STC A-141 is not a reflex set

I refer to page 96 of the Vintage Radio article in the June issue of SILICON CHIP. The circuit shown is described as a 4-valve reflex set, however there is definitely no reflexing. It is a straightforward superhet with no first audio amplifier. Obviously there is sufficient signal developed across the diode load PI to drive the speaker via V3. There were many sets manufactured with circuits similar to this one.

There were also many reflex sets whereby the detected audio was fed into the grid of the 6G8G IF amplifier via the first IF transformer coil and the audio extracted from resistor in the B+ feeding the 6G8G plate via the second IF. Thus the 6G8G becomes both an IF and an audio amplifier. Some of these sets did have problems with audio distortion etc.

Ted Baker, Bathurst, NSW.

You are right. The A-141 was not reflexed whereas the B-141 and C-141 were reflex sets. The person who sub-edited the story wrote the caption and made the mistake.

5-minute Araldite not recommended

I have just been reading the July 2001 issue and wish to thank you for such an enjoyable and well-produced magazine. There are several things I would like to comment on, if I may. First, some of the projects in your magazine feature PICs. Unless these projects are offered as kits, readers who do not possess a computer cannot build them. Indeed, there are one of two projects featured recently that we would like to build but cannot.

Secondly, in the Vintage Radio column, Mr Champness suggests using 5-minute Araldite in the repair of Bakelite cabinets. In the book "Electronic Classics" written by Andrew Emmerson, he writes: ". . . five minute epoxies produce quite brittle joints and should probably not be used where thermal expansion is likely (eg, in a valve radio cabinet, my addition). The 24-hour epoxies are far more flexible when cured."

On another subject, in the December 2000 issue, one of the projects featured was a LED Torch. We have built several of these from kits with varying degrees of success. As such, may we please offer the following construction hints:

(1). The first thing that should be soldered to the PC board should be the PC pins and the M3 washer adjacent to Q1. If left until later, heat from the soldering iron will "cook" Q1.

(2). The wire link from Q1 to the 330pF capacitor should be fitted next, otherwise it may prove impossible to fit if Q1’s body covers the mounting hole.

(3). The kit includes an M3 washer intended to be mounted adjacent to the 4.7μF capacitor We can’t really see the need for this. All it seems to do is slice through the insulation surrounding the leads to the LED thereby shorting them with results that can be imagined!

T. Robinson VK3DWZ, Woodend, Vic.

Programmed PICs for SILICON CHIP projects can be obtained from RCS Radio Pty Ltd, 41 Arlewis Street, Chester Hill, NSW 2162. Phone (02) 9738 0330.

www.cia.com.au

Component availability

I have taught private hobby electronics classes on and off over the last five years, mainly to senior primary and junior high school children. Almost always, each group of participants has started with an easy to make crystal radio set, as a primer, just to let them and their parents know what they’re getting themselves into. While this project has taken on different design formats over the years, the basic outline is still there.

I now find that some of Australia’s major electronics hobby suppliers have gone out of some of the parts required for even such a simple project. I haven’t been able to buy all of the items from one supplier for some time now and, lo and behold, when I open up some of the 2001 catalogs, I find that their entire range of ferrite products (ie, rods, balun cores and aerial coils) are missing. Jaycar seems to be the only retail supplier who, at this point in time, have enough "bits and pieces" available off the shelf.

Maybe some people will see this as a bit of a storm in a teacup but after being in the hobby for 32 years and having been in communications and business machines for 15 of those years, I think that there is a danger that many younger people who would otherwise made a start via simple projects may well be turned off by the latest batch of "computer gizmos" that have all but replaced them in some catalogs. If you think that crystal sets are out of date, do a web search (eg, www.midnightscience.com plus links, etc).

I think it’s a bit strange when major suppliers import kitsets from other countries, when there’s more than enough hardware and know-how staring them in the face down at their own warehouses.

A. Hellier, Warilla, NSW.

More comments on poor DVD quality

I was at a friend’s house recently to see his new you-beaut Fujitsu plasma display. The aspect ratio was 16:9, 1.5m wide. The unit was running in XGA mode and he tells me it has 1,000,000 * 1,000,000 pixels.

My friend played a couple of DVDs ("6 Days, 7 nights" and "As Good As It Gets"). In bright scenes the image was very good, however I must admit the picture quality of some scenes with this monitor’s resolution was dreadful.

Backgrounds were very poor indeed, especially night beach scenes and those where Harrison Ford is trying to deal with the wild pig in the lake scene of 6 Days. The only comparison I can draw is switching one’s computer monitor to 256 colours.

The general impression of the image was that it was continually digitally "raining" on the picture (this can sometimes be seen even on an analog TV set with an off-air signal). When I commented on the poor picture quality he said "Yes, that’s DVD for you. If you want to spend between US$8,000 and US$10,000 you can get a player which will rebuild the picture back close to what it was on the film".

Even my 15-year old son was appalled at the picture quality from these DVDs on this monitor. Our general consensus was that the lack of picture quality and digital rain was really annoying and at times unwatchable.

DVD is being touted by sales personnel as the "ant’s pants" video system. Obviously from what I saw recently, it’s not. Am I correct in stating that we have a current system which is better than VHS but still poor quality when compared with film? Do you know if there are plans in the offing to bring in a newer system which will replace the current MPEG compression system used on DVD?

I must admit that, after this experience, I’m totally disillusioned with DVD.

Brad Sheargold, Collaroy, NSW.

200 watt Mosfet amplifier

With regards to the 200W Mosfet audio power amplifier in the August 2001 issue I noticed that the distortion performance is not exceptional and wondered if there could be a reason. In the high current circuitry, the PC board layout violates a few rules of good practice as mentioned by British power amplifier expert Douglas Self and others. Where heavy currents, such as ripple currents to the power supply reservoir capacitors and the class B audio currents to the load are flowing through printed board tracks or indeed, ordinary wiring, it is not sufficient to assume that they have no resistance.

Of particular interest is what happens when a division of current occurs such as the power transformer centre tap carrying ripple current to one reservoir capacitor and then the other. As this lead is the 0V or power supply centre point we need a high degree of symmetry in the wiring for it to be a "quiet" 0 volts. This is sometimes called the "star point" where C2+, C3-, C4-, C5+, transformer centre tap, and the outgoing 0V lead should converge. Another place where heavy currents divide is between the load (speaker) and the two halves of the output devices.

The negative feedback should be taken from the true output track and not from the track to one or other of the output devices where track resistance is likely to impart an asymmetrical voltage drop to the signal on one half cycle. Likewise the Zobel network C12, R22 should be wired to the output terminal and returned to the 0V terminal rather than part way along the ground connection for C16. The star layout is relevant here also.

Keith Taylor, Hawthorn, SA.

This contributed design was presented as a reliable and affordable workhorse rather than as a very low distortion amp. If the amplifier had been one of our designs, the PC board would have more along the lines of what we used in the Plastic Power and Ultra-LD amplifier modules.

Electricians should document their installations

I want to comment on that bad old electricity argument! What I’ve witnessed in Mailbag to date is emotional. I ask this question: if laws are in place, will that stop unlicensed individuals undertaking electrical work? I think not. And if the unlicensed individual works for a large consumer organisation repairing VCRs, is the large organisation going to stop them doing so? I think not, because that organisation as part of their due diligence processes should be ensuring that the individual is undertaking the appropriate work practices to ensure they don’t injure themselves.

I don’t believe the electrocution problem will ever be solved, irrespective of licencing laws. I am a licenced electrician, though I don’t practice much these days and yes I’ve had severe electric shocks a number of times both because of my own stupidity and that of others.

My background is electrical and electronic controls, and I have undertaken plenty of domestic installations. I won’t deliberate on the problems I’ve encountered. What I will say is that if all the people who work on an electrical installation did their job properly there wouldn’t be an issue.

What I do have a problem with is when someone completes an electrical installation without leaving enough information, so the next individual who undertakes a modification to the installation doesn’t need to spend half of their time working out what has been done - and then, if need be, fix any bad workmanship prior to undertaking the job.

Completing an installation is only half the job. Documenting what has been undertaken is the other half. In the commercial area of the electrical industry, it is mandatory due to industry/client requirements that the installation is usually properly documented; ie, schematic diagrams, switchboard layouts/locations, properly labelled outlets, etc. And even then a lot of these are not 100% correct (I’ve had the displeasure of working on some of these installations). The installations that are properly documented occur because the client won’t pay the contractor until adequate documentation is provided.

On all the domestic installations I’ve worked on, I have not once witnessed documentation that adequately defines the installation. If any licenced electrician believes that a diagram drawn in indelible pen on the bakelite back board on the meter enclosure and labelled circuit breaker/fuses adequately define the installation they have to be kidding!

When I’ve completed an installation, I provide the client with documentation defining what I have done. As a minimum, there should be a plan of the installation defining: lighting, fixture and outlet locations; circuit ratings; and wiring paths. This should be a regulatory requirement for all new installations and local councils should be enforcing this as part of the final inspection processes.

Mal Land, via email.

We thoroughly agree with your comments on documentation.

More on the electric wiring debate

To comment on the electrical wiring debate, I need to throw a couple of items into the ring:

25 years ago, a "professional" wired an extension on my home. He tightened only two out of three screws on the back of each power point. The electrical noise on AM radio was horrendous and three power points burnt out in less than six months. The Licensing Authority told me that they could prosecute me for removing a power point from the wall but would take no action against the electrician as he had a licence!

In the ACT, a plumber can get a "Restricted Licence" on proving competence! The plumber can then connect new Hot Water Services right back to the main distribution board. With this licence, however, a plumber cannot change a power point at his own home.

Brian Wilson, Curtin, ACT.

Not all technicians deal with low current

I have been reading with much interest all the letters relating to the debate about non-qualified people performing their own wiring and the view that technicians, despite their qualifications, should not be licenced as electricians. What has really egged me however, is the ‘sparkies’ stereotypical portrayal of techs. Mr Raffaelli’s parting comment in his letter in the September 2001 issue was the straw that broke the camel’s back and prompted me to write this letter.

Not all techs are involved in small signal work or deal with less than 0.5A. In my 13-year career as a radio/electronics technician, I have worked on radar systems that are fed with 415V 3-phase and generate in the final stage 60kV at 8A feeding a magnetron that develops 2.4MW of microwave energy! Does that require a different mindset? Or how about a 10kW HF transmitter that has a final plate current of 80A?

Matthew R Clarke, Darwin, NT.

More on the widescreen "scam"

Simon Kareh has raised some very important points in the September 2001 Mailbag. I’m one of the disillusioned DVD buyers as well – the Claytons widescreen, or ‘scam-screen’, I call it. You get it on TV (FTA) all the time. The stations can’t decide how much to crop, so they adjust it on the run, especially at night when the Tea-Lady is in charge. I’ve seen the ABC do it mid-interview and crop off heads, etc. I believe the whole thing is a conspiracy to save bandwidth.

The FTA stations could send more data in their wretched MPEG format between transmitters if 44% of the lines aren’t coded with anything but black? The same on the DVD discs. It is scam-screen, saving heaps of data space on the DVD. Who would buy a new 16:9 TV? Not me, so I can have my 281 lines blown up on a ‘widescreen’ set.

I have owned a DVD player for two years and guess how many DVDs I own? Not one, because the format was over-hyped and under-delivered. Where are the camera-angles promised, the switchable widescreen/pan and scan and the fast access promised? Most new movies won’t let you skip tracks until their ‘startup program’ is finished. I still hire VHS over DVD – the sound quality is as good (good quality Pro-Logic off a hifi soundtrack is great) and my 71cm set doesn’t look like two 34cm sets side by side.

I don’t even use the DVD player for CDs any more. I got an old CD player and repaired it. It works more efficiently – no time spent deciding whether a DVD or CD is inside it. It’s just another attempt at consumer manipulation for profit, not improvement. We’re losing analog TV, we lost the best analog mobile network in the world and what’s next? Amateur bandwidth, AM radio, UHF CB?

John Richardson, via email.