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You'll get a buzz out of this one Many years ago, as a factory hand I learned the difference between field faults and production line faults. Faults in the field tended to he simpler; at least one knew that the set had once worked correctly. But production line faults - well, it was anybody's guess as to what could have happened. Although this column will reach my readers a good deal later, I am writing it in the month of March. More precisely, it is March 17 and that is very appropriate for, as all good Wearers of the Green will know, March 17 is St Patrick's Day. And why is that so appropriate? Because this story deals with one of St Patrick's countrymen; one we all know and love to hate. Yes, that's right - Murphy! This is not my first story about Murphy's antics of course; there have been plenty of those. But this was a particularly vicious attack by that worthy which created a very weird and potentially frustrating situation. It all started when a dealer colleague for whom I do warranty repairs asked me to stop by his showroom and check a new set. In greater detail, it was a National Panasonic model TC 2969 which seemed to have excessive hum in the sound channel. And, as an afterthought, he added, "It'll probably only take you a few minutes to fix". He was probably being a mite sarcastic, but the remark had more sting in it than either of us realised at the time. The TC 2969 is a large screen model (68cm) and features a stereo sound system with external speaker outlets. It has been on the market for about two years now; time enough for most of the inevitable bugs to have surfaced and been duly swatted. In fact, it has been remarkably free of such beasties, so it came as 50 SILICON CHIP something of a surprise when a new set, just out of the carton, exhibited such an obvious fault. Still, these things do happen. So it was that I duly faced up to the offending set in the showroom. My colleague switched it on and I flicked it through several channels, expecting an obvious fault. Truth to tell, I was not immediately aware of any problem and it was only when I paid particular attention to the sound that I detected the background hum. But while it was more than there should have been in a set of this quality, it was not at . \ ~ 7)1~ .,111,,, . r: o d o I -- W~'( ~ M~-1'1 STUFF UP A, VJ01<.\(~ anything like the level my colleague had implied when he first called me. When I raised this point, my colleague looked puzzled but was adamant that the hum was "much worse than that" when he first switched the set on. So that was puzzle number one; the fact that the fault seemed to be variable. Or was it? In a showroom situation, where ambient sound can vary considerably, subjective assessment might be suspect. But for now, I had to assume the worst. Puzzle number two was something I couldn't quite put my finger on but the nature of the offending sound was worrying me. Before I could think much more about that, my colleague introduced a third factor - a second set with the same fault. It had been sold to a customer, delivered in something of a hurry late one afternoon and on the basis of a quick check, had seemed OK. However, the customer soon complained of excessive hum level and this had subsequently been confirmed by my colleague. So not only did we have a strange (variable?) fault but we had it in two sets. And this changed the scene completely. Instead of a oneoff component fault, I was now thinking in terms of a design or production line fault. But why should such a fault rear its ugly head at this time, after nearly two years? Of more immediate concern was what to do about the customer's set. My colleague had left the set with him on the basis that I could fix it, as he put it, "in a couple of minutes". But there was no way I was going to tackle a fault like that out of the workshop, with the real possibility that I would need help from the National service department. So my colleague decided to simply replace the customer's set. And this was another surprise; he had a third set on hand which was totally free of the problem. Anyway, the sets were duly swapped and the customer's set was delivered to my workshop. Frame buzz In the meantime in fact, as soon as I left the shop - I realised what was nagging me about the hum. Although my colleague had said "hum" - and I had accepted the term - it wasn't hum in the strict audio sense. Hum in an audio amplifier can be caused by induction into improperly shielded leads or by earth loops, etc. This hum will be at 50Hz and sounds low pitched and reasonably pure. Alternatively, it can be due to faulty power supply filtering in which case it will be at 100Hz and sounds higher pitched and rather rough. But it came to me in a flash that this was neither. It was low pitched - 50Hz - but harsh. That meant only one thing; it was frame buzz. I mentally kicked myself for being puzzled but frame buzz was the last thing I had expected to encounter in this set - even as a fault. A quick check on the customer's set confirmed both the nature of the buzz and that it was quite bad. I fished out the manual as a first step and in the process, came across a number of modification sheets which the firm had issued. And I must give National full marks on this score; they do make every effort to keep their servicemen up to date on any problems or modifications as they occur (unlike some other companies, which practise the mushroom treatment). Anyway, one of these mod. sheets caught my eye. It didn't nominate this particular set but I recognised the two boards involved; the " E" board and the "B" board. Both are used in this set and both needed modification, although no reasons were given. The "E" board mod. involved the audio output IC and on the "B" board, the IC for the earlier stages. And part of the mod. involv- ed fitting extra "earth" leads between sections. In the light of my problem, all this seemed to make sense. There wasn't a lot involved and I had the whole job done in about 20 minutes. But it didn't work; the buzz level was exactly the same as before. I was back to square one - and the realisation that I needed help. I rang one of my contacts in the National service department, outlined the nature of the problem, and described the changes I had made. His response to this latter point was not what I had expected. In mock sarcasm he told me to go back and take another look at the mod. sheet, because the mods. were not intended for that model set. In greater detail, he explained that they were intended for an M14 chassis whereas mine was an M15 chassis. I was well aware of the number of my chassis and said so, going on to point out that there was no chassis identification on the mod. sheet. Initially, he wouldn't buy this, insisting that there was a message at the top of the page clearly stating that these mods. were intended for an M14 chassis only. I eventually convinced him that there was no such message on my sheet. How it had been omitted, and from how many sheets, remains a mystery. But it was strike one to Murphy. The upshot of all this was (1) that I should remove the mods. as they had nothing to do with the problem; and (2) that he was well aware of the frame buzz problem and knew how to correct it. The cure was simple but the story behind it was quite complex. If I cared to listen he would fill me in. Always anxious to know the reason why, I said, "Go ahead" . Supply circuitry Fig.1 is the relevant portion of the circuit showing, in particular, the network providing the voltage regulating function for the switchmode power supply. This starts at IC801 at the bottom centre of the diagram. As shown, this IC consists of a transistor, a zener diode, and three resistors. It is connected to the 113V rail (pin 1) and a proporJUNE 1990 51 IL.!!£.J T801 ~ p For 21rnch L807-"- ET549K512A TSC937 ~ For 26/29rnch EXCELDR35C ET849K511A R813 330K Jp - & C849 0.01 500V D806 0-01 TVBESIC SOOY ® D827 IIA649RL Jl18 C818 680P 2KV I 86 8V~ C814 LBIO ,.&. TSC925-4 EXCELSA35B 5V 'I' TBC937~ EXCELDR35C IW [Hy] , r 0 1.0 1 8 ::==================-,---:----, [..806 & R837 I 113V B5 r 62 ft, I 64 C815 330P2KV L803 ,"TSC925-4~ EXCELSA35B ER 20V \ +113V E C842 E 22P 0850 aRZC10DK621 25V -~- [!ill R35C 825 I 7~P I ONI.Y FOR R8I9 S.8K RB40 180K Jt V 0815 El/2 r- R823 }';II 2SD965 0 0825 IIA4240 ------- R835 0-33 0808 1-5 2W :Z.'f-V 0806 2S0965R i I = -------------- --------l & CA-2969 - ___ , I R82Q 270 IC801 ~- s1854LBM-2: - - - __ .J 0802 25C1685/0R5 2SC331\A/OR5 Fig.1: the power supply circuit for the National TC 2969 TV receiver. IC801 is at bottom centre, with the optocoupler, Q803 and Q806 to its left. The additional diode, drawn in, is at the top right corner, near D827. tion of this voltage is fed to the transistor's base. The zener diode in the emitter circuit provides a voltage reference against which the base voltage is compared. The collector of this transistor (pin 2) is connected to the 18V rail (right of the diagram, half way up) via D823, R828 and the LED in opto-coupler D826 (ON3105R). The transistor in the opto-coupler is coupled to Q803, which is coupled to Q806, which is coupled to Q807. And Q807 is the " regulator" or "switching" transistor in the switchmode power supply. (Its collector connects to the OUT pin of the 52 SILICON CHIP switchmode transformer, T801). In normal operation, the IC801 LED will assume a brightness level determined by the actual voltage on the 113V rail. If this rises above 113V, the LED brightness will rise and its associated transistor will draw more current. This will increase the current through Q803 and in turn, the current through Q806. And this will pull down the base of Q807 and thus bring the 113V rail (and others) back to the correct value. (This network is also controlled by Q802 which, in turn, is controlled by the remote control receiver to turn the set on or off. But this a secondary function and is dormant at all other times). Well, if you've digested all that, let's go back to that 18V rail. As well as supplying IC801, this rail goes off up the diagram to perform another function. That other function is to power the audio output chip. And, as already mentioned, this set features outlets for external speakers. So what happens if John Customer fouls up the connections and puts a short across the speaker terminals? Well, for starters it takes out the audio chip, which is bad enough, but there's worse to come. When the audio chip fails, it shorts and takes out safety resistor R827 which shuts down the 18V rail. And that's when the fun starts. Since the 18V rail is involved in controlling Q807, loss of this voltage causes Q807 to turn hard on, with drastic consequences. The resultant over-voltage can take out transistors and ICs left, right, and centre. In a couple of cases, sets had been returned to the National service department which required extensive work to salvage them. Not all sets so abused were as seriously affected as this but this is what can happen. (Yes, the set has protective devices and circuits designed to prevent such mayhem but it appears that they do not always work, or work fast enough}. Back to the buzz And what has all this to do with the frame buzz problem? In case readers imagine that I've wandered off along some irrelevant sidetrack, let me assure you that such is not the case. Be patient and all will be made clear. Because of the problem just of the diagram, above Q802} and the idea was to connect this to the junction of D823 and R828 via a diode. Normally, this would have no effect because the diode would be reverse biased. But if the 18V rail failed, the diode would then be forward biased and so the 14V rail would now feed the LED in the optocoupler. It was not intended to keep the set working but it would maintain the opto-coupler circuit sufficiently to prevent Q807 from running amok. The scheme worked very well and had the advantage of being simple to implement; just one inexpensive diode tacked into the circuit. TETIA TV TIP Pye T29 chassis Symptom : Almost no vertical scan. Picture' reduced to a 2. 5cm band across the centre of the screen . Circuit diagram gives no useful voltages but those measured appear to be reasonable . Cure: 0504 (BC638) faulty. The transistor checks OK both in and out of circuit but will not amplify signals applied to its base. TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania 7015. Strike two outlined, someone hit on a clever circuit modification which would protect the rest of the circuit should the 18V rail be knocked out. I'm not sure whether this was instigated by the parent company in Japan or whether it was a local development but it was very simple. There is a 14V rail in the set (top Enter Murphy for strike two, in the form of a wiring hand who couldn't tell one end of a diode from the other and wired a number of sets with the diode reverse connected. And that was what created the frame buzz. The fault was not discovered until some sets had gone into the field and there was no easy TV TEST EQUIPMENT PCB POWER TRANSFO RMERS (AUSTRALIAN MADE) SHORTED TURNS TESTER Built in meter to check EHT transformers including split diode type , yokes and drive transformers. $78.00 + $3.00 p&p HI-VOLTAGE PROBE Built-in meter reads positive or negative 0-50kV. Can also be switched to check negative voltages. For checking EHT and Focus voltages as well as TVs and Microwaves. Hi-tension voltages. $89.00 + $5.00 p&p LOW-VOLT AGE PROBE Ideal tor checking microwave ovens and TVs. The ~ ranges are from Oto 5kV and from Oto 1 OkV. Double insulated for safety. $79.00 + $5.00 p&p D Proudly manufactured in Australia by HARBUCH ELECTRONIC S P J 90 George St. , HORNSBY, N.S.W. , 2077 Phone (02)476-5 854, Fax (02) 476-3231 td DEGAUSSING WAND Strong magnetic field , larger than usual coil with multicore centre. Double insulated for safety with momentary switch operation. 240VA/C 2.2 amps. As important as having a soldering iron! $75.00 + $10.00 p&p TUNER REPAIRS We repair most tuners on an exchange basis. From only $1 7 .00 + p&p Cheque, Money Order, Bankcard or MasterCard Y.V.TuNERs) 216 Canterbury Road, Revesby, NSW 2212 Phone (02) 77 4 1154 JUNE 1990 53 \\1~,l/ ,~ ~ ~\Ii~\ ~ ;? , A -=--•- [l, . ,:;? . iii)> ~ ~ -, ' way to identify them. Fortunately, there weren't many but of course, a couple had to land on yours truly's doorstep. Don't ask me to explain why the reversed diode did what it did. I was happy enough to confirm that this was the fault in these sets and to put things right. All of which carries a clear message; such modifications, without the benefit of printed board markings, are risky. Introduce something new to a production line hand and the original Murphy's Law prevails; if something can go wrong, it will go wrong. A nudder story And now, in lighter vein, here's a short story from my colleague J.L. in Tasmania. Remember the story in the March issue a bout a colleague who had to re-orient his thinking in order to service the local sausage machine? Well, this is along similar lines, only this time it is a transition from TV sets and video recorders to 54 SILICON CHIP the much more mundane subject of dairies, cows and the associated milking procedures. Here's how J.L. tells it. Most electronic servicemen are "townies" and spend their time repairing domestic or commercial equipment brought to them by other townies. It's not often that we need to put on gumboots and solve farmyard problems. This story is about one of those situations where you leave dropped tools just where they fall. I mean, ankle deep in ordure is no place to go fishing for a $2 screwdriver, is it? (Speak for yourself, J.L., I come from a long line of Scottish ancestry). Imagine, if you can, a modern dairy farm where all cows carry electronic identification tags and their milk is instantly analysed so that appropriate feed supplements can be delivered to the computer controlled manger. Imagine further an automated milking line where the farmer has only to attach the cups to begin the extraction process. After the udder is empty, the lack of milk triggers a removal process which releases the cups and pulls the assembly away so that the cow can back out of the stall, all without further attention by the farmer. And one last point before we get to the story proper. Cows don't let down their milk immediately. They need half a minute or so to settle down before the milk flows, so the cup removal system needs to be inhibited during this period. This is accomplished with a simple automatic timer. So now to our story. Strawberry's problem It's early one wet and windy morning. Strawberry enters her stall and contentedly munches hay while farmer Giles washes the udder and attaches the cups. He moves the lever to start the suction and the cup assembly falls off the udder and is pulled upwards by the retracting mechanism. Farmer Farmer Giles rang Arthur to ask him if he would have a look at the you-know-what milking machine. It would be next week before the manufacturer's serviceman could reach him and poor old Strawberry could explode before then! Arthur knew a little about milking machines but none of the details of this one. But he was prepared to try, so it was on with the gumboots and a screwdriver on a string around his neck. Float switch Giles doesn't get out of the way in time and is clobbered by all four cups in turn. He lets fly with some unparliamentary language (and we all know what that means, don't we), then tries again, with the same result. The cups are retracted with a fair amount of force, so farmer Giles is beginning to feel rather battered and in no mood to argue with the "blankety-blank" machinery. So poor old Strawberry doesn't get milked that morning. Cows are only contented in their own stall and won't co-operate if put in any other. Nor will they co-operate if milked out of turn, so Strawberry can't be brought back later and milked by hand. Enter Arthur, a qualified electronics technician. Arthur is also one of those people who are happiest with their hands in the soil. He has a small hobby farm within commuting distance of town and grows fruit and vegetables to help keep the family fed. The cup release mechanism is activated by a float switch in the collector bowl above each stall. When the milk runs out, the float drops and closes a switch which operates a valve to release the vacuum that holds the cups in place. At the same time, a catch is released so that a spring loaded cord can pull the cups off the udder and up to the parked position at about shoulder height. The inhibit function, at the beginning of the milking cycle, is initiated by a switch attached to the vacuum lever. After the lever is operated, a 555 timer begins a 60-second countdown, during which the release mechanism cannot function. Our gumbooted serviceman learned all this by studying one of the other properly functioning stalls. So it took him very little time to realise that the faulty stall was not producing the inhibit signal due to a l000µF capacitor on the circuit board with a broken pigtail. As simple as that! Arthur's reward So endeth the technical part of the story but the best is yet to come. Farmer Giles was delighted with the prompt and efficient service and insisted that Arthur submit his bill for the time he had spent on the job. In the meantime, would Arthur let him show his appreciation with a small gift? Which is how Arthur came to drive home with a bucket of fresh milk, a side of lamb neatly butchered and packed in a cardboard carton, and a promise of all the fresh cream his family could consume. And a week later there was a cheque for the cash part of the deal as well. Why don't I ever get customers like that? Dunno J.L., but everyone up here had a good laugh over that story. And it brought back memories. As a callow country youth, when times were tough, I once earned my bread and dripping milking cows - and not with the aid of any new £angled milking machines either; it was all very rough and ready. And I learned about cows; just how stubborn, contrary, and cantankerous they can be. Nothing in your story surprised me one little bit. Truly is it said, they are "cows" of animals. ~ Fitting a Fax Card to Your PC: ctd from page 21 least cost) to you. Letterheads, signatures, logos, etc can all be transmitted provided you have access to a scanner (although you could fax your signature, etc back to your PC) and Fax89 supports most popular graphics file formats. The fax card, however, cannot be a total replacement for the fax machine. If you don't j:i.ave access to a scanner, there's no way you can send brochures, promotional material or pages from a manual. If the bulk of your work is non PCbased, then the fax card is really not for you. Rather, the fax card should be seen as an adjunct to the fax machine. Perhaps the scenario will be for a cheap lower-featured fax machine to transmit faxes while the PC fax card receives all incoming faxes. Also, the very nature of a fax suggests round-the-clock access. Are you prepared to leave your PC running all day and all night? However, for the PC user who works from home or a small office and mostly communicates via PCbased documents, a fax card probably meets all the specifications while presenting a major cost saving over a normal fax machine. The price of our sample fax card, the Fax89, was $599 from Eskay Printernet, Suite 3, 124 Marsden Street, Parramatta, NSW 2124. Phone (02) 891 1282. ~ JUNE 1990 55