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SERVICEMAN'S LOG Chuck it away and buy a new one No, that’s not my advice but it is the philosophy from one of this month’s stories – a story from the USA, where the servicing scene is very different. Even so, it has a very inter­esting connection with the local scene. My first story concerns a National Panasonic model TC-1407, 34cm colour set using an M12H chassis. This model can be any­thing up to 10 years old and is a very well made and reliable set, regarded by many as one of the best that National ever made. But of course faults do occur and, in this case, the com­plaint was total lack of colour. This is not an unusual fault in itself but the actual cause was unusual, as we shall see. When tackling colour problems, I automatically reach for the CRO leads. And the first thing I checked was whether a colour signal was coming into the chroma decoder (IC601) – see Fig.1. In fact it was, on pin 7. It was only about 0.8V p-p but this tal­lied with the circuit. Next, I checked the crystal oscillator. Again there was no problem, with plenty of 4.43MHz signal on pin 16 of the same IC. Well, that ruled out the more obvious possibilities. The next thing to check was the gating pulses (horizontally derived) which control the burst gate and similar functions. Such pulses should appear at the burst gate terminal (pin 14) of IC601 and are shown on the circuit as waveform 30, with a p-p amplitude of 4.6V. But not in this set – pin 14 was dead. Well, at least I was on the right track. Unfortunately, the track wasn’t very clear, which is just another way of saying that the best circuit diagram I could find left much to be desired. It wouldn’t have been so bad if I had been able find an original circuit but the best I had was a much copied copy of a copy – if you follow me. To be fair, the original circuit was undoubtedly very good, featuring lots of information in the form of voltages and wave­forms, but it had been much reduced and, when copied, a lot of fine detail was lost. Anyway, I was faced with the task of tracing the circuit to find where these pulses originated and at what point they were lost. And I thought I had cracked it in one as soon as I started. Not far from, and to the left of, pin 14 is a diode, D602, which connects to chassis. Naturally, I check­ed it and it was a dead short. Unfortunately, my jubilation was short lived because when I replaced it and switched on there was still no colour. A literal drawing board So, back to the drawing board –almost literally. The line split into two at this point, both moving parallel down the page for a short distance. Then one turned left and one continued down. I turned left and finished up on pin 10 of the video chip (IC301). There was supposed to be a waveform here also (designat­ ed waveform 21) which was similar to the one on pin 14. But there wasn’t. Fairly obviously, pin 10 was supposed to receive this waveform, not supply it. I followed the other line down until it spilt, going left and right. I went right, but drew another blank. It finished up at the base of transistor Q601, a blanking pulse generator – which didn’t have any pulses either. OK, back to the junction and turn 54  Silicon Chip Fig.1: the chroma decoding circuitry in the National TC-1407. IC601 is at top right, IC301 at top left, and IC501 at bottom centre. Many of the IC pin numbers are quite difficult to read. left. This brought me to pin 16 of the jungle chip, IC501. And this, I felt, had to be the source of the missing waveform. This chip performs a whole host of functions, including horizontal AFC and sync separation, both requiring a reference to pulses from the horizontal output trans­former. In fact, pin 16 was connected to the sync separator block within the IC. What’s more, this IC appeared to be performing these, and its other functions, because we had a perfectly locked picture but in monochrome. So it had to be receiving pulses from the horizontal transformer. In fact, after much more laborious cir­ cuit tracing, too complex to detail here, I confirmed that the pulses at pin 2 of the horizontal transformer were applied to pin 1 of IC501. And the CRO confirmed that the pulses here were as they should be. So why wasn’t it delivering a pulse at pin 16? I could only conclude that there was a fault in the IC. And, since I didn’t have any on hand, I had to order one. But while waiting for it, I still had an urge to confirm that this was the problem. Referring again to IC301, I noticed that on pin 11 there was a waveform (No.24) of similar shape and amplitude to the waveform that should have been present on pin 10, though somewhat smoother. So what would happen if I connected pins 10 and 11 togeth­er. At best I might get some kind of colour response. At worst, I could blow up IC301. After studying the voltages on the two pins (1.2V on pin 11 and 0V on pin 10) I decided that the risk to the IC was minimal and connected the two pins together. And it worked – well, partly. It did produce colour on the screen but it was not locked, drifting through the spectrum and producing some weird coloured scenes. But it was enough to sug­gest that my diagnosis was probably correct. And in fact it was. When the new IC arrived, I fitted it and everything came back to normal. Another satisfied customer. The American in-laws And now for a change of scene; quite a big change in fact, because my next story comes from the USA. But it also has a very close relationship with some of my previous notes and one story in particular. By way of background, my regular readers may recall that, from time to time, I have featured stories from a colleague who worked down the south coast of NSW. And I was always pleased to feature these stories because they involved factors peculiar to the area; UHF almost exclusively, long distances in many cases, and much hilly terrain. And, of course, we exchanged technical experiences and picked each other’s brains from time to time. So it came as rather a shock when, some months ago, my colleague announced that he had decided to retire and move to the US where he June 1996  55 being that muggins, “who knows all about TV sets”, could probably fix it. So, it finished up on my doorstep. I’m afraid I agreed to the idea with mixed feelings. On the one hand I had brought all my test instruments and tools with me, had organised some work­shop space, set up a bench, and begun to sort things out. However, it was all very well for the rest of the family to assume that muggins “knows all about TV sets”. The truth was that all I knew about projection TV sets was secondhand and did not even involve the same model. Nor did I have a manual, have any idea of where to find one, what it would cost if I did, and whether such an outlay could be justified. I short, I would have to fly by the seat of my pants. A bright spot had various in-laws and other family connections. So, no more stories from that source. But do TV servicemen ever retire completely? Significantly, my colleague shipped all his equipment, which was considerable, to the US with him. He had no intention of setting up in business but, I imagine, he knew he would feel lost without the means to look after his own devices and ap­ pliances, along with those of his relatives and friends. So that is one part of the background. For the other part I would refer readers to one of my own stories which appeared in the May 1995 notes under the heading “All it needs is a new fuse”. In greater detail it concerned a Mitsubishi VS-360A pro­jection TV set and the difficulty of convincing the owner that it needed a lot more than a new fuse. Linking all this together is the fact that my colleague was “in” on that story from the beginning. Neither of us had tackled projection TV sets before and he was anxious to learn all he could from what I had to learn. It was, therefore, sheer coinci­dence that one of the first family jobs he encountered after settling into his new home involved a Mitsubishi projection TV set. 56  Silicon Chip Anyway, here is my colleague’s story as he tells it. The set involved was a Mitsubishi VS-405R projection type, featuring a 100cm (40in) screen. It was about 11 years old, roughly the same age as the one my colleague had dealt with in Australia. And it came to me by a somewhat round about route. It had originally belonged to a friend of one of my in-laws and had failed some time before I came on the scene. The owner had called in a local serviceman who repaired it for $170 (I’m quoting $US, of course). It was quite a reasonable charge and the set performed perfectly. Unfortunately, it did not perform for long, failing again after about six months. And this time the owner did what so many people do in this country when something fails (particularly something 11 years old which has failed for the second time) – he chuck­ ed it away and bought a new one. And, as a matter of interest, the new set – a larger 125cm (50in) model – cost about $1200, roughly half the price of a similar set in Australia. At the same time the old set was “chucked” in the direction of my inlaws – not literally I hasten to add, because the thing weighs a ton – the idea One bright spot was that the device appeared to be fairly easy to get at; much easier than was apparently the case with the one my Australian colleague worked on, due to a slightly differ­ ent layout. One important difference was that the top part of the cabinet back could be removed, as well as the lower part, giving much better access. In addition, the 3-gun assembly was mounted on a steel subframe supported on runners on the inside of the cabinet. This allowed the subframe to be withdrawn, although not without some difficulty – more on that later. The whole system was made up from a collection of PC boards, each mount­ed on a light metal frame which served as a nominal chassis. These included a power supply board, a horizontal and vertical scan board, a signal processing board, a stereo sound board, and a convergence control board. The scan and signal boards were secured to the floor of the cabinet while the remaining boards were secured to the sides. They could all be easily unplugged and withdrawn. That much established it was time to apply power and see what happened. The answer was simple – nothing. This lead me to a 3A fuse in the power supply which had failed. This was replaced and power applied again. This time there were signs of life. The set tried to fire up but then would shut itself down and try again. In other words, there was a slow hiccup, suggesting that a protection circuit somewhere was taking over. I suspect that the set had been inadvertently left on in the hiccup condition which, if it continues long enough, can blow a fuse. I went straight to the horizontal output transistor, which was readily accessible, and picked it in one; it was shot. This was easily fixed. I didn’t have a direct replacement type but settled for a 2SD380. Unfortunately, when I switched on, the set was still hic­cuping. I found a HT rail check point and monitored it. It looked as though it was about 125V but this called for some judgement as it rose and fell. I suspected that the fault was either somewhere in the horizontal scanning circuit or in the protection circuit covering this section. Normally, I would check this second possibility by momentarily disabling the protection circuit and noting what happened. The trouble was, without a suitable diagram, I had no idea where to look for this circuit, so I put that on hold. I was also curious as to the nature of the previous fault and what work had been done. I could see that the boards had been pulled because the wiring looms had been released from their clips and, although most had been restored, a few had not. I examined the boards very carefully, in search of a clue, and finally concluded that no work had been done on them. One obvious indication was a fine layer of dust, as normally found in such situations, which had not been disturbed. At the same time, I went over each board and checked for dry joints, particularly around the horizontal scan circuitry and the four pin connections to the horizontal driver transformer. Dry joints to these pins have been a common problem with many sets in the past. In this case, the soldering quality was very good. I did remake a couple of joints which were vaguely suspicious but it was more of a gesture than anything. Why the failure? At this stage, I began to wonder why the horizontal output transistor had failed. I also wondered if there had been a previ­ous failure and if the faulty transistor I had replaced had itself been the correct type number? However, without a circuit I really had no way of knowing. Finally, having checked the most likely possibilities as I far as I could, there seemed to be only one positive check left that I could make; a check for shorted turns in the deflection coils. Fortunately, I had brought my trusty shorted turns tester with me and unearthed it after some searching. The deflection coils were plugged in, so it was easy to make the checks without pulling the metal frame assembly. I checked the horizontal windings first and the first two tested OK. But not so the third one; there was a clear indication of a short. So now I had to pull the metal frame assembly. This wasn’t quite as easy as it looked. To understand why it will help if I describe the device in greater detail. Imagine a rectangular metal frame running the width of the cabinet and sitting horizontally on two runners, one on each side of the cabinet. This frame extends from the back of the cabinet to about two thirds the way to the front. A second frame is then attached at right angles to the front of the first frame and this extends downwards to the floor of the cabinet. This supports the picture tubes and some associated circuitry. OK, having envisaged all that, consider how the frame is constructed. It is made from box section mild steel –actually two lengths of angle iron welded together to make the box sec­tion. All of which adds up to a lot of steel. Add the weight of the three picture tubes – they may be small, but they’re not light – plus a few odd pieces and you have a total weight of around 25kg. It’s not at all easy to manhandle in an enclosed space. Nor was the operation made any easier by the location of the picture tubes. These are mounted at about 45 degrees, below the level of the horizontal section so that, if it were simply removed and placed on a bench, the whole assembly would be rest­ing on the tube necks. Ouch! I tackled the problem by arranging some suitable blocks on the bench. I then unclipped all the connecting leads and careful­ly manhandled the frame out and onto the blocks. It was worth the effort. With everything out in the open it was immediately obvious why the set had failed in the first place – and why it had failed in the second place. In fact, one doesn’t often get an explanation presented as clearly and positively as this one. The faulty deflection coil assembly was coated with a brown varnish, the appearance of which exactly fitted the de­scription of the coating on the failed transformer in the set handled by my Australian colleague. Even without any other warning I would have been highly suspicious of this mixture. I had encountered a similar witch’s brew before – sometimes brown, sometimes yellow – in other makes of sets. It is sometimes used as a varnish on windings, and sometimes as a glue to secure an extra component on the copper side of a PC board. More particularly, I was well aware of its corrosive properties. It will eventually eat away any copper with which it comes in contact. In this case, of course, I had been June 1996  57 Serviceman’s Log – continued warned. I recalled that the technician in the Australian Mitsubishi service department had advised my colleague to check for this varnish on the trans­ former and for any damage it might have caused. The rest is history; that transformer was a write-off. There was one other interesting development. One of the three deflection coils – presumably fitted during the previous service –was quite free of the witch’s brew. Per­haps a message had finally penetrated. But that was all rather academic from my point of view. My more immediate concern was the fate of this set. Would I be justified in pressing on with the job or should we cut our losses? There had been a tentative agreement before I started that we would put a limit of around $100 on the cost of replace­ment parts. Labour, of course, would be no more than an extra serving of turkey at next thanksgiving – if I was lucky! I had determined that a new deflection assembly would cost about $100 58  Silicon Chip which, with a new horizontal output transistor, was already stretching this limit. To that would need to be added a second scan coil assembly, because it would be pointless to simply replace the one faulty one. The third coil was another failure just waiting to happen. On top of that, how much more of this brown varnish was there elsewhere in the system? So we were looking at $200 plus to repair an 11-year old set, which could be re­placed with a similar size modern set for around $1000. I am well aware that, in Australia, many people would regard such an oppor­tunity as a gift and be prepared to take the risk. Not so in this country. Because appliances are so much cheaper, in real terms, than in countries like Australia, the concept of service is very much different. Many people will simply discard an appliance at the first sign of trouble, without any attempt to determine whether the fault is minor or major, and whether a repair might be worthwhile. Against this background, and on my advice as to what would be involved, the decision was made to cut the losses, such as they were ( mainly my time, which nobody regarded as being very important). Not that this really worried me very much; it had been an interesting exercise and an opportunity to study another version of this type of set. The cabinet of the old set was salvaged, however. It was a very useful piece of furniture and one member of the family had it earmarked as a useful storage unit. I suppose the most interesting aspect of the story, from a technical viewpoint, was the coincidence of finding two sets, of the same make, so far apart in different countries, suffering from almost identical faults. Also, it is to be hoped that, at long last, the havoc caused by these witches’ brews has been recognised and that they will be suitably disposed of – with due regard to the environment, of course. Well that’s my colleague’s story; and a very interesting insight it is into the TV scene in another country. Thanks SC mate.