way as the original device, so that the heat­ sink can be refitted. Don’t use a pop rivet though; use a machine screw, lockwasher and nut. It’s also a good idea to smear a little heatsink com­pound on the Triac mount­ing tab before fitting the heatsink. Note that the heatsink will be live and there is no need for such niceties as mica washers. Before you reassemble the PC board assembly into the dimmer module, use your multimeter to check that the Triac is open circuit. If you switch your multimeter to the highest resistance range you will find that the resistance across the two dimmer module wires should be above 30MΩ. Do the same resistance check across the Diac. This is a small glass package with one lead connecting to the gate of the Triac. The Diac should also appear to be open circuit. Now reassemble the dimmer module. You can either have it reinstalled or you can use it as a soldering iron temperature controller, as outlined in last month’s issue of SILICON CHIP. SC 20 Electronic Projects For Cars $8.9s5 plu $3 p&p Yes! Please send me ___ copies of 20 Electronic Projects For Cars Enclosed is my cheque/money order for $­________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. Signature­­­­­­­­­­­­________________________ Card expiry date______/______ Order by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail the coupon to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 44  Silicon Chip Name _______________________Phone No (_____)____________ Street PLEASE PRINT _________________________________________________ Suburb/town _______________________________ Postcode_________ SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.jaycar.com.au MAILBAG Flashing lights foil hunting cats In your response to a letter from K.F., of Albion Park Rail, on page 93 of your August 1996 edition, headed “Cat Deter­ rent Not Humane”, you invite comment. It was an interesting and perceptive observation that you made in reply to the reader’s suggestion that emitted sonics would not deter birds in preference to bells attached to cats’ collars. We are given to believe by the avian experts that sonics do not indeed have any deterrent effect on birds. Your thoughts that cats should not be let out at night are not without pre­ cedent. In Victoria, there is a curfew placed on them in cities and shires that have elected to enforce what is now state law, and the bird and reptile population is now on the increase in those areas. Because cats hunt, by preference during the hours of sunset to sunrise, so we have researched and developed a tiny battery driven unit which emits a high intensity flashing light from a position behind the cat’s neck. It can then electively be used by responsible cat owners as a “Skare” for birds and other creatures. The cat is unaware of this and the “Skare” acts as a far more effective deterrent than tinkling bells but has added safety benefits for the cat in that it is illuminated if crossing roads in unlit areas. This device will shortly be marketed through our existing “K-9 Collar” outlets whose primary purpose is the humane and safe containment of dogs, by proven methods using sonics and avoidance therapy. John Foley, Canine Invisible Enclosures, Tugun, Qld. Internet can be cheap I would like to comment on the Publisher’s Letter in Octob­er 1996 issue: “Getting Onto the Internet Can Cost Big Money”. Whilst it is true that the WWW is mostly “voyeuristic” at present with few tangible benefits, the cost does not really have to be excessive. Granted, some companies spend many tens or even hun­dreds of thousands of dollars running their web sites. However, this does not have to be the case. For example, I have assisted a local (non-technical) compa­ny to set up with a local Internet service provider (ISP) with email and a simple web page, for about $50 a month. The email account was the driving need in this case; the web page was an added bonus. I suggest to you that email today is much like the fax machine of 5-10 years ago, an expensive novelty then but essential now. With regard to the cost/benefits of properly setting up a WWW site, I agree that not every organisation can justify such an investment. Commercially speaking, the WWW is just another adver­ tising medium, alongside print, television and radio. Of course, any media person will tell you that “advertising pays”; the question is which sort of advertising is best for a particular company. Given that $50 buys 92 words or two column centimetres each month in the SILICON CHIP Market Centre, or 5Mb of storage with an ISP, I’d suggest that both are economical methods of advertising, especially for technically-oriented organisations. B. Low, Gwynneville, NSW. Underestimating the Internet I was rather stunned by your editorial in the October 1996 issue regarding the Internet. While your comments regarding copyright are quite correct, I think you are underestimating the benefits of the Internet. Do you realise that all the major electronics manufacturers have their full technical data library on line? This is the best thing that has happened in electronics for years. You can down­ load the full data sheets on just about anything. While I am sure the electronics companies are quite helpful when SILICON CHIP magazine wants some data, I can assure you this is not the case for the hobbyist or small manufacturer. I cannot comprehend how you can say it costs a lot of money to get a company on the Internet. Do you realise that with an Ozemail membership you get 5Mb of space for your home page. Because HTML is basically text and providing you don’t have stacks of graphics, you can have a stack of stuff in 5Mb of text. Email is great – I regularly correspond with companies and indi­viduals all over the world. A SILICON CHIP Web site could have your Bookshop and back issues listings, kit listings, subscription rates, etc – all stuff that would not detract from magazine sales at all and it would be available to the world 24 hours a day, seven days a week. I recently heard about an article on port I/O in Windows 3.1 in the American magazine “Windows Developers Journal”. It was some time ago and my friend had lost the issue. I did a search on the Internet and found their Web site, scanned the article list­ing for old issues, found the article and ordered the back issue. How the hell would I have done that without the Internet? It cost me about half an hour Internet time which is (on Microplex) $2.00 + 25 cents call. You state that “most of the information on it is pretty trivial”. I gather you don’t include the complete National Semiconductor Data library! I am amazed at your attitude. M. Boxsell, Kellyville, NSW. Comment: we published an article entitled “Electronics On The Internet” in the August 1996 issue. The cost of having a presence on the Internet is minimal. The cost of a large, well-supported web site can be very high. Phototimer has wrong bridge rectifier I refer to the article “A Photographic Timer For Darkrooms” published in the April 1995 issue of SILICON CHIP. There is an unfortunate error on the PC board or in the specification for the bridge rectifier. On page 27, that portion of the PC board in which the rectifier is to be installed shows that the two AC connections are both at one end of the rectangular BR1. The parts list on page 29 and the circuit diagram on page 26 specify the rectifier as a type WO4. This has a circular encapsulation with diagonal AC connections. In order to obtain the 12V from the rectifier, two of the legs had to be sleeved with spaghetti and crossed before in­stallation. D. Paule, Glenhuntly, Vic. Comment: the bridge rectifier we used, shown in the photos, comes in a 4-pin DIP package. It is stamped DI104 and is available from Altronics in Perth. Their catalog number is Z-0070. November 1996  53 RADIO CONTROL BY BOB YOUNG AM versus FM: the real facts in the argument This month, we will take a look at some of the myths surrounding FM transmitters and receivers and see just how well they stack up against the old AM system. Some people really believe that AM is obsolete and will go so far as to claim that AM sets should be banned from flying. They are dead wrong. The Mk.22 series of articles brought forth a host of let­ters and telephone conversations, almost all of which were very positive. It certainly stirred up some interest around the coun­try. Yes, we do get the odd stinker but many are simple letters asking why 29MHz AM for the Mk.22, when all other manufacturers are producing 36MHz FM? However, the saddest letters are letter even indicated that flight training would cease unless he stopped using “inferior” AM sets and changed to FM. Very often the theme is that beginners keep crashing models; they are using AM sets, therefore the fault lies with AM. Well, I can state that there are plenty of reasons why models crash and the method of modulation is the last item that need be considered. This column What’s all the fuss about FM and AM? How did all of this start in the first place and why? We flew safely and successfully for 30 years on AM, so what has changed? those in which there is a genuine plea for help, usually from beginners who are under intense pressure from some club members to sell their “inferior” AM system and buy the latest FM-type transmitters, complete with LCD, bells, whistles and buzzers. They usually have a simple plea. “Should we sell and why? Your help please.” These letters often point to ridicule or lectures on why AM is dead. One 54  Silicon Chip is dedicated to those people who are under pressure from “experts” who should know better. What’s all the fuss about FM and AM? How did all of this start in the first place and why? We flew safely and successfully for 30 years on AM, so what has changed? Before we start I should point out that we are dealing with an extremely complex subject and it is easy to become entangled in a circular argument in which the main points keep getting lost. There are three branches in this discussion. The first concerns the relative merits of AM over FM under normal operating conditions. The second is the effect of interference on both systems and finally, there is the level of technology ap­plied to each system by the manufacturers. The question of oper­ating at 29MHz instead of 36MHz is a separate issue and we will deal with that another time. Cheap AM sets Let’s talk about the level of technology in both systems. As AM is much cheaper to produce than FM, it is the preferred system of modulation for those manufacturers going after the price conscious market. These manufacturers sometimes use dubious techniques to further reduce costs and the result is a system that provides minimal performance and reliability. This has more to do with the design and manufacturing approach than the method of modulation. To complicate matters there are also AM systems produced for model car operation, using short antennas. These were never intended by the manufacturers for aircraft use but were sold by the model trade as general purpose sets and thus found their way into model aircraft. These sets have played a large part in giving AM an undeserved poor reputation. Now “everybody” knows that FM is better than AM and, of course, so it is. But this applies to the FM used for radio and TV sound broadcasting. Fig.1: spectrum analysis of a 4-channel AM R/C trans­­mit­ter. This shows the occupied bandwidth as ±12kHz at -60dB. FM stereo radio is far superior to steam- age AM radio, and so it should be, with its frequency deviation of ±75kHz. That amounts to a channel bandwidth of 150kHz! That is true FM. By contrast, AM radio has a bandwidth of a mere ±9kHz; no wonder it is inferior. FM is not FM What “everybody” does not know is that model R/C equipment does not use true FM! To use the term FM to describe the method of modulation in an R/C transmitter is quite wrong. The system of modulation used in FM R/C sets is actually NBFSK. This stands for Narrow Band Frequency Shift Keying. This system is a form of direct frequency shift keying and is not to be confused with AFSK (audio frequency shift keying). This form of modulation uses narrow-band carrier deviation to transmit the data and let me tell you the emphasis is on NARROW! Typical frequency shifts are around ±1.5kHz to ±2.5kHz for a max­ imum channel bandwidth of 5kHz. That’s in theory. In practice, the deviation is more usual­ ly -400Hz and +2.5kHz for a system bandwidth of about 3kHz. In other words, the 36MHz carrier is shifted back and forth by a mere 3kHz. That is a world away from the 150kHz deviation applied in FM radio. Nor can anything better be expected with NBFSK. How are we ever going to get down to the coveted 10kHz channel spacing if we occupy more bandwidth? So why isn’t the correct term of Fig.2: spectrum analysis of a 5-channel FM R/C transmitter. This shows the occupied bandwidth as ±8kHz at -60dB (narrower than the AM transmitter shown in Fig.1). NBFSK used instead of FM? It really is misrepresentation. It never began as a deliberate policy but merely came into being as a matter of convenience to distinguish frequency-shift sets from AM sets. After all NBFSK is a form of FM and FM rolls off the tongue much more nicely than NBFSK, doesn’t it? The problem is, in the minds of many people, FM has come to mean something quite distinct from NBFSK. The term FM conjures up visions of wideband high fidelity stereo sound transmission systems, completely free of noise and interference. This is the underlying theme in the AM versus FM argument; AM is “inferior” because FM is so much better. But the argument is spurious and the question should be, “Is NBFSK better than AM?” or possibly, “Is NBFSK as good as AM?” Do you think I am being deliberately controversial here? Well, stick with me because you might be surprised. AM is really not AM Not only is FM not FM but just to confound the argument, there is one other thing that “everybody” does not know. The system of modulation commonly referred to as “AM” in the model trade bears no more relationship to AM radio than model “FM” bears to broadcast FM! Model AM is not AM! It is really a gated carrier system and many of the objections that apply to AM broad­ casting just simply do not apply to this system. It is a very robust system of modulation. Add to this receivers designed specifically for noise elimination and pulse shaping, with ceramic IF filters, audio slicers, audio filtering and decoding enable. What broadcast AM receiver is designed along these lines? The modern AM R/C receiv­er might look simple but it has had a long history of development and it works very well. Comparison tests With all of the above in mind we embarked on a series of tests to demonstrate FM and AM performance. We used a Silvertone Mk.22 receiver which is ideal for comparative testing as we could plug in the AM or FM modules ahead of the audio slicer. We used a loose form of antenna coupling to the signal generator which gave a practical dynamic range of 80dB. Both receiver modules were identical in sensitivity. The AM receiver circuit is that published earlier in SILICON CHIP and the FM receiver uses one of the Motorola receiver chips. The signal generator was set at 100% modulation for the AM testing while the FM modulation was set at -400Hz and +2.5kHz, mimicking a popular Japanese “FM” R/C transmitter. The external modulation was supplied from a Silvertone Mk.14 7-channel encoder. Measured under these conditions the signal-to-noise ratio of the AM receiver at the detector was -14dB at -70dB signal input, the point at which the audio slicer was about to shut off the pulse train to the decoder. The FM receiver measured -12.5dB (also at the detector), a figure 1.5dB worse than the November 1996  55 AM Receiver Fig.3: recovered modulation from the detector of an AM receiver, taken at a transmitter relative signal level of -60dB. Note that the waveform is clean and virtually noise free. AM Receiver Fig.4: same waveform as Fig.3 but with a transmitter relative signal level of -80dB. AM Receiver Fig.5: this shows the recovered data after the slicer, for a transmitter relative signal level of -60dB. AM receiver. Again the 70dB point is significant as it is the point at which the squelch is about to shut down the audio output of the receiver. Some idea of the relative signal-to noise-ratios of the two receiver modules may be gained by referring to the accompanying oscilloscope waveforms in Fig.4 & Fig.8. These were taken at a carrier level of -80dB, the lowest point at which a readable signal is present in both detectors. We took the above figures at these points because they are of interest when flying through weak signal areas. It is here that things will go pearshaped very quickly indeed if noise or interference are present. As you can see, these figures are completely at odds with the theoretical noise figures so widely available in text 56  Silicon Chip AM Receiver Fig.6: same signal conditions as for Fig.5 but with interference from the commutator of an electric motor. books and which form the basis of the “FM” versus “AM” argument. To my mind, the anomaly arises from the fact that the “FM” system uses such small deviations with simple receivers and the “AM” system uses a gated carrier with unusual receivers. They also take no account of the ambient noise levels in various receiver designs. In this case the FM receiver had a much higher ambient noise level than the AM receiver and it shows quite clearly in the scope waveforms. This point is important in the “level of tech­nology” discussed previously. So here we are well into the story and so far the AM system is ahead by a nose. It appears that we must dig deeper to find out why “everybody” believes that “FM is better than AM”. Never in its long history was AM ever considered perfect. The main weaknesses with the AM system from an R/C point of view are the AGC system and occupied bandwidth. The wider bandwidth of the AM transmitter is a result of the edge conditioning (ie, pulse shaping) of the carrier blocks which contain many harmonics. This is a most difficult factor in AM transmitter design. Blow the edge conditioning and you can end up with a spectrum a mile wide. If the time constants are not correct on the AGC rail, fast models can ex­ perience momentary glitches as the signal strength gyrates wildly on close passes to the transmitter. Here is a problem not experi­enced by communications receivers. The move to NBFSK, which began FM Receiver Fig.7: this waveform shows the recovered modulation from the detector of an FM receiver (before the squelch stage), taken at a transmitter relative signal level of -60dB. FM Receiver Fig.8: same condition as for Fig.7 but with a transmitter rela­tive signal level of -80dB. Note that noise is intruding serious­ly onto the signal and is much worse than the equivalent AM receiver condition shown in Fig.4. FM Receiver Fig.9: this shows the recovered data after the slicer of an FM receiver, for a signal level of -60dB. Note that this waveform is virtually identical to the AM slicer signal shown in Fig.5. largely in the very busy clubs in Europe I am told, was largely driven by the above two points plus the problem of electric motor noise. Electric flight is very big in Europe. They needed the narrowest channel spacing possible and stories circulated for years about 10kHz operation in Europe. Yet I was at the MAAA committee meeting last year that examined all of the latest sets, including the best from Europe and America, and that committee ruled that 10kHz operation was not safe with the current generation of NBFSK radios. So what are they doing in Europe? It is difficult to get a true story on how they are managing the frequencies in Europe but it appears that they allow FM Receiver Fig.10: same signal as for Fig.9 but with interference from the commutator an electric motor. Note that the data has been seriously disrupted, with an extra pulse appearing the fourth data block. the use of 10kHz spacing but only allow every second channel to fly simultaneously. Er, isn’t that 20kHz? Just recently, the MAAA has adopted a similar spacing for Australia. We were getting better results with AM sets in 1969 when I began testing systems for narrow band spacing. We actually flew Silvertone Mk.7 receivers on 5kHz and 10kHz spacing but we deemed this unsafe and settled on 15kHz as the closest safe spacing. We flew this spacing for many years without incident in several Sydney clubs. This is still true today as proved by the MAAA meeting in Melbourne last year. Yet there is a mystery here as a quick glance at Fig.1 and Fig.2 will confirm. The NBFSK transmitter has a slightly lower bandwidth and the receiver is more developed with very narrow filters, so how is it that the more simple AM system delivers comparable results from a band spacing point of view? Ironically, the big difference between the AM and NBFSK receivers in regard to band spacing is that the AM receiver has AGC (automatic gain control). This drastically reduces the signal levels arriving at the IF filters and thereby reduces the stress on these filters. In contrast, the NBFSK receiver with no AGC always runs at full sensitivity and the filters are subject at all times to heavy noise, carrier and November 1996  57 SATELLITE TV EQUIPMENT     Receivers  Feeds Positioners  LNBs  Actuators Dishes And much much more! C-Band Systems from $1495 Ask us for a catalog! B&M ELECTRONICS 469 Light Street, Daniella WA 6062 Phone/Fax: (09) 275 7750 Mobile: 041 99 0 55 00 Binders IF frequencies. Fig.8 shows the recovered data from the detector of a typical NBFSK receiver operating at a very low signal level (-80dB). Note the extremely high level of white noise. With the carrier off, this noise is very high. NBFSK receivers need to resort to trickery in order to get rid of this noise because if the transmitter is turned off or moved out of range and that noise got through the decod­er, the servo gears would be reduced to pulp within 30 seconds. The trickery consists of adding a squelch circuit which detects the loss of carrier and shuts down the audio preamp, thereby removing the noise to the decoder. All of this costs money of course and it all adds to the expense of an NBFSK re­ceiver. AM virtually noise free Fig.4 shows the detector of a Mk.22 AM receiver at the -80dB point and therefore running at maximum sensitivity. Here we are at the very edge of the range and yet note the almost complete absence of noise. This drops to a straight line once the carrier noise is removed. There is no need for squelch for there is not sufficient noise to get past the audio slicer in the decoder. The decoder shift register is fitted with a pulse omission detector to catch whatever stray spikes slip past the slicer. Servo gears are safe here. The reasons for this vast difference in two receivers of my own design is primarily the fact that I have no control over the choice of design and transistors in the FM receiver chip, whereas I had total control over the AM design and components. The tran­sistors in the AM receiver were the best I could find. Limiters These beautifully-made binders will protect your copies of SILICON CHIP. They are made from a dis­tinctive 2-tone green vinyl & will look great on your bookshelf. Price: $A11.95 plus $3 p&p each (NZ $8 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 58  Silicon Chip At this point it is probably opportune to raise the issue of limiters. One of the major advantages of the broadcast FM system over broadcast AM is the fact that the FM system makes use of very effective limiters which remove much of the unwanted amplitude modulation and noise from the system. The Mk.22 AM receiver is fitted with an audio slicer which also removes much of the noise from the system. This does essentially the same job as the limiter in an FM radio receiver. So here we are well into the story, having dealt with the two major complaints against the AM system. And what have we found? Technically, the balance is about equal with shortcomings in both systems, but the AM system is much cheaper to purchase and maintain. For those with more money than sense, I suppose this is not sufficient reason to give the AM system the good housekeeping stamp of approval. Therefore, let us dig a little more deeply. Here we move on to point two. A serious problem is that interference to the AM system tends to reduce the depth of modu­lation; if the interference is strong enough the modulation depth can be reduced to zero and all control lost. The equivalent effect in NBFSK sets is the capture effect. If the inter­fering signal is stronger than the carrier, then the receiver can lock on to the interfering transmitter, completely blocking out the wanted carrier. Again, all control is lost. The big difference is that the AM system is gradual whereas the FM system is abrupt. Capture is a strong point for FM radio broadcasts but a real drawback in NBFSK sets. When two signals are comparable in amplitude, the moment one signal becomes even a trifle stronger the response changes and the stronger signal assumes control. A similar effect occurs at low signal levels (almost out of range). The AM system will work right down to very low signal levels. Control deteriorates gradually at the lower levels, thus giving some warning that things are beginning to go pear-shaped. By contrast, the NBFSK system will often switch off abruptly with no warning when the squelch cuts in or capture takes over. These points are terribly important for it was demonstrated by Phil Kraft back in the very early days of proportional system development (1960s) that a system that shuts down abruptly was not as good as one that allowed the pilot to battle his way through noise and interference. There is a great deal more to this discussion but it all tends to reinforce my argument that FM is greatly oversold against AM. When you look at the true nature of each of the modulation systems, gated carrier (AM) versus NBFSK (FM), what the argument really boils down to is “weak FM versus SC super AM.” Get big sound from your computer with this . . . Multimedia Sound System Last month, we introduced this high performance Multime­dia Sound System and gave circuit and constructional details of the sound card which fits into a slot in your PC. This month, we complete the story with the constructional details of the speaker boxes. Design by RICK WALTERS Not only does this system deliver much higher sound quality than the vast majority of Multimedia speakers with their tinny little drivers but the all-up cost is quite a bargain. In fact, this month we present the details of not one but two different versions of the speaker boxes. Both employ fully magnetically shielded drivers and both have been designed with the help of BassBox CAD software. The first system to be featured uses magnetically shielded drivers from Jaycar and has a speaker box with external dimen­sions of 224mm high, 160mm wide and 212mm deep. A perspective diagram of the enclosure is shown in Fig.2. It is made of 12mm thick MDF (medium density fibreboard). Two of these boxes can be cut from one sheet of 12mm MDF, measuring 1200 x 450mm. Fig.1 shows a proposed cutting plan which provides all sides of both enclosures, with a little offcut left over. If you are cutting this with a circular saw, don’t just mark up the sheet and slice away otherwise you will end up with wrong-sized sheets. You must allow for the kerf (width of the saw cut) otherwise you will end up with unusable pieces. Better still, if you are not an expert November 1996  59 Fig.1: two boxes to house the Jaycar drivers can be cut from one sheet of 12mm MDF, measuring 1200 x 450mm. This diagram shows a proposed cutting plan which provides all sides of both enclosures, with a little offcut left over. PARTS LIST 2 5-inch magnetically shielded woofers, Jaycar CW-2102 2 magnetically shielded dome tweeters, Jaycar CT-2006 2 35mm speaker ports, Jaycar CX-2678 or equivalent 2 9-pin “D” plugs 2 9-pin “D” backshells 1 short cable with 3.5mm miniature stereo plugs 1 1200 x 450 x 12mm sheet of MDF 2 pieces of Innerbond, 220mm x 160mm 14 3mm x 20mm screws 4m heavy duty figure-8 cable (woofers) 4m light duty figure-8 cable (tweeters) 8 12mm square self adhesive feet grille cloth PVA wood glue speaker sealant (mastic) carpenter, take your sheet of MDF to your local cabinet maker and ask him to cut it for you. Mine only charged me $5, which seemed a very reasonable price to get all the panels square and the exact size. Actually, while you are there, you might ask the cabinetmaker if he will make the boxes up for you, together with the holes cut in the front baffle. You will save quite a bit of work. The two front panels (one for each box) each measure 160 x 224mm. When you are marking the positions of the tweeter, port and woofer holes, mark one for a left channel speaker and one for a right channel speaker. Fig.2 shows the hole positions for the right channel speaker and these should be flipped over for the left channel speaker, to keep the tweeter at the outside edge of the enclosure. Cut the large holes with a jigsaw and sand the edges with a medium grade of emery paper. Temporarily sit the speakers and the plastic port in their respective holes and mark the mounting screw positions. Then drill each hole with a 3mm drill. Once this is done the box can be assembled. You may or may not decide to assemble the box with timber cleats but either way, the back panel fits between the sides; it should hold the box square as you assemble it. We suggest that you place the back panel on a flat surface and then “wrap the two sides and top and bottom panel around the rear panel. Use plenty of glue and panel pins to hold the panels while the glue dries. Ideally, you should use clamps to hold the panels together while they are assembled as this makes it easier to hold everything square. Make sure that the all edges of the side panels are flush with each other at the front, so that they make a good join with the front panel when it is attached. It is the last panel to be screwed into place. When all the glue joints are dry, drill a 6mm hole for the speaker leads in the rear panel of both boxes. The next stage it to finish the boxes either by painting, veneering or possibly even by covering them with 60  Silicon Chip Fig.2: this perspective diagram shows how the enclosure for the Jaycar drivers goes together. 224 wall paper. If you paint, you will first need to fill any holes and any surface imperfections with a product like Spak-Filla and then sand smooth. We suggest a satin or low gloss paint for a washable finish. Gloss paints show every surface imperfection while flat finishes tend to show dirt. We painted ours with a water based driftwood colour which matches the computer reasonably well. You may wish to leave the speakers exposed as they look quite impressive but they are then prone to damage. Many commer­cial units use aluminium mesh for protection but this is not ideal for the best sound. We bent and soldered lengths of November 1996  61 The drivers can be left exposed but since they are going to be used on your desktop we suggest that they be covered with a grille cloth frame (as shown at left) to avoid accidental damage. This grilled cloth frame can be bent up from coat hanger wire and then covered with a light material. coat hanger wire to make a speaker cloth frame the size of the front of the box and support­ed it at the top on two flathead nails 10mm in from each edge. Two round head nails were used on each side to locate the frame, so it would not move sideways. A piece of suitably coloured light material was made into a miniature pillow case and slid over the wire frame. The open end was tucked under the front edge of the box and held with a couple of staples. Then the rubber feet were stuck on. Final assembly Once the box is finished to your satisfaction you can begin the final assembly. The tweeter can be mounted directly on the front panel; its large flange gives a good seal. The same applies to the woofer flange. However, before both woofer and tweeter are mounted you will need to connect their respective wires. Use thin figure-8 flex with a stripe to connect to the tweeter, wiring the 62  Silicon Chip stripe to the negative lug. The woofer has two pairs of connections; connect the negative terminals together and the positive terminals together and then wire the paired terminals with thick figure-8 striped wire; the stripe goes to the paired negative terminals. Feed both woofer and tweeter leads through the hole you previously drilled in the rear panel. Apply a large dob of mastic around the hole, both inside and out, pushing it in as far as you can, to prevent air leaks. Remove and discard the outer sleeve from the plastic port. Cut the remaining port to a length of 20mm, measured from the face, and mount it on the front panel using mastic seal around it. Fig.3: the pinouts for a DB9 plug. Wire the speaker leads to it as described in the text. Place one piece of Innerbond, measuring 220 x 160mm, loosely in each enclosure. This is used to help damp any inter­nal reflections. Cut the speaker leads to a suitable length for your comput­er setup and then solder them to the DB9 plug. Fig.3 shows the pinouts on the plug. Solder the woofer positive leads to pins 1 & 6 and the negative leads to pins 3 & 8. This leaves plenty of room to solder the two thin figure-8 leads; positive to pin 4 and negative to pin 5. You can now test the speakers by using a multimeter switched to the x1 Ohms range. A click should be heard from each speak­er as the prods are applied to the appropriate plug pins. Equalisation resistors Before you install the amplifier card, you will need to install the equalisation resistors R1-R8. As it happens, the values are the same for both versions of the speaker box presented in this article. R1 & R3 are 1.5kΩ, R2 & R4 are 10kΩ, R5 & R7 are links, and R6 & R8 are omitted. To install the amplifier, first remove the mains lead from your computer, open the cover and remove the blanking Fig.4: this is the cutting plan for the enclosure to house the alternative Altronics drivers. This version of the Multimedia Speakers is taller but not as deep as the other version and does not use a port tube. The hole in the baffle is sufficient to tune it. Version 2: The Altronics Alternative This second version of the speakers uses magnetically shielded drivers supplied by Altronics in Perth. These speakers are a little larger than the other version presented in this issue so the speaker box has different dimensions and its volume is six litres (versus five litre for the other version). It is taller but not as deep. Fig.5 shows the perspective view. The arrangement of the front panel is different from the first version and the front panel is now glued in place, while the back panel is screwed. It was made this way to enable all panels to be cut easily from the same 1200 x 450mm sheet of MDF – see Fig.4. No plastic port is required for this second design. The box is tuned to the correct frequency with just the front panel hole and does not need any additional porting. Both speakers have spade terminals and if you don’t wish to solder the wires to these you could fit spade lugs to the ends of the wires. The woofer only has one 8Ω voice coil and you will not have to paral­lel the voice coils as detailed in the text. PARTS LIST (Ver. 2) 2 5-inch magnetically shielded 8Ω woofers, Altronics C-3085 2 magnetically shielded 8Ω dome tweeters, Altronics C-3005 2 9-pin “D” plugs 2 9-pin “D” backshells 1 short cable with 3.5mm miniature stereo plugs 1 1200 x 450 x 12mm sheet of MDF 2 pieces of Innerbond, 270mm x 190mm 20 3mm x 20mm screws 4m heavy duty figure-8 cable (woofers) 4m light duty figure-8 cable (tweeters) 8 12mm square self-adhesive feet grille cloth PVA wood glue speaker sealant (mastic) November 1996  63 Fig.5: this perspective diagram shows the enclosure for the Altronics drivers. 64  Silicon Chip Fig.6: this printout shows the BassBox parameters for the Jaycar version of the MultiMedia Speakers. Fig.7: the BassBox parameters for the Altronics version of the Multi-Media Speakers. The two versions sound very similar to each other. plate from an empty slot, preferably one closest to the power supply connectors on the main board. Plug the amplifier card into the slot and secure it in place with the retaining screw. Insert the speaker plugs in the sockets and tighten the retaining screws. The left speaker is fed from the top connector. Close the comput­er and reconnect the mains lead. You will have to make, or buy, a connecting cable to link your sound card to the amplifier card. The 3.5mm stereo socket we used on the PC card seems to be becoming the new standard so you will need a cable with a stereo 3.5mm socket at each end. The presets on the amplifier card should be adjusted to give a suitable level when your sound card volume control is at its normal setting. Finally, please note that the 22kΩ bass boost resistors on pins 2 & 6 of IC1a & IC1b on the amplifier card should be changed to 10kΩ, while the associated 150kΩ resistors on pins 3 & 5 should be changed to 68kΩ. Also, note that the 0.1µF monolithic capacitors specified in the parts list are used as supply bypasses. Their positions can be clearly seen as small blue capacitors on the colour photo on SC page 67 of last month’s issue. November 1996  65 600W DC-DC converter for car hifi systems Despite its heavy-duty circuitry, the 600W DC-DC Converter is easy to build. Provided you correctly follow the step-by-step details for winding the transformer, it should work first time. PART 2: By JOHN CLARKE V IRTUALLY ALL the parts for the 600W DC-DC Converter are mount­ed on a PC board coded 05308961 (310 x 214mm) and this is installed in a 2-unit rack case. A small label affix­es to the front panel to provide the legends for the LED indica­tors. Begin the construction by assembling the case. This done, insert the PC board so that its front edge sits against the front panel. Position the board so that there will be a 16mm gap between the edge of the heatsink (when this is mounted in posi­tion) and the lefthand side of the case. Now mark out and drill 3mm holes 66  Silicon Chip in the base of the case for the seven PC board mounting pillars. One mounting hole is located adjacent to each corner of the board, one is in the centre of toroid inductor L1, another is located between the two bus bars to­wards the rear, and one is adjacent to transformer T1. The next step is to check the various hole sizes on the PC board. Note that 2mm holes are required at all locations where 1.78mm wire is inserted. These include the source connections of Mosfets Q3-Q5 and Q8-Q10, the interconnections to diodes D3-D6, the connections from the 6 x 10µF capacitor bank to the bus bars, and the link from T1 to the centre of the 2200µF capaci­tors. The 2200µF capacitor lead holes need to be 3.5mm diameter, while the mounting holes for the bus bar securing screws need to be 4mm diameter. In addition, 4mm holes are required for the output supply rails (-V, 0V & +V) adjacent to the 2200µF capaci­tors. The mounting holes for fuse F1 should be 8mm diameter. Next, check that all the Mosfet and diode screw mounting holes are 3mm and that the four holes used to secure L1 to the PC board are large enough to accept the cable ties. The holes for the L1a and L1b leads and for the transformer pins need to be at least 1.5mm in diameter. Fuse F2 requires a 2mm hole, while the holes for the Mosfet leads should be about 1.5mm. The large heatsink specified is a fan-type with fins run­ ning down either side of a central flat area. For this project, one set of fins is removed using a hacksaw, so that the heatsink measures 69mm wide. If necessary, the length should be trimmed so that Fig.7: install the parts on the PC board as shown here, taking care to ensure that all polarised parts are correctly oriented. November 1996  67 TABLE 2: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.47µF   470n   474 0.1µF   100n   104 .0056µF   5n6   562 .001µF   1n0   102 The Mosfet mounting holes should be drilled to 3.5mm, so that they accept insulating bushes (see Fig.8). You also need to drill mounting holes for the thermal cutout switch (TH1). This mounts on one of the fins, as shown in Fig.7. Deburr all holes using an oversize drill and check that the heatsink mounting holes for the Mosfets and power diodes are smooth and free of any metal swarf. PC board assembly Begin construction of the PC board by installing the resis­tors, diodes (except for D3-D6) and ICs, plus trimpot VR1 – see Fig.7. Table 1 shows the resistor colour codes but it is also a good idea to check the values using a digital multimeter before installing them. The wire links associated with the low-current circuitry (bottom of Fig.7) can also be inserted at this stage. Take care with the orientation of the diodes and ICs. The next step is to install six PC stakes to accept the exter­nal low-current wiring connections. Two of these stakes are installed at the TH1 wiring points (7 & 8); two at the fan wiring points (9 & 10); one to accept the +12V ignition lead; and one at the ground point (11). The fuseholder clips for F2 (1A) can Fig.8: this diagram shows the mounting details for the power diodes (top) and the BUK436 Mosfets (bottom). Note that the metal tabs of these devices must be electrically isolated from the heatsink using insulating washers and bushes. the heatsink is exactly 214mm long. File all edges to a smooth finish after cutting. The heatsink can now be positioned on the PC board and the various hole positions marked. Drill 3mm holes at the two mount­ing pillar locations and for the diode mounting screws. TABLE 1: RESISTOR COLOUR CODES ❏ No. ❏  2 ❏  1 ❏  2 ❏  1 ❏  6 ❏  4 ❏  3 ❏  1 ❏  7 ❏  2 ❏  6 68  Silicon Chip Value 1MΩ 470kΩ 47kΩ 27kΩ 10kΩ 6.8kΩ 4.7kΩ 2.2kΩ 10Ω 4.7Ω 1Ω 4-Band Code (1%) brown black green brown yellow violet yellow brown yellow violet orange brown red violet orange brown brown black orange brown blue grey red brown yellow violet red brown red red red brown brown black black brown yellow violet gold brown brown black gold gold 5-Band Code (1%) brown black black yellow brown yellow violet black orange brown yellow violet black red brown red violet black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown red red black brown brown brown black black gold brown yellow violet black silver brown brown black black silver brown Fig.9: this diagram shows the step-by-step winding details for transformer T1. Note that the primary is wound using copper sheet and this must be cut to the shape shown – see text. November 1996  69 This is the completed prototype, ready for installation in the boot of a car. Note that holes must be drilled in the front and rear panels in line with the heatsink, so that the fan can do its job. now be installed. Note that each clip has a small lug at one end to hold the fuse in place, so be sure to install them the correct way around. LEDs 1-3 and the four small-signal transistors (Q1, Q2, Q6 & Q7) go in next. Note that LEDs 1-3 are mounted at full lead length so that they can later be bent over and pushed through the front panel. Take care to ensure that they are oriented correct­ly – the anode lead is the longer of the two. LED 1 is the red LED, while LEDs 2 & 3 are green. Be sure to mount the correct transistor type at each loca­tion. Q1 & Q6 and NPN types while Q2 & Q7 are PNPs, so don’t get them mixed up. At this stage, the capacitors can all be installed on the PC board. Install the small MKT capacitors first (see Table 2 for the codes), then move on to the larger values. The 10µF 100VW capaci­tors between the bus bars are bipolar types and can be mounted either way around. However, the two 10µF 16VW capacitors must be mount­­ ed with the correct polarity, as must the four 2200µF 100VW units. Note that the latter have terminal 70  Silicon Chip numbers on their bases. Pin 1 is the positive terminal, while pin 5 is the negative terminal. Their bodies also have unusual arrow markings down the negative side. Brass link bars To cater for the heavy currents involved in the output stage, the circuit board carries two brass link bars and these are mounted using 3mm screws into tapped holes from the underside of the PC board. Once these bars are in place, run the connec­tions to the adjacent capacitor bank and to the sources of the Mosfets using 1.78mm diameter solid core wire. This same wire should also be used for the interconnections between D3D6 and for the connections between these diodes and transformer T1. In addition, a link using this wire is run from the transformer to the centre of the 2200µF capacitors. Once this wiring has been completed, solder three 4mm nuts to the underside of the PC board at the (+), 0V and (-) output terminal positions near the 2200µF capacitors. This is best done with the nuts attached to their 4mm screws, so that they line up with the board holes correctly. Transformer winding Transformer T1 is wound using copper sheet for the primary and enamelled copper wire for the secondary. Fig.9 shows the details. First, use a pair of tinsnips to cut the copper sheet to size, as shown in step 1 of Fig.9. This done, solder suitable lengths of 3.3mm2 insulated copper wire to the tags as shown in step 2. When doing this, flatten the stripped wire strands with a pair of pliers, so that they sit right down on the tags. Note that where two connections are shown to a tag, it’s best to use a single length of wire bent in half. Remove the insulation from the centre point and bend the wire into a sharp U-shape, so that the leads emerge at right angles from the copper strip. Once all the connections have been made, cover the top of the copper sheet with a layer of insulating tape. Be sure to also cover the soldered tags. This done, label the relevant leads with the numbers 1-6, as indicated on the diagram. You can do this by attaching a small piece of masking tape to each lead and writing on this. Make sure that you don’t get the leads mixed up, otherwise the connections to the drains of the Mosfets will be wrong. The red leads do not need labelling since they are all connected to the positive link bar. The copper strip can now be wound onto the former as shown in step 3 of Fig.9. Start from the top of the former and slide the solder tags into the slotted plastic flanges (you will need to make these slightly wider using a file or a pair of sidecut­ters). This done, wind on two turns and check that the solder tags with the red wires now slot into the flanges on the top of the transformer, along with the solder tags at the start end. Finally, complete the primary by winding on the next two turns, finishing again at the top of the former. Secure the winding with a layer of insulating tape, taking care to ensure that the solder tags are not shorting to each other. The secondary is wound directly over the primary winding. First, check Table 3 for the number of turns required to obtain the desired output voltage from the converter. This done, cut four 1.5-metre lengths of 1.25mm diameter enamelled copper wire and terminate one end of each onto pins 4, 5, 6 & 7, respectively (the wire ends are easily stripped by using a soldering iron to melt the enamel). Now wind on all four wires simultaneously in the direction shown, with each wire sitting directly alongside the others (ie, not jumbled up). Insulate each layer with a layer of electrical tape and continue until the requisite number of turns has been wound on. Terminate the ends onto pins 17, 16, 15 & 14 respective­ly. This done, use a multimeter to check that pin 4 connects to pin 17, pin 5 to pin 16, pin 6 to pin 15 and pin 7 to pin 14. The transformer is now assembled by sliding the cores into the former from each end and fitting the metal clips. Once the transformer has been completed, strip the ends of the primary leads and crimp eyelet lugs to the black leads only. Inductor L1a, L1b Fig.8 shows the winding details for L1a and L1b. These are wound on a common Neosid 17-745-22 ring core using 1.5mm diameter enamelled copper wire. The fan is mounted on the rear panel, in-line with the heatsink, using 9mm tapped brass spacers. Orient the fan as shown here, so that it blows the air out through the holes drilled in the rear panel. sure to wind the coils in the directions shown in Fig.8. Copper strap Fig.10: inductors L1a and L1b are wound on a common toroid former, as shown here. Table 3: Transformer Wiring Required Output Turns On Secondary ±65-70V ±60-65V ±55-60V ±50-55V ±40-50V ±40-45V ±35-40V ±30-35V ±20-30V 12 11 11 10 9 8 7 7 6 You will need about 600mm of wire for each coil on this ring core. Wind on the 14 turns for inductor L1a first, then wind on the turns for L1b. Be The copper strap connecting the link bar to fuse F1 is made from 0.6mm thick copper sheet. Begin by cutting a 75 x 18mm piece, then drill a 12mm hole in one end and an 8mm hole in the other. This done, bolt the end with the 12mm hole to the link bar, as shown on Fig.7. The copper strap is then bent down so that its 8mm hole lines up with the adjacent fuse mounting hole. Fuse F1 can now be mounted in place on the copper strap and secured using an 8mm bolt, nut and washer. An 8mm bolt, nut and washer should also be used to temporarily secure the other end of the fuse. Heatsink mounting The next step in the assembly is to fit the heatsink to the PC board. This is secured at the two main mounting points using 15mm tapped standoffs on the PC board side and 3mm screws from the heatsink side. Next, bend the leads of the Mosfets (Q3-Q5 & Q8-Q10) at right angles so that they go through the PC board holes (check also that their metal tabs line up with the heatsink mounting holes). This done, mount each Mosfet on the heatsink using an insulating washer, bush, spring washer, eyelet lug and nut as shown in Fig.8. The eyelet lugs used are the ones that were previously crimped to the November 1996  71 & 8 (near F2) at the far end of the PC board. The wiring from T1 can now be completed by connecting its red wires to the link bar as shown. The toroid inductor (L1a & L1b) can also be mounted at this stage. It is held in place using two small cable ties which pass through holes in the PC board. Current sensing resistor (Rsc) The Rsc lead is made using a 55mm length of 3.5mm2 wire. Each end is terminated by connecting it to a large eyelet using generous amounts of solder. The lead is then insulated using heatshrink tubing. For the time being, attach one end only to the link bar as shown in Fig.7. This close-up view shows the mounting details for the power diodes, Mosfets and the thermal cutout (TH1). The toroid inductor (L1a & L1b) is secured to the board using a pair of cable ties. black leads from transformer T1. These leads were all numbered, from 1-6. Be sure to connect the correct lead to the metal tab (drain) of each Mosfet – see Fig.7. Note that the type of insulating bush supplied may have a flange attached – if so, this should be cut off using a sharp knife. Note also that if mica washers are used, it will be neces­sary to smear heatsink compound on both sides. If silicone wash­ers are supplied instead, then you do not need heatsink compound. When all the Mosfets are in place, use a multimeter to confirm that their metal tabs are correctly isolated from 72  Silicon Chip the heatsink. If you do find a short, remove the device and correct the problem before proceeding further. The power diodes (D3-D6) are mounted next. These are in­stalled in a similar manner to the Mosfets, again using an insu­lating washer and a bush (with a flange) – see Fig.8. As before, use your multimeter to confirm that the device tabs are correctly isolated from the heatsink. When everything is correct, solder all the Mosfet and diode leads to the PC board. This done, bolt the thermal cutout (TH1) to the heatsink as shown in Fig.11 and use light-duty hookup wire to connect its leads to points 7 Final assembly Begin the final assembly by attaching the PC board to the case baseplate using 15mm standoffs and screws. This done, posi­tion the fan so that its blades line up with the heatsink fins and mark out suitable mounting hole positions on the rear panel. Drill these holes to accept 3mm machine screws. Next, mark out the positions for the Rsc wire connection to the rear panel, the adjacent ground wire connection and the two cable gland holes. These holes can now be drilled or punched, as appropriate. A large hole must also Below: we dressed up the large hole cut for the fan in the rear panel by adding some aluminium trim but this can be considered optional. Make sure all cables are firmly secured. Fig.11: the output cables and the positive battery cable are secured to the rear panel using cable glands. Be sure to use heavy-duty cable where indicated. November 1996  73 Fig.12: this PC etching pattern is shown 71% of actual size. It can easily be reproduced full-size using a photostat machine set to a standard enlargement of 1.41. be cut in the rear panel in line with the fan blades, to allow the air to escape. If necessary, the larger holes can be made by drilling a series of small holes around the inside diameter or 74  Silicon Chip the marked area, then knocking out the centre piece and filing to a smooth finish. The fan can now be mounted on the rear panel using 9mm tapped brass spacers and machine screws. Be sure to orient the fan so that it blows the air out of the case. This done, attach the rear panel to the case and bolt the free end of Rsc to the case, along with the negative battery lead. The cable glands can now be fitted, along with the ground eyelet. The ground eyelet connects to point 11 on the PC board. This connection can be run using medium-duty hook­ up wire. Moving now to the front panel, you will need to drill a series of airflow holes in line with the heatsink (see photo) plus three holes for the indicator LEDs. The latter are best drilled after first attaching the front panel label and this should be carefully positioned to ensure that it lines up with the onboard LEDs. Make the LED indicator holes just large enough to accept the plastic bezels. Once these have been fitted, attach the front panel and push the LEDs into place. All that remains now is to complete the wiring as shown in Fig.11. First, connect the fan to points 9 & 10 on the PC board. To do this, you will need to extend the existing fan leads, taking care to ensure that the joins are well insulated with heatshrink tubing. Next, attach the output leads to the screw terminals using crimp eyelets and 4mm screws, star washers and nuts. The star washers bite into the copper on the PC board, thereby ensuring good contacts. The +12V ignition lead should also be connected at this stage – use red medium-duty hookup wire for easy identifica­tion. The ignition lead and the three output leads pass through one of the cable glands on the rear panel. Tighten this gland firmly to prevent cable movement. Finally, the battery cables can be installed. These are run using heavy-duty 4GA cables (red for positive, black for nega­tive) which terminate into large eyelets. You will need a heavy-duty soldering iron to solder these, if they haven’t already been connected. The positive lead passes through the second cable gland and is bolted to one end of fuse F1, while the negative lead is bolted to the rear panel. Testing If you have a power supply capable of delivering 12V at 1A or more, it can be used to test the inverter. Alternatively, you can use a car battery with F1 initially replaced by a 10A automo­tive fuse. This can be wired in using hookup wire around the 8mm bolts and by using large clips for the battery connection. First, connect the positive and negative leads to the supply terminals, then connect the ignition lead to the positive terminal. The power supply will be loaded down for a few seconds if a low current supply is used as the output capacitors charge. Once the capacitors are charged, the standby current should be around 300mA due to the fan. Check that the power LED and (+) and (-) supply LEDs all light, then adjust VR1 to obtain the correct positive and negative output voltages. Next, check that the converter switches off when the input voltage is reduced to less that 10V. Of course, this is only really practical if you are testing the converter using a vari­able supply. If you are using a battery, test that the unit switches off when pin 2 of IC1 is pulled to ground (you can do this using a test lead but be careful not to short any of the adjacent IC pins). If the 10A fuse blows when a battery is used, the problem probably lies in the transformer wiring. Alternatively, the Mos­fets or diodes may be shorted to the heatsink. Recheck all wiring and component placement if you strike problems. Assuming everything works correctly, reconnect the 63A fuse for F1. The unit can now be installed in a vehicle but be sure to follow these guidelines: (1). The heavy duty supply wiring to the converter must connect directly to the battery terminals. You can purchase battery terminals that will allow the converter connection plus the normal automotive battery wiring. (2). The wires should be run through the engine bay fire­wall via grommets and pass under the vehicle carpet or mats. Entry to the boot should be via grom­mets as well. (3). The ignition connection should be made at the fusebox so that the converter will be powered whenever the ignition is on. Alternatively, a dashboard switch can be installed so that it is turned on separately. (4). Make sure that the polarity is correct when connecting the output supply rails to the amplifier. (5). Run the loudspeaker connections using twin cable. Don’t use the vehicle chassis as a return connection for the loudspeaker since heavy circulating currents can occur within the ground wiring and this could lead to SC noise problems. YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM SATELLITE ENTHUSIASTS STARTER KIT YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● ● ● ● ● ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: __________ Phone: (_______) ________________________ ACN 002 174 478 November 1996  75 SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.altronics.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.altronics.com.au SILICON CHIP If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.altronics.com.au PRODUCT SHOWCASE Hung Chang 20MHz dual trace oscilloscope trols are three pushbutton switches to select Channel 1, Channel 2 or both channels added together (ADD). By pushing the INVERT button for channel 2, the ADD button will provide a display which represents the difference between the two channel inputs. This is not quite the same as having a scope with differential inputs but it’s a handy facili­ty. Above these controls and to the right of the screen are the Power switch, Intensity, Focus and Scale Illumination (for the screen graticule) controls. Also in this section of the panel is a screwdriver preset for trace rotation. This can be used to ensure that the traces are exactly parallel to the horizontal lines on the screen graticule. As well, there is a Calibration terminal providing a 0.5V peak-peak square wave reference signal. This is used for setting the compensation of the scope probes (ie, for optimum square wave display). Timebase & triggering This attractive oscilloscope has a 20MHz bandwidth, dual trace operation and a maximum vertical input sensitivity of 1mV/ div. Its timebase extends up to 0.1µs/div. While many electronic hobbyists might yearn for a 200MHz digital storage oscilloscope with screen save to disc capabili­ ties, a great deal of measurement work can be done with a much lower priced instrument such as this Model 6502 from Hung Chang in Korea. It is quite compact, with dimensions of 140mm high, 335mm wide and 420mm deep. The front panel manages to feature all the usual controls while appearing to be very clean and uncluttered. Even so, there are no less then 11 knob controls, 11 pushbuttons and four 3-position toggle switches. The active screen area is 10cm x 8cm and the screen phosphor is blue. To the right of the screen are the vertical attenuators. These have nine settings ranging from 5mV/cm to 5V/ cm in a 1:2:5 sequence. By pushing the x5 MAG switch, the sensitivity can be increased to a maximum of 1mV/ cm while knobs concentric with the attenuator switches give continuous variation of sensitivity. To the right of each attenuator is a 3-position toggle switch allowing AC or DC coupling and Gnd while above each atten­uator is the trace position control for the respective channel. The vertical input BNC sockets are beneath the input attenuators with an earth terminal between them. Between the vertical position con- At the far right of the panel is the Time/Division switch giving 20 sweep from 0.1µs/cm to 0.2s/cm in the usual 1:2:5 steps. Below this and to the left are two 4-position toggle switches. The first is for Trigger Mode allowing selection of Auto, Norm, TV/V and TV/H triggering, while the second switch is trigger Source. The positions provided here are Internal, Ch2, Line and External. Note that if the oscilloscope is being used in the single channel mode, either Ch1 or Ch2, then sync is provided by selecting the Int source. To the right of these switches is the Trigger Level control. Above and to the right of the time­ base switch is a timebase Variable control and two pushbutton switch­es for trigger slope (either + or -) and XY mode. When pushed in this causes the Ch1 and Ch2 inputs to be displayed as an XY Lissajous plot. Trace magnification Two more pushbutton switches provide timebase magnification (MAG x5) and ALT-MAG. November 1996  79 The first is self explanatory while the second is not. When pushed it displays an additional trace with the timebase being five times normal speed. If the scope is already in Dual mode and ALT-MAG is depressed, then four traces will be shown. This is quite an unusual feature and can be handy in a number of situations. One control missing is that for selecting Alternate or Chopped mode for dual trace operation. Instead, these functions are selected automatically as the Timebase speed is altered. Trigger sensitivity for the Model 6502 on internal sync is quoted as three divisions for DC to 20MHz bandwidth. Our sample easily exceeded this spec. Reliable sync was obtained up to 2MHz with a displayed signal of less than one division and at 20MHz with a two division signal. This is good. Another interesting point about this scope is its very high maximum sensitivity of 1mV/cm although this is not useable in some situations where the vertical input amplifiers become unstable and oscillate. This is mentioned in the instruction manual and can be cured by good grounding of the KITS-R-US RF Products FMTX1 Kit $49 Single transistor 2.5 Watt Tx free running 12v-24V DC. FM band 88-108MHz. 500mV RMS audio sensitivity. FMTX2A Kit $49 A digital stereo coder using discrete components. XTAL locked subcarrier. Compatible with all our transmitters. FMTX2B Kit $49 3 stage XTAL locked 100MHz FM band 30mW output. Aust pre-emphasis. Quality specs. Optional 50mW upgrade $5. FMTX5 Kit $98 Both a FMTX2A & FMTX2B on 1 PCB. Pwt & audio routed. FME500 Kit $499 Broadcast specs. PLL 0.5 to 1 watt output narrowcast TX kit. Frequency set with Dip Switch. 220 Linear Amp Kit $499 2-15 watt output linear amp for FM band 50mW input. Simple design uses hybrid. SG1 Kit $399 Broadcast quality FM stereo coder. Uses op amps with selectable pre-emphasis. Other linear amps and kits available for broadcasters. 80  Silicon Chip signal source as well as placing a 220Ω resistor in series with the scope probe. Sweep linearity of the instrument was good and the trace sweep times within specification of ±3%. The displayed trace was very fine, with even focus across the full screen area. In short, the Hung Chang model 6502 oscilloscope is a good performer, especially at its low price. Recommended retail price is $799 including sales tax. Dual sensitivity 80MHz probes are available for $39.95 each, tax paid. Our review instrument came from Altronics, 174 Roe Street, Perth 6000. Phone 1 800 999 007. Price reduction on Fluke graphical DMM Fluke’s 860 series Graphical Multi­ Meters have proved very popular since their release in 1995. Combining advanced multimet­er capabilities with waveform display, in-circuit component testing, trend plotting and logic activity detection in one easy to use handheld instrument, they represent good value. The Fluke 865, the mid-spec model PO Box 314 Blackwood SA 5051 Ph 0414 323099 Fax 088 270 3175 AWA FM721 FM-TX board $19 Modify them as a 1 watt op Narrowcast Tx. Lots of good RF bits on PCB. AWA FM721 FM-RX board $10 The complementary receiver for the above Tx. Full circuits provided for RX or TX. Xtals have been disabled. MAX Kit for PCs $169 Talk to the real world from a PC. 7 relays, ADC, DAC 8 TTL inputs & stepper driver with sample basic programs. ETI 1623 kit for PCs $69 24 lines as inputs or outputs DSPTH-PCB and all parts. Easy to build, low cost. ETI DIGI-200 Watt Amp Kit $39 200W/2 125W/4 70W/8 from ±33 volt supply. 27,000 built since 1987. Easy to build. ROLA Digital Audio Software Call for full information about our range of digital cart players & multitrack recorders. ALL POSTAGE $6.80 Per Order FREE Steam Boat For every order over $100 receive FREE a PUTT-PUTT steam boat kit. Available separately for $19.95, this is one of the greatest educational toys ever sold. in the family of three, is currently being offered for a limited time at the reduced price of $749.00 excluding sales tax. The 865 features a basic DC accuracy of .04%, selectable display modes, including Meter Mode, Waveform Display, TrendGraph Mode, In-Circuit Component and Logic Test Mode. It is supplied with test leads and battery eliminator and housed in a shock-absorbing protective holster. For more information, contact Obiat Pty Ltd, 129 Queen St, Beacons­field, NSW 2014. Phone (02) 9698 4111; fax (02) 9699 9170. Mini-Circuits Designer’s Guide To encourage radio amateurs to build and develop their own communications equipment, Mini-Circuits USA are offering, free of charge, their RF/IF Designer’s Guide. Mini-Circuits manufacture an extensive range of mini-modules, such as RF amplifiers, fre­q uency m ix er s, po wer splitters, filters, detectors, attenuators, etc. Circuits are available in various case style packages to suit applications from surface mount to external equipment mounts. Typical mixer frequencies range from 10kHz to 4.3GHz, making them ideal for LF to SHF work. If you would like a free copy of RF/IF Designer’s Guide, contact Mini-Circuits’ Australian representatives and stockists, Clarke & Severn Electronics, PO Box 1, Hornsby, NSW 2077 or call (02) 9482 1944. PCB POWER TRANSFORMERS Programmable multimeters Meter International has added the MIC-3130 to the 3000 series of programmable multimeters. These allow the setting of low and high levels of current, voltage or resistance. When a measurement falls within the set values, a warning tone is sound­ed. This can free an operator from referring to the display when looking for a particular measurement. The Automatic Data Hold allows the operator to concentrate on probe placement, updating the display every three seconds on valid inputs. A “beep” indicates the reading is locked in. This feature significantly reduces measurement difficulty in confined areas and increases safety when measuring dangerous voltages. A special feature of the MIC3130 is the option of connect­ing K or J-type thermocouples, with the display showing either °C or °F. The MIC-3130 can also measure capacitance from 1pF to 4µF over four ranges. A frequency counter to 4MHz is also included. The unit comes in a rugged case with pushbutton range selection. The display has large 4000 count 1VA to 25VA Manufactured in Australia Harbuch Electronics Pty Ltd 9/40 Leighton Pl. HORNSBY 2077 Ph (02) 476-5854 Fx (02) 476-3231 digits with range annunciators. A bargraph is incorporated into the large 4000-count display. A standard feature is the fast display mode, which updates the display every 50ms instead of the normal 300ms peri­od. For further information, contact Computronics Corporation Ltd, 31 Kensington Street, East Perth, WA 6004. Phone (09) 221 2121; fax (09) 325 6686. ® Design low frequency loudspeaker enclos­ures fast and accurately with BassBox® software. Uses both Thiele-Small and Electro-Mechanical parameters with equal ease. Includes X. Over 2.03 passive cross­over design program. $299.00 RS-232 chips meet ESD requirements Analog Devices Inc has introduced a family of +5V, RS-232 transceivers which meets the European EMC (Electromagnetic Compa­ tibility) requirements. The ADM2xxE product line is the indus­try’s first family of robust, RS-232 and V.28 interface tran­ sceivers which provide ±15kV of ESD (ElectroStatic Discharge) and ±2kV of EFT (electrical fast transient) protection. Designed for modems, printers, lap­top and notebook comput­ers, the devices offer low EMI (electromagnetic interference) emissions in accordance with EN55022 and provide compliance with the IEC-1000-4-x immunity standards. These standards must be met for all electrical or electronic equipment carrying the “CE” (Comm­ BassBox Plus $6.00 postage. Pay by cheque, Bankcard, Mastercard, Visacard. EARTHQUAKE AUDIO PH: (02) 9948 3771 FAX: (02) 9948 8040 PO BOX 226 BALGOWLAH NSW 2093 un­aute Europeene) mark sold into the European market after January 1, 1996. Operating from a +5V supply, all five devices come with latch-up protection and immunity to high RF (radio frequency) fields to allow for operation in unshielded enclosures and electrical­ly-harsh environments. The product line offers a variety of driver/receiver combinations and all devices are available in SOIC, SSOP and TSSOP packaging The ADM2xxE family offers a pin-compatible upgrade for existing, industry-standard +5V, RS-232 transceivers. Additional features include a low power consumption of 17mW, a guaranteed high data transmission rate of 230 kbits/second, a 1µW shutdown mode and operation over a temperature range of -40°C to +85°C. For further information, contact Hartec, 205A Middleborough Road, Box Hill, Vic 3128. Phone 1 800 335 SC 623. November 1996  81 COMPUTERS Adding An Extra Parallel Port Recently, we added a second parallel port to one of our old PCs and while we were at it, we upgraded the serial ports with faster UARTs as well. What we did can be applied to other machines. By GREG SWAIN I N ITS DAY, our old 25MHz 386 computer was a very impres­sive machine. Purchased new just six short years ago, it came with a 120Mb SCSI hard disc drive, a fancy Radius graphics card and a whole 4Mb of RAM. Of course, those specifications would be sneered at today but at the time, it really was the latest and the greatest. After a couple of years use in a desktop publishing role, the 386 was relegated to more mundane tasks, such as wordprocess­ing, running payroll software and These are the two cards that we added to our 486. The X-2233 serial card from Dick Smith Electronics (above) gave us upgraded serial ports, while the old printer card at right gave us a second parallel port so that we could permanently connect two printers. 82  Silicon Chip maintaining a subscriptions database. And, earlier in the year, we added an external fax/modem so that faxes could be sent directly from the computer. When it recently conked out, it didn’t take long to discov­er why. A distinctive burn mark in the middle of one of the main chips on the multi-I/O board told the tale and, as we subsequent­ly discovered, the mother­ board had failed as well. Strangely, everything else in the machine proved to be OK, including the RAM, the disc drives, the Radius video card and the power supply. As a result, the old 386’s role was taken over by a 50MHz 486 machine that had been sitting unloved in a corner of the office, itself the victim of a recent upgrading. This replacement machine came with a 250Mb hard disc drive, 8Mb of RAM and Windows 3.11. We swapped its graphics card for the Radius card from the defunct 386 (so that we could still use the Radius monitor) and we were up and running. After that, it was simply a matter of reinstalling the necessary software and copying our backup data files onto the replacement hard disc. Parallel ports Having come this far, we decided to take a closer look at the 486 to see if it could easily be improved in any way. The first thing that could be done was obvious. For various reasons, we wanted to permanently connect two printers to the 486, one a laser printer and the other a dot matrix machine (don’t laugh; dot matrix machines are still good for printing out address labels and multi-part forms). In the past, with the old 386, we had simply swapped printer cables when ever the alternative printer was to be used but that’s a clumsy way of going about things to say the least. Perhaps even more importantly, there’s now a whole raft of non-printer devices designed to run from a parallel port. These include such things as the Snappy video capture system described last month, as well as a huge range of mass storage devices such as Iomega’s Zip and Jazz drives, external CD ROMs and even scanners. The problem is, how do you connect these devices without disconnecting the printer? One solution is to use a printer The existing serial ports were disabled by changing a couple of jumper settings on the multi-I/O card. We also reassigned the existing printer port from LPT1 to LPT2, to avoid a conflict will the “new” printer card. switcher but the most elegant way is to add a second parallel port. That way, both printers (or a printer and some other peripheral) can be permanently connected to the computer and the various applications can be set up to print to a preferred default printer. A quick rummage through our computer junk box soon turned up a dedicated printer card. Just where it originally came from is now a mystery but it proves that if you keep something for long enough, it will eventually come in handy. More to the point, there was no literature with the card and in any case, there are no on-board jumpers to configure. Assuming that it still worked, it would give us our re­quired second printer port. We’ll come back to that shortly. Faster serial ports The other thing that could be done was to improve the performance of the serial ports. As with other machines of its vintage, our 486 used 8250 UARTs (universal asynchronous receiver transmitters) in its serial port circuitry and these are only good for about 9600 Fig.1: to check which type of UARTs your machine has go to the DOS prompt, type msd to run Microsoft Diagnostics and click on Com Ports (or press C). This particular machine uses 16550 UARTs but if yours uses the older 8250s, it will need upgrading to take advantage of the latest fast modems. November 1996  83 baud. Anything faster and you need the later 16550 UARTs. Because our fax/modem is capable of operating at 14.4Kb in fax mode and 28.8Kb in modem mode, we decided that an upgrade would be well worthwhile. The answer of course is to add a new serial card with 16550 UARTs to the motherboard. This time, we weren’t so lucky with our junk box but a quick check in the Dick Smith Electronics catalog soon turned up a suitable RS232 serial card (Cat. X-2233; $39.95). It comes with two 16550 UARTs, two serial port connectors (9-pin and 15-pin) and a manual with all the IRQ (interrupt request) and address configuration details. By the way, you can easily check which type of UARTs your machine has. Just go to the DOS prompt, type msd to run Microsoft Diagnostics and click on Com Ports (or press C). The UARTs used in the computer will be listed at the bottom of the display –see Fig.1. Resolving hardw Installing the hardware Unfortunately, it’s not just a matter of plugging in the new cards and expecting everything to work. If you do that, the existing parallel and serial ports will conflict with the new ones. More precisely, you will get IRQ conflicts and conflicts between memory addresses which could lead to problems. The trick is to reassign one of the parallel ports from LPT1 to LPT2 and to disable the existing serial ports. Before doing that however, we ran Microsoft Diagnostics (type msd at the DOS prompt) to check on the IRQs used by the existing serial ports (COM1 & COM2) and their addresses. This showed that COM1 and COM2 used IRQs 4 and 3 respectively, while their memory addresses were 03F8-03FF and 02F8-02FF respectively. These settings are pretty much standard and a quick check in the manual soon revealed that these were also the default settings on our new serial card. If the latter had been different, it would have been necessary to reconfigure the jumper settings to match the existing ports. For good measure, we also used Microsoft Diagnostics to check on the address and IRQ status of the current parallel port (0378 and 5, respectively). This was mainly a precautionary 84  Silicon Chip Clicking on Computer and then Properties brings up the Computer Properties dialog box below. This lets you view current IRQ assignments and address allocations, so that you can easily choose free resources before adding new hardware. ware conflicts in Win95 Provided you have all Plug and Play (PnP) devices, Windows 95 will successfully allocate resources to avoid device conflicts. The presence of older (legacy) cards can lead to conflicts however, which you’ll need to sort out yourself. To bring up the System Properties dialog box, go to the Control Panel and double-click the System icon. The system properties dialog box will immediately indicate any resource conflicts. To view the resources assigned to a particular item, select it in the System Properties dialog box, click the Properties button and then click the Resources tab. Any device conflicts are indicated in this panel. To change resource settings, deselect Use automatic settings, select the Resource type to be changed (eg, Interrupt Request) and click the Change Setting button. November 1996  85 The two additional cards were inserted into vacant slots above the multi-I/O card. It’s a good idea to remove the connectors for the old COM ports if possible to avoid confusion. If they can’t be removed, label them clearly with a sticker. measure, in case we had to change things later on. Next, we pulled the power plug, removed the back from the PC and pulled the multi-I/O card. In addition to providing one parallel and two serial ports, this particular card also provides two IDE hard disc drive controllers plus two floppy disc drive controllers. Because there was no way of config­ uring the old printer card, the obvious thing to do was to make it LPT1 and reassign the port on the multi-I/O card from LPT1 to LPT2. This simply involved changing one of the jumpers on the card. At the same time, we changed the positions of two other jumpers to disable the serial ports (it really pays to keep the manuals that come with computers). And that’s really all there was to it. We replaced the I/O card, plugged in the additional serial and printer cards, recon­nected everything and found that it all worked. Of course, you have to remember to plug the mouse into one of the new COM ports, since the old COM ports no longer function. In fact, it’s a good idea to remove the old COM port connectors if possible to avoid confusion, or at least label them with a disabled sticker. If you strike trouble, it’s probably due to a resources conflict. To resolve 86  Silicon Chip the problem, check the IRQ settings and the memory addresses carefully and try again. If your printer ports don’t work, for example, try changing the address on one of the cards to the alternative setting. Variations on a theme All the foregoing is just one variation on a number of possible configurations although the basics still apply in each case. For example, a single multi-I/O card could be used to provide both the additional printer port and the new serial ports – just remember to disable the functions that aren’t required on both the new card and the existing card. As a matter of interest, Dick Smith Electronics sell a multiI/O card with four serial ports and three parallel ports (Cat. X-2573) for $129 or you can buy a 2-port parallel printer card (Cat. X-2548) for $49.95. On late-model 486s (and Pentiums), the I/O functions are integrated onto the motherboard but these invariably use 16550 UARTs anyway so you won’t have to upgrade the serial ports. If you do need to disable ports or change address settings, this can be done via the system BIOS. This should only rarely be necessary if adding another printer port, however – in most cases, you will be able to configure the add-in card to avoid conflicts. What about Windows 95? In this case, Windows probably won’t notice the new hardware when it boots and you’ll have to run the Add New Hardware routine from the Control Panel. You can then elect to have Win95 automatically detect the new hardware or, if you are like me and are too impatient to wait through several minutes of hard disc rattling, you can manually select the hard­ware to be added. In the latter case, just say No to the auto-detect routine, then select Ports (COM & LPT) in the next dialog box and finally Communications Port or Printer Port as appropriate. Windows 95 will then assign resources to the new hardware. If these resourc­es conflict with those used by another device, you can change them manually later on – see panel. At least, that’s the theory. We haven’t tried adding an additional printer port on a system running Win95 but the above procedure is routine. Finally, if you have a computer that’s already crammed with multiple disc drives, a network card, a sound card, a SCSI card and any other devices, check your system resources carefully before trying to add more hardware. There are only 16 IRQs avail­able to begin with and over half of these are taken up by essen­ tial items before anything extra is added. Once all the IRQs are gone, that’s it – you can’t add new hardware unless you’re will­ing to sacrifice something SC else. ORDER FORM BACK ISSUES MONTH YEAR MONTH YEAR PR ICE EACH (includes p&p) Australi a $A7.00; NZ $A8.00 (airmail ); Elsewhere $A10 (airmail ). Buy 10 or more and get a 10% discount. Note: Nov 87-Aug 88; Oct 88-Mar 89; June 89; Aug 89; Dec 89; May 90; Aug 91; Feb 92; July 92; Sept 92; NovDec 92; & March 98 are sol d out. All other issues are currently i n stock. TOTAL $A B INDERS Pl ease send me _______ SILICON CHIP bi nder(s) at $A12.95 + $5.00 p&p each (Australi a only). N ot avail abl e elsewhere. Buy five and get them postage free. $A SUBSCRIPTIONS  New subscription – month to start­­____________________________  Renewal – Sub. No.________________    Gift subscription  RATES (please tick one) 2 years (24 issues) 1 year (12 issues) Australia (incl. GST)  $A135  $A69.50 Australia with binder(s) (incl. GST)**  $A159  $A83 New Zealand (airmail)  $A145  $A77 Overseas surface mail  $A160  $A85  $A250 Overseas airmail  $A125 **1 binder with 1-year subscription; 2 binders with 2-year subscription YOUR DETAILS Your Name_________________________________________________ GIFT SUBSCRIPTION DETAILS Month to start__________________ Message_____________________ _____________________________ _____________________________ Gift for: Name_________________________ (PLEASE PRINT) Address______________________ _____________________________ (PLEASE PRINT) Address___________________________________________________ State__________Postcode_______ ______________________________________Postcode_____________ Daytime Phone No.____________________Total Price $A __________ Signature  Cheque/Money Order  Bankcard  Visa Card  Master Card ______________________________ Card No. Card expiry date________/________ Phone (02) 9979 5644 9am-5pm Mon-Fri. Please have your credit card details ready OR Fax (02) 9979 6503 Fax the coupon with your credit card details 24 hours 7 days a week Mail order form to: OR Reply Paid 25 Silicon Chip Publications PO Box 139, Collaroy 2097 No postage stamp required in Australia November 1996  87 VINTAGE RADIO By JOHN HILL A pair of Astor valve radios Occasionally, I get to repair vintage radios for other people and that’s what this month’s column is all about. It concerns a couple of interesting old Astors. Noel, a new collector of my acquaintance, phoned me recent­ly with a problem. He had found a couple of receivers in a local antique shop and wanted my advice before buying them. As both were priced at $75, I conjured up a mental picture of a couple of cracked Little Nippers with missing knobs or something equally undesirable. $75 doesn’t buy much from an antique dealer! To my surprise, the radios were by no means cracked Little Nippers but a genuine 1940 Astor “Mickey Mouse” and an early post war Baby Astor. The latter is known to collectors as an Astor Football. Both are very collectible items! My first thought was one of annoyance that I had missed out on two good radios, neither of which was in my collection. But, on second thoughts, I considered it better for Noel to have them because I would A 1940 BP model Astor “Mickey Mouse”. Although a very collectible item, it is a fairly awkward receiver to work on and has no really outstanding features apart from being very compact. No, the knobs are not the originals! 88  Silicon Chip still get the job of repairing them and, with a bit of luck, a Vintage Radio story as well. That would keep us both happy. After a bit of bartering, the Baby Astor was dropped to $50 and I took both receivers home to see if Noel had bought himself a good deal. Fixing Mickey The “Mickey Mouse” was the first to be repaired. It was quite dead and gave no response whatsoever, although the valves lit up, which always gives a little hope. A closer inspection revealed that the output transformer was open circuit and that someone had previously disconnected the electrodynamic loudspeaker and then mixed up the leads when reconnecting it. The original 6Q7 detector/first audio valve had also been replaced with a 6SQ7 which is just about the same valve except that it is a single-ended type and lacks a top cap connection. Unfortunately, those characteristic “Mickey Mouse” control knobs were missing. Apart from that, the rest of the set looked fairly original. Now while these so-called collectible radios are eagerly sought after, those who repair them often see them in a different light. This particular “Mickey” is not what one would de­ scribe as easily worked on and it has a few undesirable features. In its original form, the bulk of paper capacitors under­neath the chassis makes it impossible to gain access to the valve sockets, to check voltages, etc. This situation improves greatly once the old capacitors have been replaced with smaller, modern types. Removal of the loudspeaker requires the dial, dial cord and the dial light to be either removed or disconnected, as If it wasn’t for the “Mickey Mouse” aspect of these radios they would be just another old radio. Astor used the Disney name without permission and later dropped the “Mouse” bit. The Astor “Mickey” was a very popular post-war receiver. is appro­priate. And finally, fitting a new speaker grille cloth is fairly tricky due to the fact that the dial is in the centre of the speaker opening. All things considered, the Astor “Mickey Mouse” is not the most convenient valve receiver to work on –especially when doing a full restoration, as there are so many things that need atten­tion. Apart from being a very small radio (hence the name, “Mickey Mouse”) there is nothing really outstanding about this particular model at all. The court wrangle with Disney over the unauthorised use of the name is what makes the set collectible. As far as the receiver is concerned – it’s only average! After doing everything that needed doing, it was tryout time. The set burst into life and there was that feeling of relief knowing that all the checking, replacing, and repairing had finally produced a positive result. But that feeling of relief was short-lived because, 30 seconds later, the sound had faded to nothing. So many restorations have some strange little quirk to them that hasn’t been encountered before. In this case, none of the parts overheated and there were no crackles or hum. In fact, there was nothing obvious at all – just a volume fade off to nothing. Although the valves tested OK, I have learnt not to rely completely on any valve simply on the basis of This close-up shows the dial and speaker opening. Having the dial in the centre of the loudspeaker makes a new grille cloth rather difficult to fit. Note the hole in the dial (at 12 o’clock) where the dial lamp has “burnt” through the celluloid. The Astor “Mickey” required several component replacements before it worked satisfactorily. Some of the old resistors had tripled in value. an emission test. This test does not check a valve for all working functions and other faults can be overlooked. A replacement 6A8 sorted out this particular problem but it took a while to locate. (Never overlook the limitations of an emission tester. It does just that; it tests a valve’s emission – nothing more. Since the emission eventually fails in any valve, it is a most logical test to make. But that’s to say that this is the only manner in which a valve can fail; there are a whole range of possibili­ties other than that of emission failure. Ed.) Noel was pleased that the tattered grille cloth was re­placed with a piece of original material obtained from an old console cabinet. There were a couple of good corners in this piece of cloth and having the right fabric in a collectible receiver such as a “Mickey Mouse” really sets it apart. From a performance point of view, the 5-valve “Mickey Mouse” was only average and any of Astor’s post-war 5-valve receivers would outperform it by a fair margin. One man & a baby The second set, the model GR Baby Astor was next and as this little receiver was in “working order” the job was November 1996  89 ing the grid bias on the variable Mu control grid of the 6G8G. So the 6G8G performs quite a few functions. Despite the clever circuit, the Baby Astor needs a good aerial if it is to give any worthwhile performance. Even then it is basically a local station receiver and if a station is too close it can give rise to a fair amount of interference. The little Astor is not very selective. Parts replacement The 1948 GR model Baby Astor was known to collectors as the Astor “Football”. It is a 3-valve reflexed TRF receiver of fairly limited performance. started knowing that it should be fairly straightforward. Even so, the Astor required quite a few hours of work to complete the restora­tion. On removing the chassis from the cabinet it was surprising to see that it was not a superhet but a simple 3-valve TRF (tuned radio frequency) receiver. The valves used are: 6G8G, 6V6GT and a 5Y3 rectifier. There is some trick circuitry in- volved in the little Astor, which is a reflexed TRF receiver. In a reflexed receiver, one particular valve performs a dual function, being used to amplify both radio frequency and audio frequency signals simultaneously. In the case of the Baby Astor, the 6G8G valve does these two operations and it detects the signal as well, using one of its twin diodes. The volume is controlled by vary- This photo shows the rear view of Baby Astor chassis. The three valves are: 6G8G, 6V6GT and 5Y3 rectifier. 90  Silicon Chip Replacements were confined to the usual components, mainly paper and electrolytic capacitors One thing that could not be replaced was the strong odour of mouse urine which was nearly overpowering when ever any sol­dering was being done. There are few things worse to work on than a well-saturated chassis. It really turns my stomach! There is a small bracket at the front of the chassis near the bottom of the loudspeaker. Mouse activity had been intense in this area and the bracket has obviously been a popular spot for the relieving of bladders. Unfortunately, the overflow had seeped down onto the edge of the speaker cone and had rotted out some of the cone. The speaker was repaired using Silastic silicone rubber compound. In fact, the whole outer rim of the cone was reinforced as the edge had become very thin and fragile. These speaker repairs proved entirely satisfactory and the receiver could not have sounded better had a new speaker been fitted. It is amazing the cone reconstructions that can be done with a bit of perseverance. The silicon rubber treatment may not look very dainty but it is usually effective and long lasting. The reason for all the previously mentioned mouse infesta­tion was the fact that the speaker grille cloth and, presumably, the cardboard baffle to which it was attached, was missing. This gave front door entry to any rodent wanting to call the little Astor home. A replacement grille cloth baffle was made from cardboard and, once again, covered with another corner of the tattered console grille cloth so as to look as though it was the original fabric. One big advantage of using old speaker grille fabrics is that such a replacement doesn’t look too new and is RESURRECTION RADIO VALVE EQUIPMENT SPECIALISTS AVAILABLE RADIO & AUDIO * Circuits * Valves * Parts * Books Fully restored radios for sale The Baby Astor’s control knobs are chassis mounted and the edges of the knobs protrude through the sides of the cabinet. WANTED for CASH * Valves and radios Send SSAE for Catalogue Visit our Showroom at 242 Chapel Street (PO Box 2029), PRAHAN, VIC 3181. Phone: (03) 510 4486; Fax (03) 529 5639 cracking the dial at the stud holes. As luck would have it, I had a broken cabinet with the dial and studs intact. Transferring these components to the good cabinet did much to improve the general appearance of the receiv­er. "Silastic” silicone rubber was used to reinforce the outer rim of the speaker cone. It mightn’t look too neat but it certainly makes an effective and longlasting repair. The dial cord was difficult to re-string. more in keeping with the odd scratch or chip in the cabinet. A restora­tion that incorporates a good secondhand grille cloth has a very genuine appearance to it. Dial problems The dial cord nearly always needs attention and this par­ticular one was a bit tedious to string. It is one of those with a hole through the tuner control shaft and as the cord unreels on one side of the hole, it winds up on the other. The right number of turns needs to be on the shaft and it must also be wound in the right direction before success is possible. Of course, the best thing to do with tricky dial cords is to sketch the layout before unstringing the cord. But when the cord has broken or, worse still, is missing, one has to start from scratch and work it out the hard way. There are some incredibly difficult dial cord setups in old receivers, with some taking several metres of cord to make the long journey around all the pulleys, etc. The dial was also a problem on the little Astor Football. Originally the acrylic dial strip was attached to the cabinet by a couple of split studs that just “thumb-push” into the cabinet. These had fallen out and had been replaced with countersunk screws which had been over-tightened, thus Alignment There is not much to align when tuning up a Baby Astor. You just adjust two trimmers on the tuning cap­acitor and that’s it! The performance is reasonable when the set is connected to a good antenna and sound quality is excellent for such a small receiver. But without an antenna it is a dismal thing, to say the least, although it would work OK in a capital city situation with an indoor aerial –and that’s what it was originally designed to do. Radio receivers such as the “Mickey Mouse” and the Astor “Baby” are very collectible items today and Noel has done well to pick up these radios. Although Noel has only four valve receivers at this stage, he is putting together an interesting collection and SC is off to a good start. November 1996  91 ASK SILICON CHIP Got a technical problem? Can’t understand a piece of jargon or some technical principle? Drop us a line and we’ll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097. Efficiency gain seems magical I was curious to read the article on the High Quality PA Loudspeakers in the September 1996 issue. In there you state that having the four speakers in a series/parallel connection could increase the system efficiency by 6dB (an increase of 3dB for every doubling of the number of speakers?). While I agree that the system overall maximum volume is increased by 6dB, how could efficiency increase by 6dB? Does this mean that if I had 4,000,000 of the drivers in a series/parallel arrangement I would have a sound pressure level of 153dB/1W/1m? (P. K., Buranda, Qld). • In mathematical terms and using Thiele/Small parameters, efficiency is related to the loudspeaker free air resonance cubed (fs)3 multiplied by Vas and all divided by Qes. With two loud­speakers connected in series, fs and Qes remains the same while Vas is doubled. Thus, the efficiency is doubled. However, the impedance is also doubled and the current is halved, so the sound pressure level is the same Stereo simulator can’t be varied I have been having problems with the Stereo Simulator from your June 1996 issue. Also, a friend who has built this unit has had the same problem. On both units, the setting of the DIP switches makes no difference to the stereo effect. The notches in the left and right channels stay at 150Hz and are not affected by the DIP switch settings. I believe the fault is in the oscillator chip IC3. I have changed IC3, IC4, IC5 and IC6. (L. E., Deacon, Qld). • We are not exactly sure what you mean by “the notches in the left and right channels stay at 92  Silicon Chip as for a single driver. In other words, while we are obtaining the same sound pressure level compared to a single driver we are only providing half the power. This is equivalent to a 3dB improvement. When another series pair of speakers is connected in paral­lel with the first series pair, the total impedance is now the same as a single driver. For a 2.83V supply, the power delivered to the four speakers is the same as for a single unit but the sound pressure level rises by 6dB. This is only true for a small number of drivers when the total radiation mass (air moved) is much less than the total mass of the loudspeaker cone and coil system. As the number of drivers is increased, the efficiency gains are reduced. The law of dimin­ ishing returns applies here just as everywhere else. Using multiple train detectors I was interested in the notes on the train detector pub­ lished in the September 1995 issue. In anticipation of building my railway, I have made 150Hz”. Possibly you mean that the right channel has a notch at 150Hz while the left channel passes this frequency with the delay being 3ms. The problem with not being able to adjust the stereo effect is possibly with IC5 and the delivery of codes to IC2. Check that the E, F and G inputs at pins 3, 4 and 5 change from +5V to 0V when switches DIP1, DIP2 and DIP3 are switched from open to closed. If this functions correctly, try changing the .012µF ca­pacitor at pin 6 of IC2 to .047µF. This will extend the delay for the DATA signal to give time for the clock signal (SCK) to trig­ ger on the previous DATA signal before it changes. up four detector modules and have con­nected them to my signal lights circuits which, while on the bench, work perfectly. I then made up a track to test the modules under actual working conditions but came to a sudden stop! I had not realised that by having a circle of track split up into four sectors and using only one controller for the computer circuits, all the train detectors will be interconnected by the fact of connecting to the common rail and the train controller. I would like to connect the track as in Fig.1 on page 62 of the September 1995 issue and still allow for reversing. Would this method of wiring cause some reaction between each detector board and, if so, how can I solve the problem? Each signal board on my circuit controls one section of isolated track. Each signal board is connected to their respective lights and isolated track. My train controller is from the April/ May 1988 issues of SILICON CHIP. (R. H., Bowral, NSW) • As you have stated, with a circle of track split into four sections and using only one controller, all the train detectors will effectively be interconnected by virtue of the common rail. However, each section of track is only fed via the diodes in its respective train detector board. The system you are proposing will work quite happily for forward and reverse operation. Power supply confusion I am writing in reference to the circuit for the 100Hz Tone Burst Generator published in the “Circuit Notebook” pages of the August edition of SILICON CHIP. After reading the article I was a little bewildered by the description for the power supply. The article states: “Power is derived from a 15V transformer. D1-D4 rectify the AC, while a 7815 regulator provides the circuit with a 15V supply”. This doesn’t seem to work, for the following reason: 15V is being supplied to the rectifier circuit D1-D4. Assuming a 600mV drop for each diode, that would leave 13.8V peak, less if you take the filtered voltage, available to the input of the regula­tor. The 7815 regulators have a dropout voltage of 2V and a minimum input voltage to maintain line regulation of 17.7V (National Semiconductor Voltage Regulator Handbook). What it boils down to is you can’t get something for nothing. 13.8V in (or less) and 15V out? Am I missing something or what? (C. B., Sale, Vic). • The circuit description of the power supply is correct. The 15V transformer delivers a sine wave which will have a peak-to-peak voltage of 42.4V or 21.2V peak. After allowing for the voltage drop in the diodes, this will produce a filtered DC voltage of about +20V. This is then regulated by the 7815 to produce 15V DC. Making safe scope connections I have a technical question. Picture the diagram of a TV or computer power supply. The mains is rectified and fed to a switchmode supply. All components here are “hot” and a scope probe cannot be grounded (to the non-isolated ground) in this section to carry out measurements. After the switching transformer, isolation has been achieved and it is safe to connect a scope and ground the probe (to the isolated ground). If I now connect the TV (or computer) power supply via a mains isolation transformer, is the “hot” section now safe to ground probes and hence carry out measure­ments? I believe this assumption to be correct but have not had the courage to try it. (I. B., Yall­ambie, Vic). • As you have surmised, the only safe way to connect a scope to the hot side of the power supply is to use an isolating trans­former. This should have a rating much larger than that of the set. Typically, for the power to work properly, the isolating transformer should have a rating of 500VA or more. Battery charger voltage shortfall I have built your 10A Battery Charg­ er as described in the June 1996 issue and have encountered lesser voltages than those in your circuit. It was Unstable amplifier draws lots of current What causes a Mosfet audio power amplifier to become un­stable when driving a very low impedance load, say one or two ohms? My suspicion is that the power supply is being dragged down, unable to supply enough current. If this is the case, would upgrading the pow­er supply enable operation into low impedances? With reference to the amplifier featured in the April 1996 issue, is it possible to increase the power output capability to around 400W/4Ω by simply adding more output devices and providing a suitable power supply? Or would it require major changes to the preamp/driver stages? I have seen commercial units capable of this sort of power output, with something like 14 output devices (Mosfets in this case) per channel but they are expensive and I am sure the cost to build a similar mainly when I tried 24V capability that I considered the voltage not high enough and looked further. I cannot achieve a final output voltage of 14.4V (on 12V range) or 28.8V (on 24V range) on the battery terminals even though the charger goes into trickle charge mode. I noticed in the article that the voltage to pin 2 of IC2 should be 1V, 2V & 4V respective­ly, for 6V, 12V & 24V batteries. I have tried to troubleshoot the circuit by removing the positive connection of BR1, removing IC2 and connecting a variable power supply to the battery connections to simulate the battery voltages but I cannot find a reason for the lesser voltage on pin 2 of IC2. In this part of the circuit, the voltage I am obtaining is +14.96V on pin 4 of IC1, +13.6V on pins 7, 1 & 8, when they are on, then +0.96V, +1.92V & +3.63V respectively on the voltage divid­er leg to pin 2 of IC2. Equating this to the voltage divider circuit to pin 1 of IC2 gives outputs of +6.86V, +13.7V & +26V. I have tried replacing IC1 and disconnecting the base connec­tions to Q5, Q6 & Q7. I also rechecked the resistors, PA stage would be much less, given that there didn’t seem to be much difference in the rest of the circuitry. Do you have any plans to publish a high power design of this nature? (S. W., Hamilton, NZ). • While it is possible that running a low impedance load on an amplifier could load its power supply and thereby cause low-frequency motorboating it is more likely that your Mosfet ampli­fier is oscillating at an extremely high frequency, possibly at 100MHz or more, and this could happen under certain load condi­tions. Unless you check this out with a wide bandwidth scope, there is little way of knowing. It is possible to upgrade the 175W plastic power amplifier to 400W but we have not done any work along these lines. You would need to run eight power transistors in the output and use supply rails of around ±75V. We published a 350W Mosfet design in the August 1996 issue. Q1 and associated D1 and D7 but I cannot get the voltage correct. (G. H., Panania, NSW). • The 330Ω resistor connected to the 4.7kΩ resistor and thence to pin 2 of IC2 can be increased to give the required output voltage. We suggest changing the 330Ω resistor to 390Ω or substi­tuting a 500Ω trimpot instead. Washing machine causes EMI I own a Fisher & Paykel washing machine that produces interference to the TV/radio and my 27MHz 2-way. The interference is to the extent that the 2-way cannot be used, the radio becomes so noisy that the program cannot be heard and the TV screen is covered in broken lines diagonally across the screen. I contacted Fisher & Paykel and they said it complies with the Australian Standards and the interference is due to the computer-controlled DC motor and associated systems. I contacted the Spectrum Management Agency who said physical screening, like mesh or aluminium foil, around the computer inside could solve the November 1996  93 Getting more bass boost I have recently completed the 50W Stereo Amplifier and preamp, and it works very well. Having stated it works very well, I would like greater or more bass boost. What components should I change to increase bass response? Also, if I wished to tape record, say, “phono”, it would seem that “phone” to “tape out” bypass the preamp and amplifier. Can you help please? (M. D., Beechboro, WA). • You can increase the available bass boost and cut at very low frequencies by reducing the 22kΩ resistors at each end of the bass pot (VR2) to 10kΩ. However, doing this is also likely to increase the possibility of amplifier overload on problem but this could then make the warranty void. Even when the washing machine is switched off at the ma­chine, there is still interference but not so severe, so the machine has to be switched off also at the power point. I have contacted other people via my CB radio that have the same problem so it is generalised and I was disappointed with the reply from Fisher & Paykel who made it sound as if mine was an isolated case. I thought that because it was a problem affecting the public in general you might be able to suggest some form of suppression that could be added externally to stop or at least reduce the interference. The suggestion by the SMA that none of the affected items be used when the washing machine is being used is unacceptable in 1996. (J. C., Waroona, WA). • We agree that the interference does seem rather severe. At this stage we can only suggest that you invest in a good line filter; it will need to have a rating of at least 10 amps and should be capable of coping with any surge current from the motor. Single channel operation for VHF receiver I am a fanatical reader of your magazine which I am getting from a relative of mine in Australia. As a student of 94  Silicon Chip bass-heavy program material. A better approach, which will increase the apparent bass boost and cut, is to reduce the .01µF capacitor shunting VR2. This will raise the turnover frequency of the bass control so that it has a more audible effect at higher bass frequencies. Try a capacitor value of .0068µF. We should point out that raising the bass turnover frequen­cy in this way will increase the interaction between the bass and treble controls. In virtually all amplifiers, when the tape monitor function is used, the amplifier circuitry is effectively out of the pic­ture. The only exception is if you are taping a record and then the RIAA preamplifier is in circuit. electron­ics, I am very interested in RF communications devices and I found an article from your magazine, the VHF Monitor Receiver from the March 1989 issue. I would be very obliged to you if you can send me any schematics to modify it in order to operate it as a single channel receiver. (L. P., Athens, Greece). • There is no need to make any modification to the circuit to operate it as a single channel. Just use a multi-turn trimpot to tune it to the wanted channel. Message recorder variations I bought the 16-Second Solid State Message Recorder at the time it was published in October 1993. Then, in October 1994, the Talking Headlight Reminder circuit was published. I kept the two magazines inside each other, with the idea of using the message recorder for the headlights at a later date. Now that I have examined the two, I notice that the updated ISD1416 has a record pin (27) and a playback pin (23). In the Message Recorder circuit, pin 27 gets connected to the positive rail to play back and to the negative rail to record. Could I still use the 555 of the later circuit to turn on a PNP transis­tor between pin 27 and the positive rail to make the older chip work for the headlights? Do you have any ideas about this please? (D. S., Caloundra, Qld). • The Solid State Message Recorder was published in July 1993, while a version from Dick Smith Electronics was published in the October 1993 issue. We are not sure which you are referring to. However, for the October 1993 version we recommend connecting the pin 27 input of the recorder to play back as selected by S1. The cathode (K) of LED1 should go to ground and the Vccd pin (pin 28) to a switched positive supply. This could be obtained directly from the pin 3 output of 555 timer IC1 for the Talking Headlight Reminder. If you are using the July 1993 recorder, the pin 3 output of the 555 can connect directly to the S1 play switch. TV pattern generator output is distorted I have a problem with the Colour TV Pattern Generator which I have put together in the correct format. The seven separate patterns, except the dot pattern, appear on the television but there is a lot of distortion on the screen and, therefore, the patterns don’t show up as being clear. I get a lot of vertical distorted lines with zig zags and it’s off frequency. Could you please advise me as to where the problem might be? (M. S., Chis­wick, NSW). • From your description of the symptoms it appears likely that the circuit is affected by hum breakthrough from the power supply. The most common cause of this is the failure to use a plugpack transformer of adequate rating. It needs to be rated at 500mA or more. A 300mA unit is inadequate. Notes & Errata 175W Power Amplifier, April 1996: to further increase the safety margin in the event of amplifier failure, we suggest that the fuses be changed to 3A instead of 5A when 8Ω loudspeakers are used. Photographic Timer, April 1995: the bridge rectifier specified as “WO4” does not have the same pinout as the DIP rectifier depicted on the PC overlay on page 27. If a WO4 type is used, two of the leads will have to be swapped and sleeved so it can be SC installed in the PC board. MARKET CENTRE Cash in your surplus gear. Advertise it here in Silicon Chip. FOR SALE SATELLITE DISHES: international reception of Intelsat, Panamsat, Gori­ zont,Rimsat. Warehouse Sale – 4.6m dish & pole $1499; LNB $50; Feed $75. All accessories available. Videosat, 2/28 Salisbury Rd, Hornsby. Phone (02) 482 3100 8.30-5.00 M-F. NEW MAGNETIC CARD READER-LOCK KIT: holds 8 cards (SC Jan 96 issue) $65.00. Tested + erased chips: 27C256 (200ns $1.20 ea; 150ns $1.50 ea). 68705P3 single chip micro $4.00 ea. Track two magnetic card readers $10.00 ea (NEW). Chips min qty 10 of $7.50 P+P. Michael (03) 9803 3535. PRINTED CIRCUIT BOARD ASSEMBLY: small to medium quantities, professionally assembled. Also, instrument case wiring and minor electronic repairs. At prices you can afford. Phone Joe on (02) 9826 0958. LARGE 7 SEGMENT DISPLAY: super bright red 660nm. 120mm x 90mm. Digit is 100mm high. Common anode. $14.50 each + P&P. Koph Trad­ing. Ph (02) 9831 8065. Fax (02) 9831 6103. SOUTHERN CROSS II: 8031 single board computer. ROM-based real-time debugger built in. Ideal software development tool. BASIC programming option. Compatible with Souther Cross I add-on boards - $165.25. Brochure available plus free catalog with over 100 kits. Mastercard / Visa / Bankcard. Ozitronics (03) 9434 3806. Email: ozitronics<at>c031.aone.net.au BOOKS: technical, all very good condition. RCA Tube Manual $3.00. RCA Transmitting Tubes $3.00. Radio Amateur’s Handbook 1941, 62, 78 $15 each. Radiotron Designer’s Handbook, Fourth Edition 1952 $30. Listener In Handbooks, No.9 Radio Sets & Circuits $20. Superhet Book No.6 $20. Constructor’s Guide No.14 $20. Radiotron­ ics CLASSIFIED ADVERTISING RATES Advertising rates for this page: Classified ads: $10.00 for up to 12 words plus 50 cents for each additional word. Display ads (casual rate): $25 per column centimetre (Max. 10cm). Closing date: five weeks prior to month of sale. To run your classified ad, print it clearly on a separate sheet of paper, fill out the form below & send it with your cheque or credit card details to: Silicon Chip Classifieds, PO Box 139, Collaroy, NSW 2097. Or fax the details to (02) 9979 6503. AWV 1947, 48, 49, 50, 51, 52, 53, 55, complete for each year $20/year. Arthur Sweatman, PO Box 95, Portarlington, Victoria 3223. Phone/fax (052) 59 2876. C COMPILERS: Dunfield compilers are now even better value. Everything you need to develop C and ASM software for 68HC08, 6809, 68HC11, 68HC16, 8051/52, 8080/85, 8086 or 8096: $140.00 each. Macro Cross Assemblers for these CPUs + 6800/01/03/05 and 6502: $140 for the set. Debug monitors: $70 for 6 CPUs. All compilers, XASMs and monitors: $400. 8051/52 or 80C320 simulator (fast): $70. NEW: Disassemblers for 12 CPUs only $75. Demo disk: FREE. All prices + $5 p&p. GRANTRONICS PTY LTD, PO Box 275, Wentworthville 2145. Ph/Fax (02) 9631 1236 or Internet: http://www.mpx.com.au/~lgrant EASY PIC’n Beginners Book to using MicroChip PIC chips $50, Basic Compiler to clone Basic Stamps into cheap PIC16C84’s $135, CCS C Compiler $145, heaps of other PIC stuff, Programmers from $20, Real Time Clock, A-D. Ring or fax for FREE promo disk. WEB search on Dontronics, PO Box 595, Tullamarine 3043. Phone 03 9338 6286. Fax 03 9338 2935. MICROCRAFT PRESENTS: Dunfield (DDS) products are now available exstock at a new low price; please ask for our catalogue. Micro C, the affordable “C” compiler for embedded applications. ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. ✂ Enclosed is my cheque/money order for $­__________ or please debit my RCS RADIO PTY LTD Signature­­­­­­­­­­­­__________________________ Card expiry date______/______ Name ______________________________________________________ Street ______________________________________________________ Suburb/town ___________________________ Postcode______________ RCS Radio Pty Ltd is the only company that manufactures and sells every PC board and front panel published in SILICON CHIP, ETI and EA. RCS Radio Pty Ltd, 651 Forest Rd, Bexley 2207. Phone (02) 587 3491 November 1996  95 Your next project will be easy, fast and satisfying with a development kit from MicroZed Computers Scott Edwards Electronics Microchip OPTO 22 NEW Micro Micro Engineering Labs (PICBASIC) MICROMINT PicStic DOMINO BLACKJACK PO Box 634, ARMIDALE 2350 (296 Cook’s Rd) Ph (067) 722 777 – may time out to Mobile 014 036 775 Fax (067) 728 987    (Credit Cards OK) http://www.microzed.com.au (we hope soon) Specialising in easy-to-get-going hard/software kits. Send 2 x 45c stamps for information package Stamp kits now have a compiler for 16C58 Advertising Index Altronics................................. 76-78 Av-Comm.....................................75 B & M Electronics........................58 Dick Smith Electronics........... 12-15 Earthquake Audio........................81 MEMORY * MEMORY * MEMORY SPECIAL! (Ex Tax) 1Mbx9 – 70ns $24 30-pin Simms Versions for 8051/52, 8086, 8096, 68HC08, 6809, 68HC11 or 68HC16 $139.95 each + $3 p&h • Now on special is the SDK, a package of ALL the DDS “C” compilers for $399 + $6 p&h • EMILY52 is a PC based 8051/52 high speed simulator $69.95 + $3 p&h • DDS demo disks $7 + $3 p&h • VHS VIDEO from the USA (PAL) “CNC X-Y-Z using car alter­nators” (uses car alternators as cheap power stepper motors!) $49.95 + $6 p&h (includes diagrams) • Device programming EPROMs/PALs etc from $1.50 • Fixed price electronic design and PCB layout • Credit cards accepted • All goods sent certified mail • Call Bob for more de­tails. MICRO­CRAFT, PO Box 514, Concord NSW 2137. Phone (02) 744 5440 or fax (02) 744 9280. 68HC705 Development System: Oztechnics, PO Box 38, Illawong NSW 2234. Phone (02) 9541 0310. Fax (02) 9541 0734. http://www.oz­technics.com.au/ EDUCATIONAL ELECTRONIC KITS: Best prices. Easy to build. Full details. Latest technology. LESSON PLANS FOR TEACHERS – see our web page. Send $2 stamp for catalog and price list to: DIY Electronics, 22 McGregor St, Num­urkah, Vic. 3636. Ph/fax (058) 62 1915. Or Email laurie.c<at>cnl.com. au and let us send details. Go WWW:http://www.cnl.com.au/~laurie.c or BBS (058) 62 3303. Download details free any­time. WE APLOGOGISE. By the time you read this the MicroZed Web page should be ready: http://www.microzed.com.au 96  Silicon Chip SIMMS (Parity/No Parity) 4Mb 30 PIN-70 $52 $54 4Mb 72 PIN-70 $56 $36 8Mb 72 PIN-70 $103 $64 16Mb 72 PIN-70 $198 $159 32Mb 72 PIN-70 $359 $324 EDO SIMMS 8Mb (1Mbx32) – 60ns $67 16Mb (2Mbx32) – 60ns $161 32Mb (4Mbx32) – 60ns $324 MAC MEMORY 8Mb P’BOOK 190 $147 8Mb DOCK DUO $249 16Mb P’BOOK $257 LASER PRINTER MEMORY 2Mb UPGRADE $150 COMPAQ 8Mb CONTURA AERO $147 All other models available $Call TOSHIBA 8Mb Portege/ Sat EDO $135 16Mb Portege/ Sat EDO $235 16Mb Tecra 500/610 Sat $298 All other models available $Call CACHE 256K PIPELINE BURST $25 256K 7200/8500 $93 VIDEO MEMORY 256K x 16 70ns (SOJ) $17 1Mb 7200/7500/9500 $83 SO DIMMS 8Mb/16Mb $93/202 Ex Tax Pricing – Delivery $8. Pricing as at 27/9/96. Phone for latest. Sales Tax 22%. Credit Cards Welcome. We Also Buy And Trade-In Memory. PELHAM Memory Pty Ltd Suite 6, 2 Hillcrest Rd, Ph: (02) 9980 6988 Pennant Hills, 2120. Fax: (02) 9980 6991 Email: pelham1<at>ozemail.com.au EDA Solutions.............................19 Harbuch Electronics....................81 Instant PCBs................................96 Jaycar ................................... 45-52 Kits-R-US.....................................80 Macservice....................................3 MicroZed Computers...................96 Oatley Electronics..........................7 RAIN BRAIN 8-STATION SPRINKLER KIT: Z8 smart temp sensor, LED display, RS232 to PC. Uses 1 to 8 DALLAS DS1820. Call Mantis Micro Products, 38 Garnet Street, Niddrie, 3042. P/F/A (03) 9337 1917. mantismp<at>c031.aone.net.au HOMEMADE GENERATORS: how to instructions. Eight pages free text and colour photos on the Internet at http:/ www.onekw.co.nz/ DATAMAN EPROM PROGRAMMERS: S4 World’s leading handheld programmer/emulator, on-screen editor, over 1500 device types including EPROMS/ EEPROM/Flash up to 8Mbits. Dataman-48 up to 48-pin DIL. DOS/Win software, free updates. Call or email for details. DIGITAL GRAPHICS P/L, PO Box 281, North Ryde 2113. (02) 9888 3105. dgriffo<at>ozemail.com.au http://www.ozemail.com.au/~dgriffo WANTED WANTED: any book on Tesla coil construction. Phone (069) 52 6396 to quote price and where to send money order. Pelham........................................96 RCS Radio ..................................95 Resurrection Radio......................91 Rod Irving Electronics .......... 33-37 Silicon Chip Bookshop...............IBC Silicon Chip Back Issues....... 28-29 Silicon Chip Wallchart..............OBC Zoom Magazine.........................IFC _________________________________ PC Boards Printed circuit boards for SILICON CHIP projects are made by: • RCS Radio Pty Ltd, 651 Forest Rd, Bexley, NSW 2207. Phone (02) 587 3491. • Marday Services, PO Box 19-189, Avondale, Auckland, NZ. Phone (09) 828 5730. electronic design, and applications. The sixth edition has been expanded to include chapters on surface mount technology, hardware & software design, semicustom electronics & data communications. 63 chapters, in hard cover at $120.00. Silicon Chip Bookshop Radio Frequency Transistors Newnes Guide to Satellite TV Installation, Recept­ion & Repair. By Derek J. Stephen­son. First published 1991, reprinted 1994 (3rd edition). This is a practical guide on the installation and servicing of satellite television equipment. The coverage of the subject is extensive, without excessive theory or mathematics. 371 pages, in hard cover at $55.95. Guide to TV & Video Technology By Eugene Trundle. First pub­lish-­ ed 1988. Second edition 1996. Eugene Trundle has written for many years in Television magazine and his latest book is right up date on TV and video technology. 382 pages, in paperback, at $39.95. Servicing Personal Computers By Michael Tooley. First published 1985. 4th edition 1994. Computers are prone to failure from a number of common causes & some that are not so common. This book sets out the principles & practice of computer servicing (including disc drives, printers & monitors), describes some of the latest software diagnostic routines & includes program listings. 387 pages in hard cover at $59.95. format and R-DAT. If you want to understand digital audio, you need this reference book. 305 pages, in paperback at $55.95. The Art of Linear Electronics By John Linsley Hood. Pub­lished 1993. This is a practical handbook from one of the world’s most prolific audio designers, with many of his designs having been published in English technical magazines over the years. A great many practical circuits are featured – a must for anyone inter­ested in audio design. 336 pages, in paperback at $49.95. Components, Circuits & Applica­ tions, by F. F. Mazda. Published 1990. Previously a neglected field, power electronics has come into its own, particularly in the areas of traction and electric vehicles. F. F. Mazda is an acknowledged authority on the subject and he writes mainly on the many uses of thyristors & Triacs in single and three phase circuits. 417 pages, in soft cover at $59.95. Digital Audio & Compact Disc Technology Electronics Engineer’s Reference Book Hard cove Produced by the Sony Service Centre (Europe). 3rd edition, published 1995. Prepared by Sony’s technical staff, this is the best book on compact disc technology that we have ever come across. It covers digital audio in depth, including PCM adapters, the Video8 PCM Power Electronics Handbook Your Name__________________________________________________ PLEASE PRINT Address____________________________________________________ _____________________________________Postcode_____________ Daytime Phone No.______________________Total Price $A _________ ❏ Cheque/Money Order r Edited by F. F. Mazda. version now available First published 1989. 6th edition. This just has to be the best refer­ ence book available for electronics engineers. Provides expert coverage of all aspects of electronics in five parts: techniques, physical phenomena, material & components, ❏ Bankcard ❏ Visa Card ❏ MasterCard Card No. Signature_________________________ Card expiry date_____/______ Return to: Silicon Chip Publications, PO Box 139, Collaroy NSW, Australia 2097. Or call (02) 9979 5644 & quote your credit card details; or fax to (02) 9979 6503. Principles & Practical Applications. By Norm Dye & Helge Granberg. Published 1993. This book strips away the mysteries of RF circuit design. Written by two Motorola engineers, it looks at RF transistor fundamentals before moving on to specific design examples; eg, amplifiers, oscillators and pulsed power systems. Also included are chapters on filtering, impedance matching & CAD. 235 pages, in hard cover at $85.00. Surface Mount Technology By Rudolph Strauss. First pub­ lished 1994. This book will provide informative reading for anyone considering the assembly of PC boards with surface mounted devices. Includes chapters on wave soldering, reflow­ soldering, component placement, cleaning & quality control. 361 pages, in hard cover at $99.00. Audio Electronics By John Linsley Hood. Pub­lished 1995. This book is for anyone involved in designing, adapting and using analog and digital audio equipment. Covers tape recording, tuners & radio receivers, preamplifiers, voltage amplifiers, power amplifiers, the compact disc & digital audio, test & measurement, loudspeaker crossover systems and power supplies. 351 pages, in soft cover at $52.95.   Title  Newnes Guide to Satellite TV  Guide to TV & Video Technology  Servicing Personal Computers  The Art Of Linear Electronics  Digital Audio & Compact Disc Technology  Power Electronics Handbook  Electronic Engineer's Reference Book  Radio Frequency Transistors  Surface Mount Technology  Audio Electronics Price $55.95 $39.95 $59.95 $49.95 $55.95 $59.95 $120.00 $85.00 $99.00 $52.95 Postage: add $5.00 per book. Orders over $100 are post free within Australia. NZ & PNG add $10.00 per book, elsewhere add $15 per book. TOTAL $A November 1996  97