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SERVICEMAN'S LOG Two symptoms – one fault or two? I’ve got a really weird one this month – two quite different visual symptoms & two faulty parts mixed up in a crazy tug-o’-war. Less traumatic was the set that went green; it fooled the customer more than me. The weird story is about an AWA colour TV set, model 4303, using one of the “Q” series chassis. It would be about 10 years old and is one of several in a local motel. As with many other AWA chassis types, the “Q” series are actually made by Mitsu­bishi. I first heard of the problem when the motel proprietor rang me, identified the set, and explained that one of his guests had reported that the set had lost colour. When he later checked the report it was quite correct. There was no colour but, as he added, there was also a black line or strip about 50mm wide at the top of the picture. That should have alerted me – well, alerted me more than it did. But I did speculate as to whether I had two separate faults – which seemed most likely – or whether it was a single fault with a funny origin; and I didn’t mean funny ha-ha. As it transpired, there weren’t any laughs anywhere in the episode. I don’t know what the record is for frustration factor but, on a scale of 1-10, this must have been nudging the nine mark. Having thus set the scene, let’s get down to details. No ordinary fault The customer delivered the set to the workshop and I turned it on while he was there. And yes, his description was fairly accurate; there was no colour and there was a black band about 50mm wide at the top of the picture. But there was more to it than that and I quickly realised that this was no ordinary fault. For a start, it was obvious that the black band was not simply a result of reduced vertical scan, involving either a compressed or non-linear image. What image was there was normal and the black band was, as it were, overlaid on the image. In other words, the scan was normal, but there was some kind of spurious blanking problem. The other thing I noticed immediately was that the junction between the picture and the black band was not a straight line, as one would have expected. Rather, it was a “wavey” line, per­haps best described a rough, shallow sinewave of about 12 cycles. I also discovered that I was able to brute force the set into momentary bursts of colour by carefully fine tuning it, although there was no setting that would hold it. But having noted all this, I was no wiser as to whether it was one fault or two, although I tended to favour the two fault theory. In any case, I could only tackle one set of symptoms at a time, so I decided to tackle the blanking problem, mainly because I felt more confident about where to start. Unfortunately, there was one other trap waiting for me. I didn’t have a “Q” chassis circuit for this particular model, which uses a 90 degree picture tube. The closest I had was one for a 110 degree tube but, as far as I knew, Fig.1: the faulty section in the AWA 4303. Part of IC201 is shown on the left, with pin 9 in its bottom right hand corner. Diode D204 is below it & to the left, while diode D203 is to the right near transistor Q213. The burst gate transistor (Q601) is at extreme right. 56  Silicon Chip the sets were iden­ tical in all other respects. And I fell right into the trap. I spent a great deal of time trying to find my way around the chassis from this circuit and found that I was getting nowhere. I eventually realised that it was almost the same but not quite, Finally, a colleague came to the rescue with the correct circuit. Having cleared that hurdle, I started all over again. I was concentrating on the circuitry around IC201 and, in particular, around pin 9, which apparently feeds blanking pulses into the blanking section – see Fig.1. There are two diodes connected to this pin – D204 and D404. D204 is adjacent to this pin on the circuit, while D404 is some distance away down near the scan coil assembly, being associated with a small resistor network (R417, R418, R419 & R420). Also under suspicion were some electrolytic capacitors, including C414, C409 and C408, in the adjacent vertical output stage. I replaced these first, without any result, and also checked various resistors in this part of the circuit. They all measured spot on. That left the diodes still under suspicion. But first I decided it would be a good idea to do a voltage check around IC201. Fortunately, all the pin voltages are shown on the circuit and, with one exception, they all measured well within tolerance. As you may have guessed, the exception was pin 9. It is marked 6.1V but I measured only about 2V. So did we have a faulty IC? I had a spare on hand and it was not a big job to fit it. It made no difference but at least I had cleared it of suspicion, a point about which I was thankful later on. That left the diodes as the next prime suspects. I went to D204 first. The simplest and most reliable way to check it was to pull it out and fit a new one, which I did. Now, at the risk of seeming to state the obvious, there would appear to be only one of two possible results from such a move: either the fault would be cured and that would be the end of the exercise, or (2) it would make no difference and the diode would be cleared of suspicion. Surely, those are the rules? At least, that’s what I thought until I replaced diode D204. But no; this circuit had its own ideas. We now had complete picture cutoff; in other words, the situation was worse than before. My first reaction was to suspect that the replacement diode was either faulty or unsuitable. I didn’t have a direct replacement but had used a 1N914 small signal diode, which I felt was adequate. I tried another 1N914, then several other types, but always with the same result; total picture cutoff. I checked the origi­nal diode for leakage and although the indication was only slight, I felt sure it was leaky. Yet when I refitted it, I could at least get the original picture. To say that I was confused would be putting it mildly. It really threw me; what on earth was going on? Although I eventual­ly decided that the original diode was faulty and that the re­placement was OK, I was no closer to an explanation. About the only thing that was clear was that there was another fault some­where which still had to be found. Apparently, the original “two-faults” concept was valid but not in the manner I had envisaged. A real clue But speculation didn’t help in a practical sense and I was at something of a loss as to what to do next. In desperation, I went back to pin 9. And this provided the first real clue; the voltage here had now jumped from a too-low value of around 2V to a too-high value of about 8.4V. Well, I suppose that made sense in a way; excessive voltage into the blanking circuit would do just that – it would blank the picture. But where was this excessive voltage coming from? In order to follow the next steps, it is necessary to study this part of the circuit carefully. First, there are three resistors in ser­ ies, R209, R223 and R224 (in that order), from pin 9 to the 12V rail. Their job is to establish the 6.1V at pin 9 shown on the circuit. Also connected to pin 9 is R210, C217 and diode D204. And somewhere via that network a spurious voltage October 1994  57 was being intro­duced. All I had to do was find out how. The first thing I did was to disconnect R224 at the 12V rail, which should have removed all voltage from pin 9. But it didn’t; we still had the 8.4V. Next I disconnected R210. Well that achieved something; the pin 9 voltage dropped to zero. Thus inspired, I abandoned the circuit and began tracing the copper pattern from R210, checking with the meter probe as I went. Of course the pattern was far more complex in reality than it appears on the circuit and I ran up a lot of garden paths and encountered a lot of brick walls over the next 15 minutes or so. But suddenly I struck oil; a diode marked D203 (near Q231), the other side of which connected to the 12V rail. More importantly, its polarity was such that it was opposing the 12V. I had no idea what its real function was – and still haven’t – but it was obvious that, if it was leaky, it could be the culprit. So out it came. And was it leaky? A sieve is the only com­parison I can offer. So in went a new one and all our troubles were over. With the benefit of hindsight, it 58  Silicon Chip appears that I might have been better advised to stick with the circuit, because the of­fending diode is right alongside IC201, connecting to the 12V rail where this emerges from the 12V regulator transistor (Q231). But of course it was a lot further away on the board than it appears on the circuit. So that was the solution. But it had been a most frustrat­ing exercise. It is bad enough to have two components fail at the same time but they usually produce distinctive symptoms. In this case, not only were both failures in the same part of the circuit but, worse than that, they were actually opposing one another in the effect they had. Thus, while the leak in D203 was attempting the raise the voltage on pin 9, the leak in D204 was pulling it down – and succeeding rather too effectively. But this was the only reason the set was producing any image at all; as I found out when I replaced D204 and made matters worse. Which is all delightfully simple to explain when looking backwards; it only we could look forward as easily. And what about the colour failure? How can that be ex­plained? Again, once the fault was tracked down and corrected, the connection became obvious (no pun intended). If we go back to the junction of R210 and C217 and follow this circuit to the right, we come first to the anode connection of diode D203, which caused all the bother. From here the circuit continues to the right, connects to the cathode of D205, and then to resistor network R586, R584 & R585 (this network connects to the horizontal output transformer, from which it picks up horizontal pulses). The circuit then leaves the main board, via connector pin 10, and goes to connec­tor pin 10 on the chroma board, then via R624 to the base of Q601. And Q601 is the burst gate transistor. Most importantly, this is a DC circuit all the way; whatev­er spurious voltage appeared on this line from D203 would appear on the base of Q601, modified only by the divider network of R624 & R625. And, incidentally, R625 is incorrectly shown as 22kΩ; it is, in fact, only 2.2kΩ. Even so, there would be a lot more voltage at this point than normal, effectively upsetting the burst gate function. Of course it was a happy ending for the customer but only partially so for Yours Truly. I was glad to have solved the problem but I wish I’d done it a little quicker. Little green pictures And now for something a little more straightforward, although it did have its period of confusion. Among other things, it demonstrated how a customer’s description of a fault, no matter how well intentioned, can set one thinking in the wrong direction. It involved an HMV colour set, model 12641. The same chas­sis is used in the model 12642 and in the JVC model 7765AU. The owner first brought the set in several months ago, with the complaint that,”... the picture goes green – but only sometimes”. Well, the “only sometimes” didn’t exactly cheer me up but otherwise I assumed it would be a fault in the picture tube drive system; either the green gun being turned hard on, or the red or blue gun (or perhaps even both) being turned down in some way. I turned the set on while he was there and, sure enough, it was producing a normal picture. I suggested he leave it with me for a few days and so the set sat in a corner of the bench and ran all day and every day for the next week or so. And it never missed a beat; there was not even a suggestion of a green cast. Finally, I suggested that as we weren’t getting anywhere, it might be better if he took the set back home until the fault became more predictable. And that was the last I heard about it for the next three months or so. In fact, I had almost forgotten about it when the owner suddenly turned up with it, saying, “It’s real crook now – goes green every day.” And so it was back into the corner of the bench. But he was right this time. It had been running for less than half an hour when the fault suddenly appeared. But as soon as it did, I re­ alised that I had been thinking along completely wrong lines. It wasn’t a green cast; instead, it was green faces, with all other colours similarly incorrect. Well that put a different complexion on things (oops, sorry about that) and that meant a completely different approach. It was in no sense a picture tube drive problem; it was phase fault which meant an inversion, shift, or upset of some kind. But the interesting aside here is that it was only the green flesh tones that attracted attention. That’s not surprising in one way, I suppose, since these are normally the centre of attention. At a more practical level, the most likely cause of such a problem would be failure somewhere in the half-line frequency (7.8kHz) chain, starting at the phase discriminator, where this frequency is generated in the process of pulling the crystal oscillator into phase with the burst frequency. The 7.8kHz frequency is used to operate the reversing switch which changes the colour phase on each alternate line in synchronism with the transmitter. And when it misbehaves, which it can in variety of ways, it can do dreadful things to the colour. (In order for the receiver to perform this switching in correct phase with the transmitter, the PAL system employs a swinging burst signal. This 4.43MHz reference burst is shifted 45 degrees, plus or minus, on alternate lines and the receiver uses this shift as a code to identify each line. The high Q of the crystal oscillator averages these two shifts, while the phase discriminator, which controls the crystal phase, also provides the half-line frequency for the reversing switch). Fig.2: the 7.8kHz oscillator circuitry in the HMV 12641. This oscillator consists of transistors X304 & X305, with X303 desig­nated as a 7.8kHz killer. The 7.8kHz signal is fed to the demodu­lator IC (IC302) at top right. In this set, one of the easiest points of access to the 7.8kHz chain is at transistors X304 and X305, described jointly as the 7.8kHz oscillator – see Fig.2. This feeds a 7.8kHz signal into the demodulator IC (IC302). There is also X303, which is described as a 7.8kHz oscillator killer. However, X304 and X305 were the most likely suspects. But ease of access was not the only reason I selected this point. The transistor type used here, 2SC458, is one that I regard as a mite unreliable, being prone to intermittent be­haviour. Finally, a brief check with the CRO confirmed that this was where the frequency was running into trouble. So it really boiled down to which of the two transistors was the most likely culprit. Well, it was a 50-50 chance and I took a punt on X305. And for once I picked it in one; I fitted a replacement and all the faces were back to normal. I ran the set on the bench for several days with no sign of the problem and, although I remembered it had done this before, decided to pass it back to the customer with the least possible delay. But I warned him to contact me immediately at any sign of the trouble. Subsequent checks have confirmed that there has been no recurrence of it. So that was it. It wasn’t a highly scientific exercise but was more a result of previous experience, plus a certain amount of luck. And it does happen that way sometimes. But the custo­mer’s description did throw me initially. Back to wireless To finish off this month, I’m breaking right away from the usual to indulge in a little nostalgia. In the hurly burly of modern high-tech electronics – and the high-tech service equip­ment which it demands – we sometimes forget, or perhaps never knew, about electronics in its infancy. It wasn’t known as electronics then of course – it was radio or, before that, wireless. Which was fair enough, because the wireless set was virtually the only manifestation of what was to become electronics. But regardless of what it was called, or the state of the art, the equipment of the day needed servicing. For the wireless enthusiast of the day, or the local garage mechanic who doubled as a wireless expert, this was more often than not undertaken on the “by guess and by God” basis. As for service equipment – well this was often limited to a few basic tools – pliers, screwdrivers and a soldering October 1994  59 iron. And diagnosis was on the basis of visible faults: loose terminals, broken leads, unlit valve filaments, or obviously defective controls. Which was OK up to a point. But wireless sets used batter­ ies – and that, as they say, was the ‘ard part. Meters were few and far between, quite crude, and very expensive. They didn’t even approach the simple 1000Ω/volt multimeter which was later to become the mainstay of radio servicing. The accompanying photograph is of one of the very early attempts at a meter for use with wireless sets. It was passed over to me by a colleague, who acquired from a non-technical friend who found it in some junk in his workshop. Apart from that, its origin and history remain a mystery. It was a highly specialised piece of equipment. With a 0-50V scale and with prominent markings at 22.5V and 45V, it could have had only one role in life: to test B batteries. For the benefit of younger readers, the B battery – or high tension battery – came as a 45V unit, tapped at 22.5V. A small set (eg, one valve with 60  Silicon Chip earphones) would use one light duty version, while larger sets would use at least two heavy duty types to give 90V, or three to give 135V. And they were horribly expensive. Advertisements from wireless magazines of that era suggest that the keenest price for a 45V heavy-duty battery would be £1/5/0 ($2.50), or £3/15/0 ($7.50) for Fig.3: this pocket meter was the latest thing in test equipment in the 1920s. All it could do was test the B bat­tery. a set of three. But the basic wage was then only around £3/12/6 ($7.25). Most people earned a little more than that, say around £4/0/0, but a set of batteries would still make a mess of a week’s wages (work that out in modern terms)! With average use, but without wastage, a set would provide 6-9 months of use. And that, by any standards, made a wireless set an expensive thing to run. So nobody discarded them until they were convinced that they really were exhausted – and that the deteriorating perfor­ mance was not due to some other cause. Hence the popularity of the little meter portrayed here. For the repair man, it provided the proof needed to sell another set of batteries. And the enthu­siast who owned one was the envy of his peers; his popularity – and a regular invitation to dinner – was assured. The meter itself is almost certainly a moving iron type, renowned for its simplicity rather than sensitivity, and recog­nised by non-linearity at the low end of the scale. The colleague who passed it over to me checked it against his “u-beaut” digital meter and, to the accuracy with which he could read the simple scale, pronounced it “spot on”. He also checked its sensitivity, and found that at 45V it drew about 10mA. This was probably more by accident than by design but it would not have been an unreasonable load with which to test these batteries. Typical current drains would have been 10-15mA. How old is it? I’ve passed it around to several old timers but no-one’s game to admit to ever having seen one in actual use, for fear of revealing their age. However, history suggests that it would have been popular in the early 1920s, or about 70-plus years ago. Having said all that, the thing that stands out most in my mind is the point I made at the beginning; the very narrow appli­cation for the device. It could do only one job – test individual B batteries. As a general purpose meter for use in the wireless set itself, it was virtually useless. Unless the set used only one 45V battery, it could not be used even to confirm that the HT voltage was present anywhere in the set itself. A meter to do that was several years down the track. But that’s how things were in the SC good old days.