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SERVICEMAN'S LOG Tread carefully with a new brand name How does one cope with a set of unknown brand? Is it an orphan brought in by an overseas traveller, or is there a local importer? If so, what backup service in the way of technical data & spare parts is available? In an extreme case, where the set is an orphan or nothing can be found out about it, there is often little option but to bow out right at the start. It may not do much for one’s reputa­tion in the short term but the alternative is to risk much great­er damage. If lack of data and replacement parts means that 32  Silicon Chip the job ultimately has to be abandoned anyway, the customer is no better off and the serviceman in down the drain for his time. Of course, all this is leading up to the fact that some time ago I encountered a set carrying a brand name I had never heard of and had to go through the above mental gyrations in order to decide how to handle the situation. While that is now history, what followed is technically interesting and I thought it worthwhile to point out the need to investigate the background in all such cases. Unknown to me at the time, the story really started a couple of years earlier when a local motel changed hands. The new owner had come from Victoria – a point of some importance as it turned out – and one of the first decisions he made when taking over was to replace all the TV sets. It was a logical decision. From what I knew of the original setup, the sets were approaching the end of their commercial life anyway (life in a motel can be pretty rugged at times) and were all VHF-only models. With a significant number of guests now wanting to watch SBS, the lack of UHF was a serious shortcoming. So, after carefully studying what was available in his state of origin, he settled for 12 Contec 51cm colour TV sets, model MSVR-5383. And this was what I was presented with when he approached me sometime later to undertake the service of these sets. From a business point of view, of course, it was an attrac­tive offer. But, initially, I hesitated to be become involved. I had never heard of the Contec brand and needed to be reassured along the lines already discussed, before committing myself. As it transpired, the owner had done his homework pretty well. The sets had been purchased from a Melbourne firm, Freecor International and, yes, he had investigated the service and backup situation and was able to give me the name and phone number of the manager of their service organisation. Which wasn’t a bad effort. But he’d gone one further; he had secured some circuits. And as circuits go these days, they are quite good for the most part. The only snag is that, in the original, -31V 5V 4 F 3 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 114.9V C 5 6 Fig.1: the power supply circuitry for the Contec MSVR-5383 (note: primary side of the switchmode supply not shown). some parts have been shaded, leaving a dot pattern which can make some values hard to read. However, with all that information to hand, I felt reason­ably confident about tackling these sets. And, initially, most of the faults turned out to be fairly routine. But then, about 12 months ago, I encountered the first difficult one, which I am about to relate, followed by a couple of real weirdos. More about those in notes to come. Lost stations So what was this one? The customer’s complaint was that if the set was turned off at the power point, as can easily happen when motel rooms are being serviced, then all the channels pro­ grammed into it would be lost. To get it working again, it had to be reprogrammed. But this only occurred if the power point was turned off; it did not occur if the set was turned off into the standby mode. With the set on the bench, I found the owner’s description quite accurate; the only additional factor, which the owner himself added, was that the power point had to be off for someth­ing like half an hour before the memory D516 C C523 C 1 2 3 Q508 Q509 was lost (the exact reason for this is still a mystery). And so, having confirmed the situation, I was faced with the problem of where to start look­ing. I had never encountered such a fault or anything remotely like it at that time. Nor had I encountered any literature ex­plaining in detail how these systems worked. I turned to the circuit for inspiration but it didn’t help a great deal. All I learned was that it was most likely in or around one of three ICs: IC801, IC802 and IC804. IC801 was the 42-pin central processing unit (CPU); IC802 was a 14-pin unit with internal boxes captioned “memory transistor array”, “address register”, “address decoder”, etc; and IC804 was a 16-pin unit that contained an oscillator, a display timing generator and various other circuits. Both these latter ICs were closely asso­ciated with IC801 (the CPU). Of those three, IC802 looked the most likely possibility. But that was – at best – little more than an educated guess. I needed more than that; I needed some real help. I decided it was time to put the owner’s backup research to the test. I rang the service organisation and was put through to the service manager. And that was a real bonus, because he turned out to be most co-operative and was familiar with many of the firms and personnel that I dealt with in Sydney. More importantly, he was a mine of information about the set. As soon as I described the symptoms, he was onto them. And I was right about one thing; it did involve IC802. He drew my attention to pin 2, which is marked as -31.65V. This voltage is derived from a small 50Hz power supply, based on transformer T501, on the main power supply board. This is the standby supply, which means that it is activated while ever the power point is turned on. It provides the -31V rail from pin 8 of T501 via a diode (D510), a 47µF filter capacitor (C512), a 1kΩ resistor (R519) and a zener diode (IC501). This goes out on pin 4 of plug/socket F. Subsequent analysis revealed that this supply also provides a regulated 5V rail for the CPU and the remote control receiver, from pin 7 of T501, via IC502 and a simple filter network. This goes out on pin 3 of plug/socket F. And there is a 12V supply for IC904 (the audio output stage) from pins 3 & November 1994  33 SERVICEMAN’S LOG – CTD 4 of T501, via a full-wave rectifier and regulator transistor Q506. This goes out on pin 1 of plug/socket E. Having pinpointed pin 2, the service manager came straight to the point. “Check that 31V rail. You’ll probably find it either zero or very low. We have had cases where IC802 has failed internally and taken out the 1kΩ resistor in the power supply”. Well, that was about the most succinct diagnosis I can remember. I thanked the gentleman and went back to the bench. And he was dead right; the voltage on pin 2 was down to a couple of volts and the cause was R519, which had gone very high. But there was more; diode D510 was also faulty. Not completely open cir­cuit; more partially broken down. It may still have been provid­ing some rectification. After that, the job was pretty much routine. I replaced R519 and D510 and fitted a new IC for IC802, and we were back in business. Of course, the set had to be reprogrammed but once that was done, it would hold the program regardless of the condi­tion of the power point. So the set went back to the customer and 12 months later it is still behaving itself. But it was a valuable experience, and I learned a great deal from it. And the experience was to prove invaluable more recently when, as I have already hinted, there were more problems in this area but with quite different symptoms and different causes. More about those in future notes. Another motel set My next story concerns a Samsung 34cm colour set, model CB-349F, one of several belonging to another local motel. In fact, these sets have featured in these notes before. The complaint – or rather the problem – was lack of bright­ ness. And I make this distinction because the complaint was poor colour; they claimed they couldn’t adjust the colour properly. Taken at its face value, such a complaint would suggest lack of colour saturation. Unfortunately, some people have diffi­culty in differentiating between colour and brightness – it’s 34  Silicon Chip all the same to them. In fact, there was no colour problem; it was simple loss of brightness. Where the loss is only slight, one might be tempted to adjust sub-brightness control VR203 – a 2kΩ pot. Another pos­sibility is to increase the setting of the “screen” or G2 voltage control, or even try adjusting both. However, these tricks smack of a quick fix approach and are best avoided in most cases. But it is wise to check these two controls, in case someone has had a fiddle (it does happen) – they should be in about mid-position. In any case, unless the G2 voltage is significantly below normal, it is best left alone. There was no temptation in this case. The brightness loss was considerable, which clearly indicated a fault of some kind. And I had a pretty good idea where it would be. The most frequent cause of this problem in these sets is resistor R208, which has a nasty habit of going high. It is part of the beam limiting circuit and is connected between the 125V HT rail and pin 4 (pedestal clamp) of the main IC, IC501. (It’s a swine of a thing to find on the circuit, being tucked away down below the horizontal output transformer). Its value is not given directly on the circuit but in a table on the side. For 14-inch and 16-inch tubes – which covers this one – it is given as 127kΩ 0.5W. For 18-inch or 20-inch tubes, the value is 110kΩ 0.5W. And it has something of a history. Back in May 1990, this resistor was mentioned in a Samsung service note, advising that it be checked for an increase in value. In fact, several such cases were found. At the same time, there was some initial confu­sion as to what value these resistors were supposed to be, since they carried a colour coding which didn’t seem to make sense with the values in the table. However, that’s all by way of background because, in this case, the resistor measured spot on. From there, I checked the G2 voltage and found that it was down significantly. And I seemed to recall that there was another common fault which produced these symptoms but, for the moment, I couldn’t remember the details. A caffeine fix helped and the memory suddenly clicked. Of course, a capacitor on the neck board – in particular, capacitor C519, a 330pF 1kV disc ceramic which bypasses the G2 line to chassis. As soon as I saw it, I was even more convinced that I was on the right track; it was a blue disc ceramic and I recalled encountering these in the power supply on several previous occa­ sions, where their tendency to leak caused some nasty problems. And I had mentioned their unreliability in these notes at the time. Well, that was it. I pulled it out, confirmed the leakage, and fitted a new one. And that was it; normal brightness returned and I had another satisfied customer. Naturally, the new capaci­tor was a different brand to the original – one about which I feel a lot more confident. It hadn’t been any big deal but it did make me think about some of those previous faults which I had temporarily forgotten. One’s memory needs to be jolted from time to time. The HMV portable To finish off, here is another story which in itself was no big deal. In fact, it was little more than routine but I decided to tell it because it presents an opportunity to discuss a couple of important points. The set involved was an HMV portable TV set, model 8010501, a 34cm unit actually made by JVC. It’s getting a mite long in the tooth now, being some 15 years old at least, but is still a goer for all that. The customer’s complaint was straight to the point; no picture and no sound. When I set it up on the bench this proved to be literally true. But there was a raster and some white noise from the speaker; it wasn’t much use to the customer but was quite valuable as far as I was concerned. Even more valuable was a very prominent hum pattern on the raster and the fact that this was also shrunken on all four sides. So it didn’t take a genius to deduce that we had a power supply problem. And finally, there was the HT rail. This should have been around 110V but was actually only about 80V. Likely causes? The first thought is almost automatically to blame the main filter capacitor but there are other possibili­ties. For example, failure of the voltage regulator transistor I had measured it proper­ly, or whether there was a fault in the tester. A few quick checks soon ruled out those ideas, so I patched it back into circuit and gave it another try. I could have saved my time; it simply would not work. Well, it’s not the first time I’ve struck such contradic­tions. And it emphasises the old rule that the final test for any component is whether it will work where it is supposed to work. So what was wrong with the capacitor? It could be one of several faults that show up in electrolytics but my guess is that it was suffering from high internal resistance. It is a known fault and it means that, even if the capacitance value is cor­rect, it cannot charge or discharge fast enough to provide the required function. Capacitor compatibility can produce a variety of symptoms, including those listed above, this depending on the exact nature of the fault. Another possibility is a faulty bridge rectifier. Failure of one diode will result in only half-wave rectification, with reduced voltage and lots of hum. But these thoughts were quickly put on hold. Time enough to worry about them if the most likely culprit was cleared. So I went straight to the main filter capacitor, a 600µF 180V electro­lytic. The easiest way to check this is simply to clip another one across it. But hold on – not while the set is turned on. A large value discharged capacitor is, in effect, a short circuit and connecting it across a HT supply with another capacitor already in circuit will create an almighty splat. And the spikes such a splat can generate on the HT rail can produce some unpleasant surprises – like defunct ICs and transistors. And so I switched the set off, fished out an appropriate capacitor and patched it into circuit via a couple of clip leads. And that was it; when I switched the set back on, we had 110V on the HT rail, normal picture and normal sound. But while I considered the point proved, there was one surprise. When I pulled the old capacitor out, I put it on the capacitance tester. And according to that it was OK; it measured just a whisker under its rated 600µF. For a moment, I wondered whether So that clarified the diagnosis. But it was not quite the end of the job; there were some practical problems still to be solved. The test capacitor I had used was physically incompat­ible, as were all the others of suitable value which I had in stock. The closest would fit in the space OK but its leads did not match the mounting holes in the board. And considering the age of the set, finding an exact replacement would, at best, call for considerable time and effort; time which would cost money and inconvenience the customer. In these circumstances, I felt that a certain amount of improvisation would be justified. In fact, it wasn’t all that difficult. There was enough space around the mounting area, on the underside of the board, to permit drilling a couple of new mounting holes to suit the replacement capacitor. The copper pattern was cleaned around these and the new lugs soldered to them. It made a perfectly satisfactory job, with a minimum of delay. But there was a rather interesting aftermath. A couple of weeks after I had finished the job, I came across the faulty capacitor on the bench and I hooked it up to the capacitance meter again. And this time it read about 200µF. Later again, when I came to write these notes, I tested it again and it measured virtually zero. Well, it had taken a long time to completely die but it had finally given up the ghost. And good riddance. SC November 1994  35