Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG When it looks easy, it often ain’t Yes, it did look easy. There it was; an obviously dam­aged component clearly visible. All I had to do was find out why it was damaged. Although this would involve some searching, it turned out to be a much bigger search than anyone could have imagined. One of the most elementary methods of servicing has always been visual observation. Way back in the very early days of radio, when bright emitter valves were the norm, the first thing one looked for in a dead set was whether all the valves were alight. In fact, there were those who bemoaned the advent of the dull emitter valves, significantly more economical though they were, because they no longer provided this visual clue. Much has changed since then of course, but the visual clue remains a valuable one, even with today’s technology. The burn marks on a PC board, the bulging capacitor, the blackened fuse, the burnt resistor; they all pinpoint a fault area. And while they don’t necessarily pinpoint the fault itself, they can show one where to start looking. All of which is leading up to a particularly frustrating problem I encountered recently; the more so because at first glance – literally – there was a typical visual clue which should have put me straight on the right track. It all started when the customer turned up with a Samsung colour set, model CB-5012Z. This is a 51cm set using a P/58SC type chassis and was about three years old. The complaint was straightforward enough; the set was completely dead, having simply failed in the middle of a program. So, at the first opportunity, I put it up on the bench. I didn’t bother to switch it on but simply pulled the back off and looked for any obvious clues. This was relatively easy because all the parts are on a single PC board, the only reservation being that the components in the power supply section are very tightly packed. Two things were immediately obvious: (1) the mains fuse, F801 (3.5A), was blown; and (2) resistor R809 (270kΩ, 1W) was badly blackened. This resistor runs from the positive side of the bridge rectifier, at about 300V, to pin 4 of the switchmode power supply control IC, IC801 (TDA4601). And, as I subsequently dis­ covered, safety resistor R801 (5.6Ω 7W) in the mains input line, just after the fuse, had also been sacrificed. cial about all that. Fairly obviously, there was a short that involved all three components and, as such, I didn’t think that it would be hard to find. Anyway, the first thing to do was to replace the faulty resistors and I did this without even testing them. This done, I replaced the fuse and switched on with everything under close scrutiny. Splat! There was a flash of flame, a puff of black smoke, and I had another blackened resistor. Fortunately, a fast reflex action by my switch finger saved the fuse and the safety resis­tor. My initial thought was simply along the lines that a short at the IC end of R809 could produce such symptoms. I even went so far as to check for a short circuit between pin 4 of the IC and chassis; it was almost a reflex action. But then, on reflection, I realised that this didn’t make sense. Even putting a 270kΩ resistor directly across 300V would dissipate only about one third of a watt. So what was destroying the resistor? All I could think of was that a much higher voltage, from somewhere else in the power supply, was finding its way to this resistor. But from where and by what means remained a mystery. I went over the circuit around pin 4 of IC801 and resistor R809 but drew a complete blank. Finally, and somewhat against my better judgement, I decided that it must be a faulty IC. In any case, replacing it would prove the point, one way or the other. The only snag was that I didn’t have this particular IC in stock, so one had to be ordered. When it arrived a couple of days later, I lost no time in fitting it. This proved to be a somewhat tricky exercise due to the rather cramped conditions on this part of the board and the fact that the IC is mounted on a heatsink. Splat No.1 Splat No.2 Well, there was nothing very spe40  Silicon Chip Eventually, the job was completed Fig.1: the power supply circuit for the Samsung CB-5012Z. Fuse F801 is at extreme left, safety resistor R801 to the right, & the bridge rectifier to the right again. R809 is below the lower left corner of IC801 at top right, while C816 is mid-way up the right-hand edge of the diagram. and I made ready for another test. A new 270kΩ resistor had been fitted and I hoped all would go well this time. I pressed the power switch. Splat! Another flash of flame, another puff of smoke, and another black­ened resistor. I gave up! Well, almost but I certainly felt like it. Unfortunately, I had no choice but to keep at it and so, for want of a better ap­ proach, I simply began checking every component around the IC, either measuring then in-situ or removing them from the board for testing where necessary. I had checked a dozen or more components in this way, with­out result, and was beginning to question the wisdom of this approach when I found myself in the vicinity of transistor Q801, the power supply switching transistor. This was removed and tested but also proved to be OK. The next component was C816, a 222pF 1000V ceramic capaci­tor connected between Q801’s collector and chassis. This compon­ent is obviously a spike suppressing device. Because of the associated circuitry around it, I decided that this it would also have to be removed for testing. In fact, pulling it out was all the testing needed. It was mounted so close to other components that I could see only one side of it. But when I pulled it out and the other side became visible, I realised that I had struck oil. The case had split open to reveal a great black gaping crack. So at last I’d found the real culprit. But what, you may ask, did it have to do with resistor R809, which appears to be in no way connected with this part of the circuit. And if you are thinking of way-out explanations involving spikes in Q801’s collector circuit, forget it. Maybe there were some spikes but that isn’t the explanation. In fact, it was much more mundane than that and simply hinges on the proximity of C816 to R809. They were sitting side by side, virtually touching, with C816 lying slightly over the top of R809. So the smoke and flame I had observed had come from C816, not R809. And the blackening of R809? This was almost certainly a burn – not from internal heat but from external heat generated by C816. Remember, I mentioned earlier that I had not even bothered to check the “damaged” resistors. That was a fatal mistake. Had I done so, I would almost certainly have adopted a dif­ ferent approach. When I eventually checked all three of these resistors, they were spot on in value. There was nothing wrong with any of them. The damage was purely cosmetic and I had been well and truly conned. Rubbing in the salt But there was still some salt to be rubbed into the wound. I replaced C816, fitted a new resistor for R809 purely for ap­pearance, and switched on. And up came a perfect picture; the only thing that had ever been wrong with the set was C816. And it had carried a perfect visual clue but one which was impossible to see. Had I been able to see it, I would have simply replaced the capacitor and the set would have been back in operation in a matter of minutes. As it was, I wasted hours on the job and, financially, it was a total disaster; something which had to be written off to experience. In that sense, it wasn’t a complete loss. Apart from the obvious lessons, one other point emerged. I realised that there was a failure pattern emerging concerning the C816 type capaci­tor. Quite recently, I had also serviced a couple of Samsung chassis which carried the Akai label. Both suffered from the same fault – failure of a capacitor across the horizontal output transistor. And it was an identical capacitor: blue ceramic, 222pF, 1000V. In both cases, July 1993  41 SERVICEMAN'S LOG – CTD But my customer knew where; onto the power mains connected to his TV set. After that, the TV set didn’t go any more. I wonder if the Greek gods know about TV sets? OK, enough! But it was classic case of a mains lightning strike and this customer wasn’t the only one affected. When I pulled the back off the set, the damage was plain to see. The most obvious was the mains fuse, F801, 4A. The inside of the glass was totally blackened, suggesting a pretty violent strike, and I had no doubt that I would find more subtle damage as I went along. The other visual clue involved a line filter, L801, in the mains lead immediately following the fuse. The filter coils themselves were undamaged but the white plastic case which enclosed them had been blown to pieces. Since it was still work­ing, I decided to leave it until later. The fuse was replaced and I moved on to the next item down the line: the bridge rectifier involving diodes D801-804. Two of these four diodes had snuffed it and these were replaced. Switch-on they had simply developed a dead short and shut the set down without any fireworks. But from now on, I’m keeping my eye out for any faults which might involve this particular capacitor. It could well be less reliable than one expects from this type in general service. Further to that observation, I have been able to secure another make of capacitor which I hope will be less troublesome than the originals. When I needed replacement capacitors for the Akai sets, it was more convenient to order them from an independ­ent supplier rather than from Samsung. These not only carry a different brand but, more importantly, are rated at 2000V. These new capacitors were fitted to the Akai sets, as well as to the Samsung set which was the subject of this month’s story. Here’s hoping that I have struck a blow for my customers. hardly the set’s fault. No, the blame really lies with the great god Jupiter. In a fit of pique, “he hurled a thunderbolt into the air, which fell to earth he knew not where” (as they say in the classics). Jupiter strikes Fig.2: parts layout for the power supply in the Samsung CB-5012Z. R809 (circled) is situated between transistor Q801 on the right & C816 on the left. Note that, in practice, the board layout is much more crowded than this diagram indicates. My next story is also about a Sam­ sung set – a model CB-518F fitted with a P50HA chassis – and it also in­­­ volves visual clues. But I must hasten to add that this problem was 42  Silicon Chip OK, time for a switch-on test. This left no doubt that there was more trouble ahead. There were loud protestations from the switchmode section of the power supply, suggesting a serious overload. I immediately checked the HT rail for any suggestion of a short to chassis but could find nothing wrong. On this basis, and because all the faults so far had been at the input to the power supply, it seemed likely that the fault was still in this area. There are several more diodes in this section and I checked all these but found nothing wrong. The next suspect was the regulator IC, Q801 (STR50103A). But before taking a final step in this direction, I made a few more checks. I was able to measure some HT rail voltage – about 68V as compared to the 103V shown on the circuit (pin 2 IC Q801 and TP103) – and I also checked the horizontal output transistor (Q404, 2SD-1555) but this appeared to be OK. At that stage, I felt that I had gone as far afield as was reasonable for a strike of this kind, so I returned to the regu­ lator IC. This is a small device, having only five pins, and I had one in stock so it was a simple matter to replace it. ing circuit, caused the two coils to be pushed apart slightly, due to magnetic repulsion. They sat only loosely on the ferrite core. The movement wasn’t very great, and would not have caused any damage in the normal way. But with the massive surge that destroyed the bridge diodes and the fuse, the movement had ob­viously been much great­ er; enough to break the flimsy plastic cover. So that solved that particular mystery. Unanswered questions This photo clearly shows the crack in the back of C816. Also shown in the blackened fuse (F801) and one of the replace­ment resistors used for R809. But all that did was establish that there was nothing wrong with the original IC; the new one made no difference. I made a few more checks and found that the 12V rail was down in about the same proportion as the HT rail loss. This 12V rail is derived from a 16.5V tap (pin 2) on the horizontal output transformer via diode D408 and resistor R225 (47Ω, 2W). But there is also a 12V rail derived from the chopper transformer via diode D820, which is used as a starting supply to get the horizontal system running. And at this point I couldn’t be sure which of these two supplies was powering the system, to the extent that it was working at all. I also realised that, while all this analysis of the circuit was very interesting, it wasn’t really revealing anything that might help solve the problem. It was time to change tactics. let-down after all the chasing around the circuit. And what about the line filter I mentioned earlier? While the coils were undamaged in any way, the white plastic cover was scattered in pieces around the inside of the cabinet. How come? I found the answer quite by chance. The filter consists of two fine wire coils (or chokes) wound on small flat plastic bobbins, about the diameter of a 5-cent piece, but somewhat thicker. These in turn are mounted side by side on the centre leg of a rectangular ferrite core. And I noticed, when switching the set on after it was re­paired, that the switch-on surge, due to the degauss- But that still leaves other questions unanswered. Why did such a massive surge, having destroyed the bridge diodes in that part of the circuit, skip over the regulator IC and the horizon­tal driver stage, to pick on the horizontal output stage? And why didn’t it spread further via the supply rails and do a lot more damage? More importantly, from a practical point of view, why did Q404 test OK when it wasn’t? I don’t have any answers for the first two questions. I doubt whether anybody has – except Jupiter perhaps and he’s not telling. I don’t have a complete answer to the third question eith­ er, but there seems little doubt that the protective devices in these transistors (ie, the diode between collector and emitter and the resistor between base and emitter) make them difficult to test reliably. The resistor, in particular (normally 35-40Ω), makes it difficult Transistors cheat Speculating on likely component failures, my thoughts came back to the horizontal output tran­ sistor, Q804. Granted, I had run the meter over it and decided that it was OK. But it wouldn’t be the first time that such a transistor had cheated the testing procedure. As always, and as they used to say in the old valve days, the ultimate test of a suspect device is to replace it. Which was what I did, it not being a particularly difficult procedure. And that was it. The set was up and running in all its original glory. Which was both a relief and something of a July 1993  43 SERVICEMAN'S LOG – CTD Fig.3: the power supply circuit for the Samsung CB-518F. The mains on/off switch is at the bottom left of the diagram. Fuse F801 follows, then the line filter L801, the de­gauss circuit L802, and bridge rectifier D801-804. Next in line is chop­per transformer T801 then and switching IC Q801. The horizontal deflection circuit is at the top of the diagram. to determine the condition of the base-emitter junction. On the other hand, they don’t always confuse the issue; sometimes faults are quite readily detected. It all depends on the nature of the failure. So the rule seems to be if it tests faulty, then it is faulty; if it tests OK, it might 44  Silicon Chip be faulty, or it might not. Must try not to get caught like that again. Circuit diagrams One final comment. Unfortunately, the quality of the dia­grams in many manuals leaves a lot to be desired, and the dia­grams for the Samsung sets just discussed fall into this cate­gory. The main problem stems from the large size needed for many original drawings, followed by over reduction in an effort to accommodate them in a typical manual. This can create major problems when trying to trace a cir­cuit, while tracking down a difficult fault. Component values are often hard to read, particularly where figures 6, 8, 9 and even 0 (zero) are concerned. In a blurr­ ed reproduction, one can easily be mistaken for the other. Even more confusion can occur where circuit lines cross. While the concept of using small circular blob to denote a con­ nection, or no blob to denote a non-connective crossing, has the advantage of draughting simplicity, it falls down badly where the reproduction is poor. A certain amount of image spread can occur where lines cross, creating the impression of a blob where none exists, or giving rise to doubts as to whether a genuine blob is really only a blur. And take my word for it; it can waste a lot of time. Personally, I much prefer the more conservative drawing convention, which uses a loop to denote a non-connective cross­ing. The stated objection, of course, is that this requires more work and is therefore more costly. Well maybe it used to be but these days, with Computer Aided Drawing (CAD) programs, I doubt whether the difference is all that great. Anyway, for my money, the differSC ence is worth any extra cost.