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SERVICEMAN'S LOG We have good news & we have bad news First, the good news. It’s not often that these stories relate a complete win; a puzzling problem, a neat technical solution & financial satisfaction for all concerned. The hard ones seldom make much profit, so this is an exception. The set concerned was an Akai CTK-107, a 34cm set which is very similar to a Samsung CB-349F. And one of the hardest parts of the job was getting a clear description of the fault from the owner. About the only thing he was definite about was that it was intermittent in operation. But intermittent what? Complete failure? Loss of picture? Loss of colour? Loss of sound? No – it was none of these. Eventu­ally, after putting him through the third degree, I formed the opinion that it was a form of horizontal tearing, sometimes accompanied by streaking. So we left it that. When I put it on the bench and turned it on I was lucky for once; it put on a display immediately and was almost exactly along the lines I had envisaged. Unfortunately, the symptoms didn’t tell me much; they could have been due to a hundred dif­ferent faults. And of course, it came and went as it saw fit, lasting anything from a few seconds to a few minutes. I let it run on the end of the bench while I attended to other jobs, glancing at it from time to time, hoping it might display some other symptom. And it did – for one fleeting second, during a particularly bad bout of tearing, the picture suddenly changed shape. This new shape could best be described as a wedge shape, or keystone. In short, it had normal scan width at the top but tapered to a much narrower scan at the bottom. And, naturally, the colour convergence went completely haywire. Then, in a flash, all the symptoms disappeared and the set was back to normal. Mental block Now I should have known what it meant and I knew I should know. But, for the life of me, I couldn’t pick it. So I simply let it run. And it ran day after day without any sign of the fault. I was on the point of giving it back to the customer until some more drastic or permanent symptom appeared. Fortunately, he had another set and he indicated that I should keep it for as long as necessary. In fact, the set had to be put aside for a couple of days. When I set it up again, it came up with a perfect picture and so I let it run. 40  Silicon Chip Then, suddenly, I looked at it and there was a perfect keystone, this time apparently permanent. And that’s when the penny dropped. Of course –a deflection coil fault or, more precisely, a shorted turn in the horizontal section. I had seen one way back in the early days of monochrome TV and even remem­bered a reference to it in the textbook of the time: “Basic Television”, by Bernard Grob. (Some textbooks, including Grob’s, describe this shape as a trapezoid but all my references describe a trapezoid as having no parallel sides, which does not fit this effect. My best referenc­es suggest that it would be better called a trapezium, although there appears to be some confusion here too). Anyway, I unplugged the neck board, removed the convergence adjustment rings, and eased off the scan coils. And one glance was enough (see photo). The wonder was not that there was a fault; the wonder was that the set had worked as well as it had for as long as it had. OK, so I’d found the fault, But what to do about it? Both cost and availability were problems. Akai replacement parts can sometimes be hard to get and a new scan coil was going to cost around $100 or more. Combined with labour, the repair could well be uneconomical. What about a Samsung unit? Well, it should be available but might still be too costly. More importantly, would it be totally compatible? The two sets were similar but not identical. While musing thus, I suddenly remembered that I had a junked Sam­ sung tucked away somewhere and, if I remembered cor­rectly, the scan coil assembly looked very similar. In fact, the set turned out to be a Samsung CB-515F, a much larger 51cm model. On the other hand, the scan coils were visually identical, even down to the plug on the cable. But were they identical? Would they So that’s the good news for the month. The bad news is in the form of a letter from a reader, Mr K. E. of the ACT. It details his problems finding competent service organisations. This is what he writes. Tale of woe work on the smaller set? Well, it was worth a try, even though I wasn’t very confid­ent. So I fitted the coils back on the tube (rather roughly), followed by the convergence rings and the neck board. With everything back in place, I switched on and, to my complete amazement, the picture came up almost spot on. There was some static and dynamic convergence error but no more than one would expect from a proper replacement coil. It looked like a goer. And so it was. After a full convergence routine, I had a picture which was every bit as good as the original. So it was a win all round. It was a rare fault, with symp­toms that initially looked as though they could be due to almost anything. And then came a breakthrough when the fault obligingly identified itself. So half the job was done with almost no man-hours expended. Finally, a I had a suitable replacement part right to hand which made the repair economical. I charged the customer a modest fee, made a reasonable profit, and everyone was happy. I also learnt (or re-learnt) a couple of important points. First, I re-learnt the symptoms of a faulty scan coil and second, I learnt that a scan coil from one set could be used successfully on a completely different make and model. It is a point worth remembering, both in terms of these particular devices and as general rule. If two scan coils look similar, don’t be put off because they come from different sets. It is worth a try. I read R. Pankiv’s letter in the March edition and it imme­diately reminded me of a couple of odd problems I have had with two different electronic units. The first was with a VCR, the second with a Commodore computer. The recorder is a Teac MV-400. It was bought in a secondhand shop, where I saw it working, both recording and playing back. It was then about 18 months old and the shopkeeper gave it a month’s guarantee. The machine must have heard him because, guess what, the problem appeared six weeks later. It’s now over five years old and probably not worth fixing. It works perfectly well most of the time. In the fault condition, no matter what is done with the remote control or panel buttons, the tape will not run forward. There is no fast forward in play mode, no play function and no fast forward without the head engaged. Turning it off and on again, even at the power point, made no difference. But it would work after the mains power was off for a day or so! I put up with it for a while, then took it down to a nearby TV and VCR serviceman. I explained minutely what the problem was and he said: “OK, give us a couple of days.” A few afternoons later, he came back with the statement: “Well we cleaned the head. $25 please”. “What, was it dirty?” “Nah, not very. In fact it was pretty clean”. “What about the refusal to run forward?” He blinked. “That didn’t happen. I didn’t see that at all”. “That’s why I brought it here. I told you all about it. I explained at length; I told you the fault was intermittent”. What followed was a long spiel about sunlight and/or room lights shining into the cabinet and confusing the infrared sen­sors. Or it could be weak batteries in the remote control causing it to send out wrong signals, without being touched. It sounded like nonsense but I couldn’t be sure. He was the bloke who was supposed to know. September 1993  41 SERVICEMAN'S LOG – CTD One glance at the scan coil was enough to identify the fault in an Akai CTK-107 34cm colour TV set. The wonder was not that there was a fault; the wonder was that the set worked as well as it did for so long. I bought new batteries for the remote control and took the thing home. A few days later it was playing up again – same fault. I took it to another serviceman, told him not to clean the heads because that had been done, and told him exactly what was wrong. After a few days he said that there was nothing wrong with it. Eventually, I ran a Teac service agent to earth (no; not in the ACT). This time I took a big luggage label and wrote the fault details on it. This was attached to the mains cord so that it could not easily be ignored. I handed the same details on a sheet over the counter. After a week I phoned. It was clear that they hadn’t even looked at it. After another week I tried again. Guess what, they had cleaned the head. Three days later I called in. The spiel this time made more sense. In the digital control area, 5% tolerance resistors have been used and this can result in one which is 42  Silicon Chip just slightly too high. It’s no problem if the resistor is at the lower end of its tolerance range. But if you happen to have a slightly high one, sometimes it’s a bit too high. This makes the control circuit think it sees the end of the tape. The resistor is buried so far in the depths that it would be a major job just to get at it. Which resistor is it? That I never found out but a tempo­rary cure is to disconnect the sensor just to the left of the tape carriage. The VCR will then play but not record. I decided that enough was enough. I took the damn thing home and it’s been playing up on and off ever since. It hasn’t been near any serviceman either. Well, that’s E. K.’s tale of woe – a little edited – about the Teac recorder. I don’t propose to deal with the computer problem. I am not “into” computers and would not do it justice. But what a tale of woe about the recorder. I think it best if I deal with it at two levels: (a) the treatment by the various service organisations, and (b) any thoughts of my own on the purely technical aspect. In regard to the service organisations, it is a tale of ineptitude, technical gobbledegook fob-offs and, overall, straight-out dishonesty. And all three organisations had one thing in common: they did not observe the fault or, more importantly, make any real attempt to observe it. It is virtually impossible to tackle a fault which cannot be observed and intermittent faults often call for a lot of patience, just to reach this point. But none of them was prepared to exercise such patience. They displayed what I regard as an “intermittent block”; a fai­lure to recognise the word, at least insofar as it applies to technical problems. The word is brushed aside, or totally ig­nored, and the equipment serviced solely on the basis of what is observed when first turned on. Which is just another way of saying that the customer’s comments are totally ignored. Granted, these can be rather weird at times and often largely irrelevant, but seldom totally so. Somewhere in their dissertation there will be some useful snippets of information, often quite vital. It is the serviceman’s job to sift the wheat from the chaff. But never ignore the customer’s story; you do so at your peril. Of course, sometimes the customer won’t talk, but that’s another story. Unfortunately, this attitude is encountered all too often, and is responsible for the many complaints by customers that a service organisation, “... charged me (so many) dollars and didn’t fix the fault”. The explanations As for the explanations offered, they are also typical of this approach; pure technical gobbledegook, designed to blind the customer with pseudo-science. The first one, about light confusing the sensors, is a partial truth. It has happened to me but only when the recorder is out of its case on the bench. When it is back in its case, it would be a strange lighting arrangement indeed which could cause such an effect. The suggestion that it was weak batteries in the remote control unit was, as K. E. suspected, pure nonsense. It is not worthy of comment. The service agent’s explanation Fig.1: this diagram, from a Panasonic training manual, illustrates the various transport control and safety functions normally found in a video recorder. While the unit discussed would differ in detail, this will help the reader follow the story. was more refined, at least to the point where, initially, it seemed to make some sense. But it doesn’t stand up to close examination. If the idea was anything more that spontaneous guesswork, then it should have been at least possible to nominate the resistor or, at least, the ones most likely to be involved. As K. E. himself asked, which resistor? And as for them being too hard to get at – well, there are many components which are hard to reach but I don’t believe there are any which are too hard. And it wouldn’t be the first time I have had to pull something apart to get at a suspect component; and then found that it wasn’t the culprit after all! All of which adds up to a situation where the three organi­ sations have performed a gross disservice – to both the customer and the industry as a whole. What more can one say? Technical aspects And what are my thoughts on the technicalities of the prob­ lem? Not very much, I’m afraid. Unfortunately, it is a make and model which I know little about. I don’t recall ever having handled one and I have no service manuals or even a circuit. I flogged the problem to a number of colleagues, hoping to score either some literature or a comment based on experience. Unfor­tunately, I drew a blank on both. And since servicing by remote control is hard enough at any time, these limitations make it almost impossible. I can only comment on the broad basis of all such machines, although the details vary considerably between makes. To help in this regard I am reproducing a drawing from a training manual put out by Panasonic, covering the NVG-20 and NVG-21 series recorders. While undoubtedly differing in detail from the Teac, the information is basic and should help the reader to follow the story. It gives a skeletal portrayal of the microprocessor, with the associated control and safety functions likely to involved in a fault of this kind. At top left are the two end-of-reel phototran­sistor sensors (take-up and supply) and their LED light source. Below this is the safety tab switch, the dew sensor, the cassette switch, and the reel movement sensor. This latter is another photosensor device, September 1993  43 SERVICEMAN'S LOG – CTD provided to shut the system down if a reel is not rotating when it should. On the right is a rotary switch, called the mode select switch. We will have more to say about this later. One of the significant aspects of this case is the fact that the failure involves tape movement in one direction only: forward. This might suggest an end-ofreel sensing failure; the only seemingly sensible suggestion hinted at by the first serv­iceman but in a nonsensical context. So let’s assume that an end-of-reel sensor fails; ie, goes open circuit. Normally, with a tape loaded and in mid-reel, neither end sensor photo­ 44  Silicon Chip tran­s­istor will see the sensing light source; they will see it only through the clear tape at one end or the other (some tapes do not even have this refinement). So, failure of the phototrans­istor, or associated circuit, to “see” the light source, would not halt the tape movement; it would have contrary effect. Now let’s consider the reverse possibility; a leakage or short circuit in or around one of the phototransistors –par­ticularly the supply reel one – such that it thinks it is seeing a light continuously. As a result, it tells the microprocessor that the system has reached the end of the tape and inhibits all forward movement. But it wouldn’t inhibit reverse movement, because this is what would have to happen in this condition; the tape would have to be rewound. And K. E. provides a clue to support this theory. He says that “... a temporary cure is to disconnect the sensor just to the left of the tape carriage. The VCR will then play, but not record”. Assuming that he has identified the supply reel sensor, then this theory would seem to fit, at least as far as the transport problem is concerned. On that basis I would suggest that replacing the phototransistor would be the first thing to do. They are worth only a few cents and it would quickly settle this point. But this still leaves the mystery as to why it won’t record with this improvised cure. One might hope that replacing the phototransistor would cure this problem also but I very much doubt it. I cannot see any connection bet­ween this part of the circuit and the recording function. So do we have two separate faults and if so, why hasn’t the recording fault been observed before? Or is the whole theory of a faulty phototrans­ istor wrong, in spite of K. E.’s observa­ tions? All right, if the theory is wrong, what else do we have? The most likely culprit – and at least one colleague plumps most strongly for this – is the mode select switch. This is a mechani­ cal switch, sometimes a rotary type, sometimes a slide type, activated by the recorder mechanism, according to the function selected by the user: play, fast forward, rewind, etc. A major reason for suspecting this is that it is a known source of trouble – not frequently but often enough to put one on guard. And when it does play up, it can produce some weird faults. So this would be the next thing to check. With a few excep­tions, they are not particularly expensive and are relatively simple to fit. But one or two are a mite pricey and at least one is quite critical to fit, creating its own weird effects if it is not precisely mounted. So there it is E. K. It’s the best advice we can offer at this distance. Maybe it will help but if it doesn’t, at least I didn’t charge anything – not even for SC clean­ing the heads!