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SERVICEMAN’S LOG Purity is not always only in the mind The last few years have seen many changes in TV receiver design, mainly in the form of “improvements” supposedly intended to make the viewer’s life easier. But one of these, the remote control facility, has turned out to have a nasty sting in its tail. This story concerns a Panasonic colour set, model TC-29V26A. This is what is known as a “large screen” set, ranging around the 70cm mark, and while the problems encountered are in no way peculiar to this type of set, there is a feeling that they might be just that little more critical than their smaller brethren. Be that as it may, it provides a good opportunity to look at an old problem – purity error – which appears to have been given a new lease of life by modern developments. It all started when I received a phone call from a local large appliance retailer. The staff member calling, having con­firmed that he had the right person, then asked if I was an accredited service centre for Panasonic. I confirmed that I was and it was then that a problem arose. “Well,” said the caller, “will you go out to Mr So-and-So’s place, at such-and-such an address, and fix his TV set”. And, in spite of the nature of the wording, it was not a request; it was a command – almost a royal command. It was not an approach calculated to put me in a good mood. I don’t take kindly to being ordered to do things, even if it involves a normal service. And in this case it didn’t. Like most of my colleagues, I am doing my best to avoid house calls these days. One cannot ignore the travelling costs and, with modern sets, more often than not the job cannot be done in the home anyway. Granted, there are exceptions but, one way or another, the extra costs have to be met. And, in the case of 72  Silicon Chip warranty service, there is no way that these can be met. Most warranty payments are pretty tight anyway. So it’s not surprising to find that almost all appliance warranties require the purchaser to return a faulty appliance to the manufacturer or his accredited service centre. So, in a nutshell, I don’t do house calls on warranty jobs in any circumstances. And I advised the caller accordingly. He became a mite shirty at this and tried to pull rank and insist that I do what he wanted. I let him carry on until he ran out of puff and then suggested that he advise his customer to contact me, so that I could liaise with him and come to some mutually convenient arrangement. So, after some mumbling and grumbling, he reluctant­ly agreed to do this. In due course, the customer contacted me and I explained the above policy to him. Rather ironically, this didn’t worry him in the least. He had suitable transport and was quite happy to bring the set into the shop. So much for the other fellow’s huffing and puffing. Having clarified that point I asked the customer what the problem was. He said that, in general, he was very happy with the set but that it had a patch of bluish colour in one corner of the screen. In short, we had a purity error. Check list I went through a standard check list with him. Were there any loudspeakers near the TV set? No, that was ruled out. Any magnetic devices of any kind on top of the set, in particular, children’s toy cars with electric motors in them? Some of these motors have powerful magnets and I have known them to create just such problems. No; so we ruled that out too. Had the set been moved recently? Many larger sets are on mobile stands these days and can be readily moved, typically to suit a changed room layout. But that was not so in this case. With all those points covered, there was one more thing to try. I suggested that, over the next day or so, he resort to switching the set on and off at the power point, rather than via the remote control system. Which brings me to a point which has been largely over­looked in modern set design. Sets using electronic on-off switch­ i ng, as with remote control systems, no longer activate the degaussing system every time the set is turned on. In fact, if the power point is left turned on – which is the normal situation to permit full use of a remote control – the degaussing circuit may not activate from one year’s end to another. As a result, any purity problems which would normally be cured at the next switch-on remain unresolved. And the fact that I had to advise the customer of this situation is another oversight; there is no mention of this problem, or how to cure it, in this set’s user manual. Nor have I been able to find it in any other manuals. Anyway, I left the customer with that suggestion, and ad­vised him to call me in a couple of days if the problem persist­ed. Well, it did persist and he called me and reported this. And so I suggested that he bring the set in. That was no problem; he had a 4-wheel drive wagon and plenty of assistance to load it. And since I imagined that it would be a simple case of overall degaussing with the degauss wand, I said I could probably do the job while he waited. He duly turned up as arranged and we manhandled the set onto the bench. And the problem was just as he had described it; a bluish patch in the top right hand corner. While not all that strong, it would be a quite an intolerable distraction in prac­tice. So I fired up the degauss wand and went right over the set; front, sides, top and back of the cabinet. And that cured it. There was no doubt in my mind, or that of the customer’s, that the bluish patch had been completely eliminated. I made out the necessary warranty claim for Panasonic and we loaded the set back into the wagon. And, before the customer left, I was most careful to emphasise that he should contact me immediately if there were any further problems. Many weeks went by and I heard nothing further, which lead to the natural assumption that all was well. It came as something of a shock, therefore, when I received a call from the service manager at Panasonic, concerning a complaint from a customer about a set. Initially, I didn’t connect this with the aforementioned customer, due to some confusion over the name, but the address provided the clue. Anyway, it appeared that he was still not satisfied with the set and had written to Panasonic to have something further done about it. By all accounts, it wasn’t an unpleasant letter but it was unfortunate that he felt impelled to do this. I thought I had made the position quite clear. I can only imagine that he thought he had to go through Pana­sonic in order to initiate another warranty call. Anyway, I eventually contacted him again and we made anoth­er appointment. But I explained to him that this time I would need to keep the set for several days. My idea was to go through a complete purity and static convergence routine. He was quite happy about this arrangement. So the set finished up back on the bench. But these large sets are no snack to handle. Just getting it up on the bench is a two-man operation and then there is the job of getting the back off. This is not the simple job that it was in the old days. The set has to be turned on its face, many screws removed, and the back very carefully lifted off, taking care not to knock the neck off the picture tube! (Yes, I understand that it has happened). And when the back is removed, there is not much cabinet left to support the works in an upright position. But everything was sorted out eventually. Checking the set’s performance con­ firmed that the original purity problem had returned. Exactly why was not clear, although subsequent discoveries may provide a partial explanation. Purity adjustments The first thing I did was plug in the degauss wand and give the whole of the inside of the set a thorough going over. Again, this seemed to clear the problem but, having been caught once, I wasn’t taking any chances. And so it was on to the purity adjustment. Old hands may recall that for the early colour tubes, using the delta (triangu­lar) gun configuration, the purity adjustment was done using the red December 1994  73 SERVICEMAN’S LOG – CTD gun. The procedure was to unclamp the deflection coils and move them back as far as possible, then adjust the purity magnets for a pure red area in the centre of the screen. This was then expanded to cover the whole screen when the coils were moved forward. These days, with the in-line gun configuration, the green or centre gun is used but otherwise the procedure is much the same. The red and blue guns are turned off and the scan coil assembly unclamped and moved back. But the result will not be quite the same. What we are aiming for now is a vertical green block, about 300mm wide, in the middle of the screen. 74  Silicon Chip And when the coils are moved forward again, the result should be an even green display over the whole screen. In fact, this didn’t happen. When I moved the coils back, the green pattern was substantially to the right of centre, facing the screen. Correction is by means of the purity rings, the first two behind the scan coils. In this case, the pattern responded as it was supposed to and was moved to the centre of the screen. It also responded correctly when I moved the coils forward and we had a nice even screen pattern. If it is not quite right, the purity magnets can be adjusted slightly again for best re­sults. The scan coil clamps can be tightened at this stage but it is a good idea to feed in a cross hatch pattern first, to make sure that the picture has not been rotated in the process. The next step is to energise the red and blue guns in turn and check that they are even and pure. Again, this didn’t happen quite according to the book. The blue gun gave an accept­able pattern but the red gun produced a faint orange cast in the top right corner. It took some more minor juggling of the purity magnets to correct this. Finally, I fed in the cross hatch pattern again and checked the static convergence in the centre of the screen. There was a slight error, which was easily corrected with the static convergence magnets. That done, I considered the job finished and judged that the customer should have no more cause for complaint. But I did take the opportunity to make one more test, which was quite revealing. The set had been sitting on the bench on an east/west line and I turned it, while running, through 90 degrees into a north/south alignment. The result was a fairly substantial purity error; substan­tial enough to risk a customer reaction. I turned the set off, waited long enough for the degauss thermistor to cool and turned it on. Result; no purity error. I repeated the exercise in reverse, turning the set back to its original east/west alignment. Again, it gave substantial purity error which was cured by a switch-off/switch-on routine. The overall conclusion was that the set was quite sensitive to prevailing magnetic fields – mainly the Earth’s I imagine. There is nothing new about this; it has been with us ever since the advent of colour. Nor am I suggesting that this set is any worse than any other set. What I am saying is that we have tended to forget about this sensitivity because the degaussing systems have kept it under control. But now, with remote control switching bypassing the degauss systems, it is rearing its ugly head again. So that’s one to watch. More from the motel My next story is a continuation of the Contec saga I start­ed in the November notes. Readers will no doubt remember the puzzling symptoms -31V 5V 4 3 F F 12V 1 F 2 F 1 E 2 E 8 1 D510 C514 47 7 6 240V D511 3 4 2 5 R519 1k IC 510 IC502 330 C515 470 0.1 Q506 T501 Q505 T502 Q507 114.9V C 5 Fig.1: the power supply circuit for the Contec MSVR-5383. The -31V rail is derived from transformer T501 (pins 6 & 8), via D510, C514 and R519. A simple fault can cause the weirdest symptoms. caused by the failure of the 31V rail supplying pin 2 of IC802. So this is about another Contec MSVR-5383 from the same local motel. And the symptoms still involved the memory function which were involved previously but there the similarity ended; they were really weird this time. The customer’s story was somewhat similar to the previous one. If the set was left in standby mode there was no problem but if it was turned off at the mains and – most important – left off for about half an hour, there was an apparent loss of memory. And that “apparent” qualification is really the heart of the story because it is about the only word I can think of which even approaches describing the problem. The only real way to describe it is to give an example. Let’s assume that the set has been programmed for five channels, using positions 1-5. Position zero is blank. Let us further assume that the set, when switched off, was running on position 2. Now, in the normal course of events, the set could be switched off, even at the mains and, when switched on again any time later, would come up 6 D516 C C523 C 1 2 3 Q508 Q509 on position 2. Not so with this set. Assuming that it had been off at the mains for about half an hour or more, the most likely scenario would be that it would come up on position zero and thus give a blank screen and white noise. So let’s try the remote control and call for position 1. Result: no response. Ditto for position 2 and so on. But suddenly at, say, position 4, there is the channel programmed for channel 4. But the sound is at full blast, prompting a frantic stab at the volume down button. And this works, allowing the volume to be set to a normal level. So let’s try position 5. It may or may not respond. Moving back down the scale, a previously dead position, say 2, might now respond. So might position 1. But go back to position 4 and it may no longer be available. Now all that is purely hypothetical, because the response at any time is completely random and unpredictable; there was absolutely no pattern of any kind. And if that isn’t enough to give a bloke nightmares, I don’t know what is. But that was it and I was stuck with it. Remembering the previous experi- ence, I went straight to the supply to pin 2 of IC802, although it was more in desperation anything else. And that qualification was justified, because pin 2 was sitting at 31V, exactly as it should be. A crook IC802? That seemed to be the next most likely pos­sibility and I had a spare on hand. It took only a few minutes to fit it and I gave it a test run, feeling fairly confident that it would come good. No way mate, as they say in the classics; it was exactly the same as before. Where to now? So where to from here? I looked at the circuit and it looked right back at me. The only likely possibility seemed to be IC804, which is obviously a companion to IC802 and performs several similar functions. The only immediate problem was that I didn’t have a replacement. Nor was it available from my normal supplier. I would have to go back to the Contec service organisa­tion. Perhaps that was just as well; it made me think a bit harder. And some of the things it made me think about were other weirdos I’d experienced December 1994  75 SILICON CHIP BOOK SHOP Newnes Guide to Satellite TV 336 pages, in paperback at $49.95. Installation, Recept­ion & Repair. By Derek J. Stephen­son. 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A great many practical circuits are featured – a must for anyone inter­ested in audio design. Optoelectronics: An Introduction By J. C. A. Chaimowicz. First published 1989, reprinted 1992. This particular field is about to explode and it is most important for engineers and technicians to bring themselves up to date. The subject is comprehensively covered, starting with optics and then moving into all aspects of fibre optic communications. 361 pages, in paperback at $55.95. Digital Audio & Compact Disc Technology Produced by the Sony Service Centre (Europe). 3rd edition, published 1995. Prepared by Sony’s technical staff, this is the best book on compact disc technology that we have ever come across. It covers digital audio in depth, including PCM adapters, the Video8 PCM format and R-DAT. If you want to understand digital audio, you need this reference book. 305 pages, in paperback at $55.95. Power Electronics Handbook Components, Circuits & Applica­ tions, by F. F. Mazda. 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Eugene Trundle has written for many years in Television magazine and his latest book is right up date on TV and video technology. 432 pages, in paperback, at $39.95.  Title Price  Newnes Guide to Satellite TV  Servicing Personal Computers  The Art Of Linear Electronics  Optoelectronics: An Introduction  Digital Audio & Compact Disc Technology  Power Electronics Handbook  Surface Mount Technology  Electronic Engineer’s Reference Book  Radio Frequency Transistors  Newnes Guide to TV & Video Technology Postage: add $5.00 per book. Orders over $100 are post free within Australia. NZ & PNG add $10.00 per book, elsewhere add $15 per book. TOTAL $A $55.95 $59.95 $49.95 $55.95 $55.95 $59.95 $99.00 $140.00 $85.00 $39.95 SERVICEMAN’S LOG – CTD let it stand, I then patched in the good capacitor and switched on. No problem; the set came up on the original channel and all other channels could be called up correctly. I repeated this exercise several times during the day, and it worked every time. In short, the ripple did not seem to pres­ent any difficulties about programming the set, only about the subsequent recalling function. A possible theory One theory that has been advanced is that erratic recall behaviour was a function of the exact moment when the remote control message was received, relative to the phase of the rip­ ple. If it occurred at the exact moment when the ripple was at its cross­over point – ie, neither adding or subtracting anything from the DC rail, then response would be normal. At all other times, there would be a risk of failure. Well, it is an interesting theory but I’m afraid that is all it will ever be. I can’t think of any way of proving it. In any case, it still leaves a lot of other questions unanswered. But at least I’d SC found and fixed the fault. TRANSFORMERS • TOROIDAL • CONVENTIONAL • POWER • OUTPUT • CURRENT • INVERTER • PLUGPACKS • CHOKES –not necessarily involving memory sys­ tems – where the most bizarre symptoms could result from rela­tively simple faults. And one of the simple faults which had tricked me in the past was ripple on a supply rail. I could hardly wait to get the CRO probe on pin 2 of IC802. And there it was – about 5V of ripple on what should have been a DC supply. An easy cure The reason was almost too obvious to justify mentioning; it just had to be C514, a 47µF 60V electrolytic capacitor in the 31V supply rail. I pulled it out and checked it and it was struggling to make 5µF. I patched in a new one, put the set though all its paces, and it came up trumps; nothing I could do would cause it to lose its memory. So, in practical terms, that was the end of the exercise. Why did it do what it did? Frankly, I have little or no idea. But I did try a few tricks before the set went home. Before permanently fitting the new electrolytic, I patched the old one back in, programmed the set, then switched it off and let it stand for the prescribed period. There was some doubt about this exact period. The customer had suggested half an hour and I worked to this for a while. Later I realised that this was more than necessary; about 10 minutes was sufficient but it had to be at least this. Anyway, having programmed it and STOCK RANGE TOROIDALS BEST PRICES APPROVED TO AS 3108-1990 SPECIALS DESIGNED & MADE 15VA to 7.5kVA Tortech Pty Ltd 24/31 Wentworth St, Greenacre 2190 Phone (02) 642 6003 Fax (02) 642 6127 December 1994  77