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SERVICEMAN'S LOG Always look on the grim side That heading describes the pessimistic service­ man. When he encounters a fault which looks easy, he automatically assumes it’s going to be hard. And when he encounters one that looks hard, he is quite certain it’s going to be hard. Of course, he’s often right – but not always. My first story this month concerns an HMV colour set, model B4803A, the “48” signifying 48cm and the “A” an Australian ver­sion. But more exactly, the chassis is actually made by JVC. This model has been around for about 15 years and I am fairly familiar with it. So, when the lady owner rang to say she had a problem, I assumed that it would be something I could handle without too much trouble. I asked her in what way the set was misbehaving and she replied that while there was a watchable picture on the screen, it was, in her words, “very red”. I pondered on this briefly, considered several possibili­ ties without reaching any conclusion, then simply advised her to bring the set in. Even then I didn’t anticipate anything 58  Silicon Chip unduly difficult. But then, one never does. Anyway, the set was duly delivered and I put it up on the bench and switched it on. The result was more or less as the customer had described it; the picture was complete and it was only the colour that was wrong. But it wasn’t red, as she had thought. It was magenta, a colour which is often mistaken for red, the difference being rather subtle. But it is an important difference, because it immediately pinpointed the real nature of the fault – loss of green, leaving red and blue which mix to make magenta. Well that seemed to simplify the situation; all I had to do was find out why there was no green. And, while there could be several reasons, failures of this kind are not normally difficult to track down. High voltage My first check was the voltage on the collector of the green drive transistor Fig.1: this diagram shows the colour decoder IC (IC302) and the neck-board circuitry for the HMV B4803. The picture tube driver transistors (X101-X103) are to the right, with the green driver transistor (X103) at the bottom. Pin 10 of IC302 connects to pin 7 of IC301 (not shown) via two resistors. (X103), which drives the picture tube green cathode. This normally sits at around 145V, with roughly similar values on the red and blue drives. However, this one measured around 180V which is the supply rail voltage, meaning that this transistor was not drawing any current. As well as suggesting a fault in the drive system generally, this also cleared the picture tube of suspicion. Further checking revealed that the voltage on the base of X103, normally around 7.4V, was only a fraction of this. And this in turn suggested two possibilities, both of which I had experi­ enced previously: (1) a fault in the drive transistor itself (they can develop some very funny faults); or (2) a more subtle fault around the colour matrix chip, IC302 (TA7622­AP), which provides the base voltages for the three driver transistors. It was toss up but the driver transistor is quite easy to change and I had one on hand, so I tried that first. But all that did was clear the transistor; replacing it made no difference. My next step was to take a look at the circuitry around IC302. The three pins involved are pin 2 (red), pin 4 (green) and pin 6 (blue). But there is a nasty trap here for unsuspecting players; not shown on the circuit is a modification consisting of three clamping diodes, one for each pin. These are designated on the board as D403, D404 and D405. In each case, the anode goes to the pin and the cathode to the 12V rail. These diodes have a nasty habit of going leaky. And when one does, it can produce symptoms very similar to these. Again it was a relatively simple job to clarify the point. I pulled the suspect diode out and, rather than waste time testing it (such tests are not always conclusive anyway), simply fitted a new one. But again, I drew a blank; the problem was still there. Which didn’t leave much, except the IC. I went over the circuit, seeking inspiration as to any other likely cause but without success; it just had to be the IC. Good news & bad Fortunately, I had this particular IC in stock and, with only 16 pins involved, it was a simple job to fit a new one. And I confidently expected that this would finally cure the fault. How naive can one be? All I had done was create a good news/bad news situation. The good news was that I had cured the original fault. There was now normal voltage on pin 4 of IC302 (and on the base of transis­tor X103) and all signs of the magenta cast had vanished. The bad news was that I now had a monochrome picture – there was no colour. By very carefully adjusting the fine tuning control, I eventually brought up some colour but it was still a long way from being right. There were several things wrong with the picture, some of them hard to describe. For example, there were patches where there was no colour, or where one particular colour was absent, to nominate a couple of minor faults. And I classified those faults as minor because the major one was a real beauty; the colour pattern was displaced by about 30mm to the right of the monochrome image to which it belonged. It produced a weird effect. The offair picture I was using happened to be coverage of a one-day cricket match, in which the fielding side was wearing bright yellow uniforms. Imagine, if you can, a fieldsman, portrayed in monochrome, chasing a ball across the screen from right to left, with the yellow of his uniform running several steps behind in what looked like a vain attempt to catch up. And then, when he turned and ran the other way, it looked as though he was trying to catch the colour! To the casual observer, it would probably have looked out­rageously funny. To me, faced with the task of May 1994  59 Fig.2: the power supply circuitry for the National TC-2658 colour TV set. The mains power enters on the left (blue & brown), while the bridge rectifier (D833-D836) is in the centre of the diagram. To the right of the bridge rectifier is switching transformer T801, while IC801 is at extreme right. Test point TPE1 is below IC801 & should normally measure 113V. finding out what was causing it, the humour of the display was somehow lost. More to the point, I didn’t have a clue as to where to even start looking for a fault like this. I had never seen, or even heard of, anything like it before. To add to my confusion, there was the question as to wheth­er there had been two faults in the set when it came to me: (1) the obvious loss of green; and (2) this “new” fault. It was quite possible that the second fault had originally been masked by the obvious loss of green but, to be truthful, I hadn’t taken all that much notice of the picture’s finer points. I had simply diagnosed loss of green and gone on from there. Alternatively, was there only one fault originally, meaning that I had created the second fault in curing the first one? It was all very disconcerting. Anyway, for want of any better ideas, I went around IC302 with the meter, checking the voltage on each pin. Everything tallied very closely with the circuit values until I came to pin 10. This is shown on the circuit as measuring a mere .08V but the meter was reading somewhere around 5V plus. I didn’t note it precisely; just 60  Silicon Chip that it was grossly wrong. Could the replacement for IC302 be faulty? It was hardly likely, seeing it was a brand new unit. But stranger things have happened and, as I had a second one on hand, I decided to make certain. So IC302 was changed for a second time. Result – exactly the same as before. That clinched it; it obviously wasn’t IC302. Next, I began tracing the circuit from pin 10 and, after running up a couple of blind alleys, I came to pin 7 of IC301, the chroma IC. This is marked with a similar value, in this case .09V, but the actual voltage was grossly high here too, being similar to that on pin 10 on IC302. I checked the circuit carefully for any other likely source of the spurious voltage and the only other possibility seemed to be diode D201, which might be leaky. To make sure, I disconnected it but that made no difference. So as far as I could see, IC301 was about the only possible place, apart from IC302 itself, from which the spurious voltage could originate. And IC302 had been replaced twice. The next logical step was to change IC301. The only snag was that I didn’t have one in stock and so one had to be ordered. I also ordered another IC302 while I was about it. In the back of my mind was the thought that a fault in IC301 might have damaged IC302, so it was best to be on the safe side. The two ICs arrived a couple of days later and, full of confidence, I lost no time in replacing IC301. It came as a nasty shock when this had no effect; the symptoms remained as weird as ever and the same spurious voltage was present. When I’d regained my composure, I did something which, in hindsight, I realised I should have done much earlier; I separated the two pins from each other. And so, at long last the truth was revealed; pin 7 of IC301 reverted to normal, while pin 10 of IC302 retained the spurious 5V. Initially, considering that IC302 had already been changed twice, I was loath to accept that the fault was actually in this IC. Instead, I tried to think of some external error which would cause it to produce this voltage. But I drew a mental blank; I could think of nothing that would do this. So there was only one thing left; change IC302 for the third time. I couldn’t believe that this was the answer but I didn’t know what I would do if it wasn’t. But it was the answer; the new IC cured the fault complete­ly. And that, from a practical point of view, was the end of the story. The set was returned to the owner and everyone was happy. Well, I was happy the problems had been solved but less happy and very puzzled about the IC situation. Statistically, ICs are very reliable and I cannot recall the last time that a new IC proved faulty. As for two new ICs being faulty – well, that would suggest lottery odds. But there was the evidence on the workbench. Granted, they had been in stock for a couple of years but that is hardly relevant. The only other point of note is that they both carried the same batch markings and, not surprisingly, these differed from those on the one I had just bought. So, if it was a batch problem, how many other unfortunate servicemen had been driven half way up the wall, as I had been? Strange symptoms My next story is not an especially profound one but is of interest because of an unusual fault in a particular component. But the fault was not only unusual; it also created some very strange symptoms. On the other hand, no great detective work was needed to track it down. In fact, this was one of those rare occasions when a job which looked as though it was going to be hard turned out to quite simple, rather than the other way round. The set was a National colour TV set, model TC-2658, which is fitted with an M14 chassis. This chassis, with minor varia­tions, has been used in a number of National models and I have dealt with it several times in the past. The customer’s complaint was simply that the set had failed completely and, when I put it up on the bench and turned it on, this appeared to be true enough, at least from his point view; there was no picture and no sound. But there were some signs of life. For starters, the power supply was giving forth a high pitched squeal of distress; the kind of sound usually associated with a gross overload. And this, initially, was what I suspected was happening. My first check was to measure the HT rail voltage, which is most conveniently done at test point TPE1 in the power supply section. The normal value at this point is 113V but, in this case, it was reading 163V. Apparently, the power supply was underloaded rather than overloaded and the sounds of distress were, somehow, due to the excessive voltage it was generating. My first reaction to the excessive voltage was to assume that some part of the circuit – most probably the horizontal output stage – was not drawing current. And, in turn, I suspected that the horizontal output transistor, Q501, might have gone open circuit. So this was pulled out and checked. No joy. It checked out perfectly and there was certainly no sign of an open circuit. So what now? As I have pointed out before when discussing this chassis, it is fitted with an elaborate protection circuit. This is designed to detect over-current and over-voltage situa­ tions in various parts of the circuit and to shut the set down to avoid more serious damage if a fault occurs. In this case, it was obvious that the excessive voltage had caused the protection circuit to shut the set down. Subscribe now to the largest faults & remedies library in Australia ✱ ✱ 1994 manuals are now available. Our database is regularly updated with information supplied by technicians such as yourself. ✱ Exclusive backup service by qualified technicians. ✱ ✱ Over 10,000 faults and remedies on file with flow charts and diagrams. Covers Colour TVs and VCRs of all brands sold in Australia EFIL Phone or fax now for your FREE information package ELECTRONIC FAULT INFORMATION Reply Paid 4 P.O. Box 969 AIRLIE BEACH 4802 Ph 079 465690 Fax 079 467038 May 1994  61 SERVICEMAN'S LOG – CTD tant point was that the set was work­ ing, with no obvious faults or signs of distress. This threw suspicion right back to the power supply; the HT voltage was not wrong because of any lack of loading in the set, so it had to be the power supply itself that was at fault. The heart of the power supply is an STR50113-M regulator chip (IC801). These devices are no stranger to me; while their failure rate is probably not excessive, I’ve had enough trouble with them to put me on alert. Except that I had never seen a fault like this before – usually, they develop an internal short to chassis, which takes out a 4.7Ω safety resistor (R841). Nevertheless, I could find nothing else in the power supply circuit which could possibly account for the excessive voltage. So out came IC801 – it has only five pins – and in went a new one. I switched the set on again – still on the Variac – and found that the HT voltage was now low. I then wound the input up to 240V, still on the alert for any signs of distress, but there were none. More importantly, I now had a HT rail that was spot on 113V and the set was running perfectly. One of the easy ones From a practical point of view, the existence of the pro­ tection circuit means that this has to be disabled in order to track down the fault. Only then will the set try to function normally and display the fault in its true colours. Disabling this circuit is simple enough. Resistor R536 (100Ω) connects to the emitter of transistor Q503, which forms part of the protection circuit, and removing this is all that is necessary. Risk of damage But there is more to it than that. If the fault is a poten­tially destructive one, disabling the protection circuit could cause additional damage. And that was exactly the situation here; with the power supply generating 163V on the HT rail, the risk of damage if the set was allowed to function with this voltage was quite high. Fortunately, the solution is fairly simple. In such circum­stances, I feed the set from a Variac. This is set initially at a suitable low voltage and then gradually wound up while the 62  Silicon Chip HT rail is monitored. There’s just one catch here – in many cases, the set’s kick start circuit will not function if the voltage is wound up from zero; it needs to switched on at a reasonable input level. I normally set the Variac to deliver about 100V, with the set switched off, then switch the set on. This is usually high enough to provide the required kick start but still low enough to avoid trouble in the event of a destructive fault. So that was the setup. The set started readily enough with an input of 100V and I wound the voltage up gradually until I had about 113V on the HT rail (note: this occurred at something consider­ably less than 240V input). And all seemed well – there was no smoke, flames, smells or other nasty symptoms and, more importantly, the set was functioning more or less normally, with a quite watchable picture on the screen. But I say “more or less” advisedly, because the HT rail voltage was quite unstable and the picture’s behaviour was some­what erratic. But the impor- So, relatively speaking, this was one of the easy ones. But I thought that it was worth recounting for several of reasons. My first reason was to restate the protection circuit situation. One must learn to recognise those sets which incorporate these circuits and know how to safely disable them without causing further damage. My second reason was simply to report the unusual fault in the regulator IC. It is a fault that I had not encountered or heard of before. And finally, I wanted to remind readers of what I had to do to finish the job – restore the protection circuit. Unfortunate­ly, I have encountered a disturbing number of sets which have had various protection circuits or safety features modified or disa­bled for one reason or another and not restored to original condition when the job was finished. It is important to always restore any protection circuits when the job is done, both from a safety aspect and to prevent unnecessary damage to the set if a fault occurs. After all, that’s what the protection circuit is there for in SC the first place.