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Ring out the ne-w, ring in the-old If that heading sounds like a new year greeting suffering from alcoholic inversion then let me assure you that it is not; it is quite deliberate and was inspired by one of the most blatant and disgraceful rip-offs that I have ever encountered. The story started with a telephone call from a potential customer who had just moved into the area and who had been given my name by a regular customer. The caller explained that he had a Philips 56cm K9 colour set which had failed completely and wanted to know how best to go about getting it serviced. Could I call and collect it, could I service it in the house, or would he have to bring it to the shop? I'm trying to avoid house calls as much as possible these days, unless there are special circumstances. They are time consuming - and therefore expensive - and with the ever increasing complexity of modern devices it is becoming more and more difficult to do a proper job away from one's own bench. One exception is very large sets, and particularly where it would be unreasonable to expect the person concerned to manhandle it. Another is where the symptoms suggest that the fault might involve the location: antenna problems, local interference, and so on. In this case I advised the caller that I would call if this was essential, but that it would save time and reduce costs if he could manage to bring the set in. He accepted the situation happily enough, and added that he could probably manage to bring the set in without too much bother. In the event, having the set on my own bench was a very wise decision. And so it was that he duly turned up with the set in the back of a station wagon, and I helped him bring it into the shop. I then took the opportunity to quiz him about the set's history; previous problems, service work &c. He turned out to be quite well clued up in this regard; much better than most. Previous jobs ~~--AND AOt>l::D 11-IA'T H~ C.OU\...P :BR,NG 71-\e: Se:.T \ N WrTHOUT 000 ,oo 38 SILTCON CHIP MUCH :BOT\-\E'R.oo The set had had two major service jobs performed on it quite recently; one by an independent serviceman and one by a large wellknown service organisation. In the first case, the picture tube had been replaced and in the second case a new tripler had been fitted. (Not many customers would even be able to remember the name of a tripler!) For his part he was anxious to get some idea of how long the job was going to take and what it was likely to cost. Naturally, I wasn't prepared to commit myself on either question, but suggested he might like to wait while I turned the set on and made a preliminary check. So I turned it on and confirmed the behaviour as he had described it on the phone; no picture, no sound, no raster. I pulled the back off the set and that was my first shock! The previous serviceman had obviouly been a European gentleman by the name of Rufus Gertz - and boy, was he ruf. The two screws which secure the two swing-out boards were missing, and the strip cover over the power supply was missing, as were the screws which hold it. I kept the observation to myself and reached for the multimeter to measure the 155V supply rail on the horizontal scan board. I fully expected to find a low voltage here, as is typical of a shorted horizontal output transistor for example, and which produces just such symptoms. Instead the voltage was high, up around 180V. The most likely explanation was that, instead of a short circuit on the horizontal scan board, there was an open circuit. Another vague possibility was a fault on the power sup- .ply board. Although unlikely, it was easy to check because I carry a spare power supply board on hand, and it takes only a few moments to make the swap. In fact, my board behaved exactly the same, putting suspicion back on the horizontal scan board. At this point the customer was still waiting patiently, hoping that I could give him some idea of the likely seriousness of the problem, so I swung the two boards out for a visual examination, in case there were any obvious clues or damage which I needed to know about. In fact there didn't appear to be anything obvious on the boards, but there was something else that caused me to reel back - metophorically speaking that is. Remembering the customer's comment about a new picture tube I fully expected to find a typical re-built tube from one of the local companies which do such a good job in this field. Instead, the set was fitted with an original Philips tube. It wasn't the tube which had been fitted to the set when new - which was fair enough, seeing that it was supposed to have had a new tube fitted - but neither was it a new tube. A ring in This model set was originally fitted with a type A56-120X, whereas the tube now in the set was a later type, the A56-410X, which is almost identical with the 120X except that f L· r"./z ,. ·- ...... "-' ~ a'nt it is fitted with a fast acting heater to bring the picture up more rapidly at switch-on. Well, there was nothing sinister about that either; the real giveaway was a little Philips sticker on the tube. Traditionally, every new Philips set carries at least two of these stickers; one on - typically - the horizontal scan board and one on the picture tube, and both carry the serial number of the set. But in this case the serial number on the picture tube sticker was not the serial number of the set. So the tube was obviously not a rebuilt tube, from which any such stickers or labels would have been removed. Nor could it be a new tube from Philips, fitted by someone else, because it would not have carried a sticker either. In fact, it was obviously a tube salvaged from a junked set which had been rung in on an unsuspecting customer. Once again I kept my own council. I needed time to think about this one. Getting back to the immediate technical problem, it was now obvious that I could not give the customer any meaningful answer as to the time or cost involved. All I could do was suggest he come back later in the day when I might have a better idea of the problem. So that was how we left it. Back at the bench I stoked up the CRO and prepared to do battle. All the indications were that the horizontal output stage was not working so I started by checking the base of the horizontal drive transistor, TS430, a BD232 which is fed with horizontal pulses from module U330, at about 1.5V p-p. In fact, this came up spot on, so all was well so far . But it was a different story at the collector of TS430. There is supposed to be 400V p-p signal at this point, but there was nothing. And for a very good reason; there was no DC voltage on the collector. This situation, in turn, was due to the sad demise of R440, a 1200 resistor in the collector supply line from the 155V rail. It was cooked to a cinder and had obviously suffered a very severe overload. But the reason for this was not immediately obvious. I first suspected that TS430 had broken down, but an in-situ check seemed to rule this out, at the same time ruling out the breakdown of any other components in the collector circuit. One possibility was that something was breaking down at the operating voltages, which were a lot higher than I could apply with the ohmmeter. So the easiest way to find out JUNE 1988 39 several more times, but only for about a second on each occasion. Pondering over all the symptoms I decided that the horizontal drive transistor, TS430, was the most likely suspect. I pulled it out and checked it as thoroughly as possible, but it gave no sign of any fault. Nevertheless, having gone this far, it was just as easy to fit a new one and remove one possible culprit. And that seemed to be it. I ran the set for several hours more, until the customer came back in the late afternoon, and it behaved perfectly during that time. There seems to be little doubt that TS430 was suffering from intermittent breakdown and, sooner or later, would have destroyed R440 once again. Follow up checks have confirmed that there has been no sign of any shutdowns, no matter how brief. SERVICEMAN'S LOG What about the tube? our ~~ W\1'\-1 11-(E: NEW.,.., IN W\TI-\ 11--\e:. OL-V seemed to be to replace R440 and see what happened. Which I did, and nothing happened - nothing untoward, that is. The set came good immediately and produced a very good picture, the only qualification being that the brightness control was obviously set a good deal higher than the average position for this set. As far as I was concerned this served only to confirm what I-had already discovered; the tube was well past its prime. It probably had a couple more years life in it, but that was pretty poor value for the price of a new tube. But my more immediate concern was to find an explanation for the failed R440. Resistors can fail spontaneously, but they don't cook themselves in this manner. Something else had failed, and would probably fail again. It was simply lying doggo for the moment. So I left the set running and busied myself tidying things up inside it. I fished out some screws to secure the swing-out boards, and salvaged a power supply cover 40 STLICON CHIP strip from a junked chassis. Then I tackled the leads associated with the tripler. Whoever had replaced the tripler had at least been a disciple of our friend Rufus; no attention had been paid to the original dress of these leads, something which can be quite important where the very high voltages are concerned. There were even a couple of bare connections which can develop corona discharge in unfavourable conditions. I re-dressed the leads and covered the bare connections with a coating of Silastic, cleaned away some dust and grime and, in general, made the set look more like a new one. With everything thus tidied up I turned the set on again and let it run on the bench while I went on with other jobs. I was still puzzling over the failure of R440. Happily, my tenacity was rewarded. The set suddenly shut down, but only for a second or so, then came good. I left it running and it shut down again - and again. In fact it did it So that was that, at least in the technical sense. But what about that bodgie picture tube? Should I tell the customer what I had found? It was a diplomatically dicy situation. Not surprisingly, many people don't like being told they have been ripped off; in their minds it implies that they have been less astute than they should have been. There is also the risk that a serviceman delivering such news may be suspected of simply trying to discredit the previous serviceman and enhance his own image. This risk is particularly high where the critisism is purely subjective or speculative, rather than based on hard fact. I was still pondering these points when the customer returned in the late afternoon. He was accompanied by his wife this time, which gave me further food for thought. Would I precipitate a domestic argument along the lines of, "I told you not to take the set to that bloke"? I bided my time, initially simply describing the fault I had found, and emphasising that they should contact me again in the event of any trouble. Both appeared to grasp the situation, and the points I was making, quite well. All of this was more or less routine, of course, but it did give me a chance sum up both parties. And having done so I finally took the bull · by the horns. I broached the subject by asking them to confirm that a new picture tube had been fitted recently and, when they did so, asked if they would mind telling me what this had cost. They were quite happy to discuss this and recalled that the replaceregular correspondent, J.L. of ment had cost them about $300; Tasmania. It is particularly inwhich is about the going price for teresting in that it relates to my supplying and fitting a re-built picstory in the February notes concerture tube - if it is the genuine artining a National TC-2004. In fact , it cle. At this point I broke the bad appears that this story provided the news and quickly followed up the clue which enabled J.L. to solve a statement by demonstrating how I completely different fault. This is knew this to be so. And I even went how he tells it. so far as to offer to back them if The story that follows relates to they felt inclined to take the matter the same chassis, but in a different to the Trade Practices Commission. model, and provides another sympI was quite genuine about this. I'm not in the habit of dabbing in tom to add to the villainies of that colleagues for what I might regard · particular capacitor. This set was a TC-1802 and came to me with the as doubtful business ethics, or complaint that there was no pictechnical incompetence, at a ture, only a line across the centre of relatively minor level, if only the screen. The owner was convincbecause it is virtually impossible to make such accusations stick. And, ed that the picture tube was shot in any case, such tactics usually (the pessimistic type froni the February notes!), and refused to reap their own reward. But something as blatant as this believe me when I tried to convince is another matter. It is hard enough him otherwise. I fired up the set and, sure for the service industry to maintain enough, there was the line across an honest image in the public eye often due to genuine misunderstan- the centre of the screen. But it dings - without characters like wasn't quite the classic frame colthis perpetrating genuine frauds of lapse symptom; the line was hiccupping slowly. At about two second inthis magnitude. The customers' response was tervals the picture would expand to the top of the screen and then colrather surprising. I am quite sure that they accepted my statement completely but, no, they didn't want ( to become involved in any such claims. More to the point, they didn't seem to be particularly shocked at my revelation and I gained the very strong impression that they had already arrived at a similar suspicion. So that was it. I didn't press the point and they seemed happy enough with the job I had done, didn't quibble over the bill, and gave every indication that they would be back next time they wanted service. Which is about as much as one can wish for. A vital clue At a more technical level, here is another contribution from my lapse again. I had never seen a fault like this, and was at a loss to know where to start looking. I suppose you have to get lucky eventually; Murphy has to take a holiday sometimes. I decided to start looking from the vertical output end, and to check the voltages first. This proved to be the right way to go and I had the answer in two minutes flat. This chassis uses a push-pull vertical output stage, TR407 and TR408, fed from ± 25V rails. I found the + 25V at the TR407 collector, but there was no - 25V at the TR408 collector. I checked back along the - 25V rail to find diode D406 (UF-2) shorted and R445 open. · R445 is a rn fusible resistor, apparently intended to protect the diode. Replacing both put the set back into working order - for a while. In fact, I had no sooner replaced the cabinet back when I noticed that the original pulsating line fault had returned. A minute later I had confirmed that the diode and resistor were shorted and open, respectively. This was quite a blow, because everything had worked normally for an hour or so before I replaced the cabinet back. All voltages had been correct, and waveforms almost exactly according to the manual. So what kind of fault could kill a 400V, 1A diode running at 25V and something less that lA? I spent the next hour minutely ex- :'(il"i. . oo.,I SPE:N\ n-\E:. NE:XT HOIJ'R. L.-OOKING FO'R. AN'-f-r\-\\NG TH~T IV\\Gt-\T K I L--l- 1'-\ C: 'Dl 01) E: ooo JUNE 1988 41 SERVICEMAN'S LOG ammmg the set, looking for anything that might kill the diode. In particular, I was looking for something that might generate high voltage spikes in the line output transformer. I found nothing. So it seemed that all I could do was get the set going again and hope that further observation might provide a clue. Unfortunately, I had used up my last UF-2 diode and had to cast around for a suitable substitute. I chose an OA636/600 on the grounds that it was a fast recovery 1A diode, with a 200V margin over the UF-2 . With the new diode and another 10 resistor in place the set came good and ran steadily for several days. I called the owner and suggested he pick it up. He couldn't make it straight away and it was more than a week before the set went home. And during all that time it ran perfectly. But it wasn't home for long. The owner was on the phone the same evening with the revolting news that it had failed again, in exactly the same way. I was back inside the chassis the next day searching vainly for something- anything- that could have killed those diodes. And one of the puzzling aspects was why it was only the negative rail that was giving trouble. Both rails used identical components and were supplied from opposite ends of the same ·transformer. I considered the possibility of a temporary short in the PNP output transistor, TR408. But if so, and if it had drawn enough current to take out R445, it should have at least distressed its rn emitter resistor, R434. But this latter was in pristine condition, with not the slightest bubble on its paintwork to suggest that it had ever been more than warm. While I was contemplating the problem the February edition of SILICON CHIP arrived and, as usual, I turned straight to the Serviceman's Log, where I found the story about the baulky TC-2004. There was little similarity between 42 SILICON CHIP +160V + C564I T +24V Simplified circuit of the National 2004 chassis power supply, showing how the 160V, + 25V and - 25V rails are derived from windings on the horizontal output transformer, T552. that story and mine, except that they both involved a form of vertical trouble. Common factor But there was one common factor; one part of the vertical oscillator stage is powered from the 160V rail, the one that caused all the trouble in the TC-2004. So, with renewed hope, I set TETIA CORNER Ran k Arena C2205 Picture varies with volume control setting. Picture becomes weak or disappears, fine tuning changes, chroma weak but noisy, and sound clicks and pops - all as the volume control is moved. Finally, set blows rude raspberry when switched off. Cure: C351 (1 0OµF 16V electroly t ic) open circuit. This capacitor is an emitter bypass on the sound output transistor. The symptoms occur because the wrong feedback information from the output emitter upsets the 1 9V rail via TR301 , the audio driver transistor. Symptom: about checking this rail. Series resistor R559 , inductor L556, and diode D553 were all perfect, but when I came to the l0µF electro, C564, I reckoned I had struck oil. The negative terminal exhibited a perfect dry joint; I could move the capacitor and see the pigtail wobbling about in a hollow in the solder What this could do to the 160V rail, particularly in regard to generating spikes, doesn't bear thinking about. And, of course, such spikes could appear in the vertical oscillator and drive stages and damage components in the output stage. I set about resoldering the pigtail, but it didn't want to be fixed. There seemed to be some corrosion on it. So I removed the capacitor and got another surprise. There was a fair bit of its inside outside, with the rubber plug pushed almost out of its seating. It also had a heavy encrustation of dried electrolyte and had only about half of its original capacitance. I fitted a new one. I had re-stocked with UF-2 diodes so a new one was fitted at D406, plus a new C424 lO00pF capacitor across it, and a new 10 resistor at. R445 . At switch-on the set came good immediately, although the height was a little excessive and the vertical centering slightly out. Both responded to the appropriate controls and the set ran for several days before being returned to its owner. And - fingers crossed - it has given no trouble since. And that's J.L.'s story, which makes a very useful addition to the February notes. That 160V rail certainly has a lot to answer for . J.L. goes on to comment about the loss of sound in the February story, which I was unable to follow up. He points out that the 12V rail, which supplies the sound IF and the brightness and contrast controls, is derived from the opposite end of the winding which provides the 160V rail, and that they share a common chassis connection via a plug and socket. And he suggests that a spurious resistance in this connection could allow faults in the 160V rail to appear on the 12V rail. Well, it's a thought J.L., and worth keeping in mind. ~