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All this • ID a brand new set Most service faults fall into a particular pattern: the fault in a brand new set; the intermittent fault; the multiple fault; and even the contradictory fault where replacing a faulty component makes the situation worse instead of better. Most of us have experienced these from time to time, but how many have found them all in one set? Well, that's the gist of my main story this month and I think it sets some kind of record. See what you think. It all started with a call from a dealer colleague for whom I do service work, including warranty work on new sets. This concerned a brand new TV set - a National TC2258 - which he had just taken out of its carton, for display on the showroom floor, only to find that it appeared to be completely dead. And, since National is one of the brands I am authorised to service, it was clearly my baby. My colleague duly delivered the set and I dug out the appropriate manual. As well as the model number already quoted, some readers may recognise the set as using the National chassis type M14H. The relevant portion of the circuit is reproduced herewith to assist readers in following the story. My initial reaction to the job was somewhat blase - I assumed that it would be a relatively straightfor- t)J ALL 11-\lS IN 62 SILICON CI-IIP ~ ~N't> ward fault without any serious hassles. After all, it was completely dead. A preliminary check revealed that there was a normal HT rail at around 113V, but no sign of horizontal deflection, EHT, or any other functions derived from this part of the circuit. I stoked up the CRO and began checking through the horizontal circuit. The horizontal oscillator is part of of a 'jungle' type chip, IC601, AN5600k-R, a 42-pin monster which delivers the horizontal oscillator signal from pin 41. This part of the circuit is not shown here but the output from this pin goes to the base of transistor Q500, the horizontal driver stage. This drives the output stage, Q501 , via transformer T500 (bottom left hand corner of circuit). The CRO established that the signal was coming out of the chip OK and was being applied to the base of Q500. But that was all; there was nothing at the Q500 collector. Nor was the reason hard to find; a quick check with the meter showed that there was no voltage at this point. This voltage is normally applied via R551, an 820Q safety resistor, although the purpose of the safety resistor in this line is far from clear. Anyway, this was the problem, the resistor being open circuit. Judging by appearances, this resistor might have been nothing more than a conventional 2W metallised type but, in any case, I had nothing like this value, quite apart from any special qualities it might have. Anxious to get the set working, I made up a string of three 1W resistors: two 330Q and a 2200, making 880Q, which I reckoned was near enough for a test. Protesting squawk ~ SE"t".... I fitted the string in place and switched on. The result was a loud ,j. D30, "'"" . .,,. IO<) - . "", · 0, 47 ffiID ''" 560P500V "" TlHl5768 IC401 AN5521 l502 =-tl.1!5lA02 r ~p SOO' Fig.1: relevant portion of the National TC2258 circuit. Transistors Q500 and Q501 are at bottom left, Q503 and Q504 at bottom right, and D510 and TPE21 roughly top centre. protesting squawk from the line output transformer, then silence; the set was as dead as ever before. Further checking revealed that the horizontal output transistor, Q501, had failed. Fortunately, I had a replacement type on hand, and this was duly fitted. But, before trying again, I did what I should have done the first time and checked the value of the substitute resistor string. I need hardly add that Murphy had been at work. What I had taken for 2200 was, in fact, a 22000 resistor with a somewhat dubious colour band. My immediate reaction was to wonder whether this mistake on my part had caused the output transistor to fail, or whether it had been faulty all along, but only became apparent when I restored the voltage to the driver transistor. This was the first of many such questions I was to ask myself before I finshed with the set. I fitted a correct value resistor, crossed my fingers, and switched on. And this time all was well - no signs of protest, normal sound, and a first class picture when the tube warmed up. The only snag now was the 8200 safety resistor. I had ordered a replacement from National, along with other parts, but when the order arrived there was no resistor - it had been put on back order. Naturally, I didn't want a brand new set to go out into the field with a bodgie string of resistors in it but, on the other hand, the dealer wanted to put it on display. The upshot was that I explained the situation to him and we agreed that I return the set to him for display but on the understanding that the resistor would have to be replaced before the set was delivered to a customer. And that was more or less that or so I thought. I'm not quite sure what happened over the next few days except that it appears that my dealer friend must have left the set switched off most of the time, turning it on only when a customer showed interest in it. Anyway, it was some time before he had occasion to run it for any length of time. When he did, he was on the phone in short order. It appeared that the set had run for only three quarters of an hour, then stopped. He had switched it off for about 10 minutes, then switched it on again, whereupon it had played normally for another three quarters of an hour. He had found that this pattern could be repeated indefinitely and that a pause of as little as five minutes could restore performance, but that this seemed to shorten the playing time. And so the set landed back on my bench. My first move was to simply run it to confirm the dealer's description, and to watch for any symptoms which might provide a clue as to the nature of the fault. This approach paid off. After about 40 minutes I noticed that the picture was starting to jitter slightly from side to side, suggesting possible instability in the horizontal circuitry. A few minutes later there was another protesting squawk and the set shut down. Then, just as the dealer reported, switching the set off for a few minutes was all that was necessary to restore performance. So what now? There was little doubt in my mind that it was the horizontal system failing, the real question being why. Once again I decided that a CRO would be the best form of attack, at least initially. And this time, I selected a triple trace instrument, a BWD Model 525. I connected one probe to the base of transistor Q500, one to this transistor's collector, and one to the collector of the output transistor, Q501. With the set running cold, the waveforms were pretty much as indicated in the service manual, so it was simply a matter of waiting until the picture started to jitter. My first attempt was not very successful. I was concentrating on the output stage waveform when DECEMBER1987 63 the picture started to jitter, but this seemed to be perfectly stable. It was only a moment before the set shut down that I looked closely at the waveform at the base of Q500, and realised that it appeared to have changed shape. The change was not very marked and I put the set through a couple more cycles before I was sure that this was so. When I was, I removed the probe from the collector of Q501 and connected it to the horizontal output pin, pin 41, of IC601. Then I put the set through another cycle. This time, results were more encouraging. Initially, the waveform at pin 41 was very close to that shown in the manual, in both amplitude and shape, although it differs significantly from that at the base of Q500, due mainly to the RC network in the base circuit. But as the set approached its shut-down condition, the waveform at pin 41 changed significantly, both in amplitude (which was decreasing) and shape. However, as I noted previously, this had only a marginal effect on the waveform at the base of Q500. Nevertheless, as the signal from pin 41 continued to change, it eventually reached a point where the set shut down. Which was all very interesting up to a point, but what did it mean? My knee-jerk reaction was to blame the chip but, while I didn't entirely rule out the possibility, I quickly put that idea on hold. I am coming to the conclusion that most chips are pretty reliable these days and I am less inclined to replace them than I once was. In any case, the idea of unsoldering 42 pins doesn't particularly appeal unless all other possibilities have been exhausted. At this point I took some time off to study the circuit and form a clearer picture of how this part of it worked. This didn't seem to help much at the time, although it did prove useful later. But it did inspire me to make a voltage check of the chip, both when the set was :.unning normally, and when it was about to fail. Initially, all voltages were within normal tolerance, so I let the set run for about half an hour, then checked them again. Most of them 64 SILICON CHIP -- . A-r Tl-lAT TIME 'I l)E:Ol)e:D lT ~s TIME TO A'SAN't>OI\) -ms SC\ ENT\l=\C A??~OAC H P\ \\\'t> Re~oR"t TO il'\E ?1<\N\\1" \V~ ••. . showed only minor differences but one had dropped significantly. Pin 42 is marked 8.5V on the circuit and had actually read 8.35V when the set was cold. But now it was down to 6V and still falling. When it reached 5.4V, the set shut down. Now I felt I was getting somewhere, and my previous study of the circuit was proving useful. As nearly as I can make out the 8.5V on pin 42 is the supply for the horizontal oscillator. It is derived from the 113V rail via R511, a 6.Bkn 3W resistor (approx. top centre of the accompanying circuit). From this point, a line runs (left) to pin 42, and right to R536 (1000} and thence to the emitter of Q503 (bottom right). Protection circuit Now Q503, together with Q504, is part of an over-voltage protection circuit, designed to operate if the main HT rail should rise significantly above 113V. It works like this. Near the top centre of the circuit is a voltage divider from the 113V rail to chassis, consisting of a 18 7kQ resistor (R527), and a 20kQ resistor (R528}, both 1 %. The voltage at the junction (no load) should be approximately 10.9V. This junction in connected to a zener diode, D510, the exact value of which is not stated but is obviously somewhere around 10.9V. The other side of the zener goes to the base of Q504, which normally has no forward bias and is turned off. If the voltage rises at the zener junction, the zener will conduct, turn on Q504 and Q503, and pull down the 8.5V rail feeding the horizontal oscillator. As a result, the horizontal circuit and those circuits operating from it are shut down. At least, that was what I deduced from my study of the circuit. Some of the details were obviously still missing, but I reckoned I had enough to go on for the time being. There was either a genuine overvoltage condition on the HT rail, causing the shut-down circuit to function as intended, or there was a temperature sensitive fault in the protection circuit which was creating the false alarm. I had already checked the HT rail on several occasions and it had always been spot on, but I went through the motions again with special attention to the shut-down condition. I also checked the voltage at test-point TPEZ 1, and even the EHT. They all remained rock steady right up until the set shut down. I did note, however, that the voltage at zener D510 was somewhat lower than I calculated for a simple divider arrangement. So it looked like a false alarm. All I had to do was find out why. My first step was to apply some freezer to what I felt were likely to be temperature sensitive components in this part of the circuit: zener diode D510, zener diode D502, diode D513, and transistors Q503 and Q504. None of the diodes responded to this, but both transistors did. If either one was sprayed as the 8.5V rail was dropping towards shut-down, the voltage would rise to normal. This proved to be a rather surprising finding, in view of subsequent events. Next I monitored the various voltages applied to Q503 and Q504 as shut-down approached. I checked them first in the cold condition and found them to be virtually spot on. Then I let the set run and watched for any changes. An obvious change was at the emitter of Q503 which was virtually the same point as pin 42 of the chip, so this fell gradually as shut-down approached. The same applied to the collector of Q504, and the base of Q503, which are connected to the same line. But the real puzzle was that there was no change to the voltage on the base of Q504 (and the collector of Q503), where one would expect to find the 'alarm' voltage real or false - needed to turn on these two transistors and pull down the 8.5V rail. At this point I decided to abandon the scientific approach and resort to the primitive; ie, check each of the dozen or so components in this part of the circuit individually. The resistors were easy enough to check in situ, and all came up well within tolerance. Transistors, on the other hand, are best removed from the circuit for testing. I pulled Q504 out first and found that it was not only faulty, but faulty in a rather unusual way. It had a base to emitter short and an open circuit collector. The discovery was gratifying, of course, but was also puzzling. I couldn't relate the fault to the symptoms and, in particular, I couldn't reconcile the discovery with the fact that spraying the transistor with freezer appeared to cure the fault. But there was no point in dwelling on this. I fished out a suitable replacement transistor and fitted it , then tried the set again. Result: the set wouldn't even start. So it was back to the component by compo- nent check. I lifted zener diode D502 and it checked OK. Then I tried D513, and this proved to have a high resistance leak. So that was replaced. Still the set refused to work. I removed Q503 and checked it, but could find nothing wrong with it. I was feeling deperate now and decided to lift zener diode D510 and thus render the protection circuit inoperative. This took only a moment and the set came good immediately, with all voltages and waveforms normal. What was more, it continued to run for the next several hours, with no significant change to any of these parameters. wrong value replacement for R551? Apart from stating that I don't think this last failure happened in this way, I really can't answer these questions. In a sense, I suppose, all this is rather academic. The faults have been found and the set repaired, and that is all that really matters. At least that is the practical approach and, as I have commented on previous occasions, there is a limit to how much time one can spend mulling over the whys and wherefores of circuit behaviour under fault conditions. No more squawks Which didn't leave much to suspect except the zener, D510. As I said earlier, there was no indication as to the exact value of this, but I reasoned that it was probably a 10.BV type. So, without bothering to check the old one, I fitted a 10.BV type out of stock and tried again. And away went the set like a bought one! Fortunately, the manual describes a check for the protection circuit. It calls for the application of a progressively higher voltage, from an outside source, to test point TPE21, until the set shuts down, which it should at about 11.BV. I did this and it came out spot on. And that was more or less the end of the job. I ran the set all day for the next couple of days, then returned it to the dealer. It has given no trouble since . Naturally; I was glad to have solved the problem and to have the set off my hands, but I was less than happy about the reasons for what I found. To start with, why were there so many faults in one set, and a brand new o:i:J.e at that? A faulty safety resistor (R551), a faulty horizontal output transistor (Q501), a faulty protection transistor (Q504), a faulty diode (D513), and a faulty zener diode (D510). Did all these faults occur independently, or did one fault trigger all the rest? Or perhaps only some of them? And if one fault triggered the rest, which one was it? And did I destroy Q501 by fitting a 'RE..\I\Vll'JG ~ FA\n\FU\Ol-!) 'Pt\\\..\~S K9 .... And now for a change of scene from a very new set to a very old one: the old faithful Philips K9. The owner had been a customer since he bought the set, over 11 years ago now, and while it has developed the usual faults common to this model, it has given good service and has plenty of life yet. This particular problem is interesting because it demonstrates that there is always something new to be encountered, even in a model about which one imagines one has seen all the tricks, as I did about this one. It also emphasises that it pays to be suspicious, perhaps more so than I was on this occasion. The fault itself was fairly straightforward; the set was completely dead due, in turn, to the fact that the power supply was also DECE/IIHE/l 1987 65 VL 170 :~ C175 ....--->,;N,--_, I I I I I !~ ;~ I I I Fig.2: skeleton circuit of the Philips K9 power supply switching transistor network. Why was it tough on the transistor? completely dead. Since this is not an uncommon situation with this model, I keep a spare power supply board on hand for a quick test. By plugging in a known good board I can quickly establish whether the power supply is itself faulty, or whether it is being shut down by a fault elsewhere in the set. In this case the test clearly indicated a faulty power board and, from there, it was a short step to pinpointing the real culprit. It proved to be the switching transistor, TS170 (2N472), which had shorted collector to emitter and blown the 2A fuse , VL170, in the process. Simply replacing these two components would have been enough to put the set back into operation, but experience has taught me that there are several other faults which can initiate this failure (note: TS 170 can be a BUl 26 in some models, or even a BU326 if the original has been replaced in the field}. The main off enders are dry joints, particularly in those parts of the circuit where they can cause sparking and spikes. Joints to transformer T182 are common offenders , as are those to the main smoothing capacitors, Cl 78a and Cl 78b. All these possibilities were thoroughly checked and ruled out, the faulty components replaced, the set given a test run, and returned to the owner. All went well for about three months and I had virtually forgotten about the job until the owner turned up at the shop again with the K9 in the back of his utility. It was completely dead again and it turned out to be exactly the same fault. Naturally, I was suspicious, and I went over the board again, determined to make sure there was nothing I had missed the first time around. In fact I found nothing, and I was forced to the conclusion that it was mere coincidence, unlikely though TETIA CORNER Rank Arena C2239 (B2 Chassis) Symp'tom: No picture but sound OK. Rather dark. raster with retrace lines on channel, but raster becomes lighter off channel. Brightness and contrast controls have no effect. Y signal disappears at pin 5, IC701. Voltage at pin 2 IC701 much higher than it should be. 66 SILICON CHIP Cure: TR402 short circuit. This transistor is one of two forming a multivibrator that generates the vertical blanking signal. When it fails the blanking is turned hard on, and IC702 is permanently blocked. This information supplied by The Electronics Technicians' Institute of Australia (Tasmanian branch). this seemed. So I fitted another new transistor and fuse, gave the set a test run, and returned it to the owner. And, again, all went well for another three months. Then the set was back in the shop with the same fault. No coincidence Well, that settled it. There was no way I could accept a third failure as coincidence; there just had to be a more subtle fault that was responsible. Since I felt fairly confident that I had excluded the more common causes of this problem, such as dry joints, the alternative approach seemed to be to get the set running and make as many dynamic tests as possible. So I replaced the transistor and fuse, turned the set on, and started with some voltage checks. Unfortunately, the circuit diagram gives very little information in this regard, so the best I could do was check the voltages which were marked - which all turned out to be within tolerance - then make as many other checks as I could think of and try to relate them to what I imagined they should be. Once again I found little to make me suspicious. Having drawn a blank with the voltages, the next thing I wanted to do was check the various waveforms around this section. But, once again, the circuit is noticeably lacking in such information. Such waveforms as are shown seemed hardly appropriate to the problem. In particular, I would have liked to have known the appropriate waveform for the collector of TS 170. Fortunately, there seemed to be a way out. I plugged in my stock power board and connected the CRO to it. In so doing, I had to take into account that the negative reference point is not the chassis but a point on the circuit marked as the reference point for certain voltage measurements (marked in red}, and which is close to the negative end of the main bridge rectifier. The resultant waveform was basically a square wave, but with some ringing on the leading edge. More exactly, the basic square wave had an amplitude of about 550V, while the ringing had a peakto-peak amplitude of about 100V, or 50V above the 550V, making a total peak value of 600V. I made a note of this and was about to plug in the set's own board when I had a stroke of luck. Another K9 came in for a relatively simple fault, providing a golden opportunity for another reference. In fact, it produced a waveform almost identical with that from my own board, so I felt reasonably confident that this was how it should be. All that remained now was to check the suspect board. And this gave me my first real clue. In general terms, the waveform was the same, at least in shape, but the values were significantly different. The square wave portion was now up to 600V while the ringing amplitude had also increased, now running at a good 200V p-p, or 100V above the square portion, giving a peak amplitude of 700V. Considering the set's history over the last few months I felt sure that this was the most likely cause. But I still had to find out why it was happening and, in the process, prove that it wasn't just a normal spread of component values. And now that I had something definite to go on I began to recall some suggestions I had heard about causes of excessive ringing and possible destructive spikes. Among the suspect components is R175 (120} and R174 (560), both in the base circuit of TS 170, which, if they go high, can cause excessive ringing. Also in the base circuit is C177 (4.7µF} which apparently can cause trouble if it drops its value. These were checked and found to be spot on. Less likely suspects are R182 (0.330 or, in some cases, 100), and R176 and R177 (18kn}. These were also checked and cleared. So what now? With the base and emitter circuits seemingly cleared, what about the collector circuit? Among other things this contains three filter networks consisting of resistors, capacitors and diodes. The failure of any one of these would surely have some effect on the waveform. I started with D176, R173 and Cl 93, all of which checked out OK. So did D179 in the adjacent network, but R190 (150k0) was a different matter - it was open circuit! I replaced this and then, before returning the board to the set, checked out the third network consisting of D178, R189 and Cl 75. All were OK. I put the board back into the set, hooked up the CRO, and switched on. Up came a perfect waveform; ie, identical to the other two boards. And that, as far as I was concerned, was all the proof I needed. I have since handled several more K9s and have made a point of checking the waveform as a matter of routine. All have been essentially the same as my own stock board. So that was it. I put everything back together, ran the set for a couple of days, then returned it to the customer. Only a few weeks have elapsed since then so it is much too early yet to boast. All I can do is keep my fingers crossed and hope. -1e1gscHlADIResistor kits for Laboratory * Service * Workshop These comprehensive resistor kits are ideal for the engineer, technician and the dedicated enthusiast. No more wasting time trying to find odd value resistors. No more high procurement costs for small order quantities. Have them at your fingertips when you need them , neatly stored for quick selection in transparent rectangular tubes or compartments, clearly marked with the IEC Series resistance values. Kits are available with 50 to 586 resistance values and containing from 1,210 to 26,000 resistors, in surface mount (MELF) or leaded types. This is our Mini-MELF (LMM 0204-24) lab assortment containing 6500 resistors. For further information contact: 1 f~ ~~\ CRUSADER ELECTRONIC COMPONENTS PTY LTD ~I//J\\\"": 81 Princes Hwy, St Peters , NSW 2044 . Phone (02) 516 3855 , 519 6685, 517 2775. Telex 123993 . Fax (02) 517 1189 . APPOINTED DISTRIBUTORS Sydney: George Brown & Co Pty Ltd , 519 5855. Geoff Wood Electronics Pty Ltd , 810 6845. Wollongong: Macelec Pty Ltd, 29 1455 . Canberra: George Brown & Co Pty Ltd , 80 4355. Newcastle: Novocastrian Electronic Supplies, 62 1358. · Melbourne: Nalmos Pty Ltd , 439 5500. Jesec Components Pty Ltd , 598 2333. George Brown & Co Pty Ltd, 878 8111. Brisbane: L. E. Broughen & Co, 369 1277 . Colourview Wholesale Pty Ltd , 27 5 3188. St Lucia Electronics, 52 7 466. Adelaide: Protronics Pty Ltd, 21 2 3111. Perth: Simon Holman & Co, 381 41 55. Protronics Pty Ltd , 362 1044. DECEf\lBER 1987 67