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SERVICEMAN'S LOG Nothing unusual happened this month I don’t have any stories from my own bench this month, since nothing sufficiently unusual has happened. So I have had to call on a couple of colleagues, who have come to the party with some really tricky ones. The first story comes from my colleague on the NSW south coast and it concerns a problem peculiar to his area. In fact, for readers not familiar with this area, it is necessary to set the scene in terms of the local TV channels. In the early days of TV, residents of Wollongong, about 80km south of Sydney, managed as best they could with signals from the Sydney channels. It was a chancey business. As well as the distance, they had to contend with less-than-favourable topography. Tall masts, high gain antennas and masthead amplifiers were the order of the day. Some managed reasonably well; others took what was there on a day-to-day basis and were grateful for it. Further south, around Nowra, Bate­man’s Bay and their surrounds, it was virtually hopeless. First relief for the area came with the establishment of a couple of VHF transmitters at Knight’s Hill, south of Wollongong. Today, the area is served entirely by five program sources in the UHF band, using five main transmitters and five translators. In order to appreciate my colleague’s story, it is neces­sary to set out these channels. The five main transmitters, at Knight’s Hill, use channels between 53 (701-708MHz) and 65 (785792MHz), while the translators use channels between 30 (540-547MHz) and 48 (666-673MHz). So, against that background, here’s my colleague’s story, more or less as he related it to me. The fussy Rank The set was a Rank-NEC model C-1413. It was brought in many months ago by one of my lady customers with the complaint that “the picture goes funny on some of the channels only”. Well, I’ve had worse descriptions although, as it turned out, it was AUSTRALIAN MADE TV TEST EQUIPMENT 12 Months Warranty on Parts & Labour DEGAUSSING WAND Great for computer mon­­­­­­it­ ors. Strong magnetic field. Double insulated, momentary switch operation. Demagnetises colour picture tubes, colour computer monitors, poker machines, video and audio tapes. 240V AC 2.2 amps, 7700AT. $85.00 + $10.00 p&p Cheque, Money Order, Visa, Bankcard or Mastercard. Phone for free product list SHORTED TURNS TESTER Built-in meter to check EHT transformers, in­clud­­­­ing split diode type, yokes & drive trans­ formers. $95.00 +$3.00 p&p TV, VCR TUNER REPAIRS From $22. Repair or Ex­change 216 Canterbury Rd, Revesby, NSW 2212, Australia. Phone (02) 774 1154 Fax (02) 774 1154 40  Silicon Chip accurate enough; it just wasn’t very helpful. In cases like this, one of the points I have to watch in this area is the need to determine, right from the start, which transmit­ters a customer uses; the main transmitters, the translators, or even combinations of both in odd cases. This fine distinction is usually lost on most people; they think in terms of names – ABC, SBS, Prime, etc – with little appreciation of how the program comes to them. But some careful questioning in this case finally pinpointed the main transmitters on Knight’s Hill as the signal source. So that was where we started. The model C-1413 is one of a series of sets which use essentially the same circuit and appear under several brand names. As a Rank, it also appears as the C-1414 and C-2020 (among others) but it also appears under the GE label as GE482 and under the General label as GC205. I have most of these circuits, including the C-1413 and the C-2020. As luck would have it, the 2020 came out of the file along with the 1413 and I left it out. As far as the symptoms were concerned, the lady was right and the pictures from Knight’s Hill were “funny”. And that wasn’t such a silly term either. The effect isn’t easy to describe; my best attempt would be random pulling and rolling, with the sug­gestion that this might have been hum related. The lady was also right in that there was no sign of the trouble on the translator channels. In fact, I explored this aspect very thoroughly to make quite sure. OK, so we had a frequency related problem. That meant trou­ble somewhere in the front end; probably in the UHF tuner itself. This set uses two mechanical tuners; the UHF tuner which down-converts to VHF, and a VHF tuner which then down-converts these signals to the IF. Of course, it can process off-air VHF signals as one carrying the supply rail, the AGC, AFT and chassis connections, one for the IF lead, and one for an auxiliary network for the UHF tuner RF bias control. With the two sets close together, there was enough lead length to allow the suspect tuner assembly to be replaced with the known good one, without any need for mechanical demounting. I was fully confident that this would confirm that there was a tuner fault. If so, it would make things easy because I had a couple of spare tuners from junked sets and a replacement would be a cheap and easy solution. But no – the set behaved exactly the same with the replace­ment tuner assembly. This was a really revolting development; any complacency I had allowed myself up to this point was completely dispelled. I had a real stinker on my hands, defying all the rules. If the fault was in the main circuit, which was carrying nothing higher in frequency than the IF, then how did it know when the set was tuned to something above 700MHz? And if use of the word “know” sounds a bit way out, it was no more so than the fault itself. Caffeine fix well. It’s all quite conventional really and I imagined that the job would be fairly routine. The first thing I tried was feeding in a signal from the colour bar generator. This happened to be set to channel 36 (575-582MHz) in the translator group and, as expected, it produced a perfectly steady picture. I then reset the generator to channel 67 (799-806MHz) in the transmitter group, fully expecting that I would be able to observe the fault under controlled conditions. But not a bit of it. The colour bar signal was just as steady on this channel as it had been on the lower one. It was a nasty setback. OK, so it was back to the real world. I switched to one of the Knight’s Hill transmitters and confirmed that the fault was still very much alive. So what next? Well, luck was with me; I had another identical set in the workshop at this time. There wasn’t much wrong with it and I could use it for a spot of swapping. In particular, I had in mind to swap the tuner assemblies, thus either confirming or rejecting this section as being at fault. It was a simple exercise. The tuner assembly is connected via three plugs, After I’d had a caffeine fix and calmed down a little, I had another thought. Could it be a power supply fault? A long shot surely – how could the power supply be involved? It was just about as far removed from the frequency selection process as anything could be. Yet I’ve had some very funny faults traced to power sup­ plies. We tend to forget that there are signal paths through or around all power supplies, usually involving capacitors other than the filter capacitors, and that failure of these can create faults a long way from the source. More to the point from a practical point of view, it took only a few minutes to patch the power supply from the other set into this one and settle the point once and for all. And it did – it made no difference. The next most likely possibility was distortion of the sync pulses. I could think of no way that such a fault could be fre­quency conscious but the idea could not be ignored. The IF signal from the tuner goes through an amplifier stage (TR201), a SAW filter (FL201), and thence to pins April 1994  41 Fig.1: the IF circuitry in the Rank C-1413 colour TV set. The IF input is at extreme left & feeds TR201, while C208 is below IC201, between pins 11 & 14. Note capacitors C221 and C222 from pin 11 to chassis. 1 & 16 of IC201 – see Fig.1. It emerges on pin 12 and a clear staircase waveform pattern is given for this point. So the CRO was hitched to pin 12 and the colour bar generator used to provide a stair­case signal. But, again, there was no cry of “Eureka” – or a triumphant dash down the main street. As far as I could see the waveform was perfect; exactly according to the circuit, with no hint of dis­ tortion or compression, from either a channel 36 or channel 67 signal. Nor did the signal level appear to matter. The generator can deliver a solid signal – stronger than most off-air signals in practice – and so I took this right down until the pattern dropped out of colour. The sync pulse remained perfect. Nor did varying the AGC adjustment have any effect. But what about off-air signals? I checked some of the transla­tor signals and the sync pulses appeared much the same as from the generator. The pulses from the Knight’s Hill transmitters didn’t look too bad either, although that “too bad” implies a qualifica­tion. Yes, the shape was still OK but one difference did catch my eye, although I still don’t know whether it was relevant. As I said, the shape was correct but there appeared to be some rub­bish, or noise, inside the pulse rectangle. It was nothing that could be resolved and is still a mystery. 42  Silicon Chip All I know is that it was only on the troublesome signals. Hard slog So now it was down to hard slogging and a lot of hope. I changed transistor TR201, the SAW filter, and even IC201. I changed the capacitors on the AGC line (pin 4), including C210 (4.7µF), C212 (0.01µF) and C209 (4.7µF), plus sundry resistors. None of these had any effect. By this stage, I had reached the point where I had to stand back and take a long hard look at the whole situation, not only technically but financially. I had spent a lot of time on it; somewhat more that could reasonably be justified for something which was now looking as though it might be a write-off. I contacted the customer and brought her up to date on the situation. She was quite co-operative, in that she was in no hurry to get the set back. To be truthful, I gained the impres­ sion that she was quite prepared to write it off. But she was prepared to spend up to $100 to get it working. So, at least the pressure was off. I had more pressing jobs to attend to and so the set was put aside in one corner of the bench. I had fully intended to get back to it reasonably soon but, as often happens, other jobs kept piling up and I kept putting it off. And so several months went by. But its mere presence provid­ed a nagging factor; every time I looked at it, I felt guilty – and apprehensive. I had no idea how I was going to tackle it. Eventually, when things slackened off over the Christmas/New Year break, I knew that the moment of truth had come. I fished it out again, determined to settle the situation one way or the other. Fortunately, I had scribbled a few notes and kept the com­ponents which had been changed, so I was soon back in the pic­ture. But I didn’t really have any fresh ideas. The best I could do was to continue replacing likely – or even unlikely – compon­ents and hope for a breakthrough. And that’s what happened. After a couple of false tries, which included capacitors C222 (47µF) and C221 (0.01µF) in paral­lel with it, I came to capacitor C208 – a 2.2µF tantalum electro­lytic connected between pins 11 & 14 of IC201. I hadn’t tried it earlier because it was hidden on the copper side of the board. Only its presence on the circuit diagram as part of the AGC circuit sent me looking for it. Its location may be significant, considering what followed. I pulled the capacitor out and checked it. It measured spot on but I replaced it anyway. Well, sort of – I didn’t have a 2.2µF capacitor handy, so I settled for 1µF. The result was quite dramatic; not a total cure but such an improvement that I could have almost let it go. There was just an occasional tendency to pull. My natural reaction, initially, was that if I fitted the correct value, it would complete the cure. It might have too, but a number of things happened to change my approach. To explain this, I have to make it clear that I had abso­ lutely no idea as to the function of this capacitor. I didn’t even know the function of pin 11; whoever drew the circuit had omitted to identify it. In hindsight, I could have easily worked it out by tracing the circuit but didn’t I realise this. But I was curious about C208. It was then that the C-2020 circuit stuck its nose in – see Fig.2. It had been updated, with pin 11 marked as VCC (the supply rail pin). On the C1414, it connects to the 12V rail via a 22Ω decoupling resistor (R223), bypassed by the previously noted C221 and C222. Suddenly, C208’s role became clear. It is a bypass capaci­tor for pin 14. Pin 14 is part of the internal AGC circuitry and, while I have no idea of its exact role, it is clear that it is held at around 7V by a 560kΩ resistor (R209) to chassis. And C208’s job is to peg this point, at RF, to chassis. Only it doesn’t go direct to chassis – it goes to pin 11. And pin 11 is pegged to chassis at all but DC by C221 and C222. It’s a rather roundabout route but a perfectly valid one – at least in theory. In the process of working all that out, I became aware that the pin 14 bypass circuit was quite different in the C-2020 circuit. In this case, the bypass capacitor, now designated C223 and reduced to 1µF, goes directly to chassis. In practical terms, this seemed to me to be a much more elegant approach and since someone a lot smarter than I had preferred it, why not try it? The rest is history, as our political commentators like to say; I made the changes and it was a perfect cure. I ran it for a couple of weeks before calling the lady and it remained rock steady. So we had a happy ending. But I realise that the story poses as many questions as it answers. While it is clear that the pin 14 bypass arrangement is critical – and that someone at engineering level must have discov- Fig.2: this is the IF circuit from the Rank C-2020. Note the changes on pin 11 & 14 compared with the C-1413. ered this – I still can’t explain why the fault was frequency conscious. But then I’m not an engineer; I’m only the poor bloke who has to try to make sense of these weird situations in the field. All I can do is learn from the experience and, by passing it on, perhaps help some other poor blighter from going round the bend – as I nearly did. Well, that’s my colleague’s story, and a good one it is too – as a story. But I can only sympathise with him over the anguish and frustration it must have caused. Nor can I answer any of the questions it poses. Any suggestions? A thorny problem And so to colleague number two; old faithful, J. L. from the island way down the bottom. Here’s his contribution. By all that’s reasonable, the old Thorn model 3504 should have been junked 10 years ago. It was released by AWA in early 1975 as the company’s first ever colour TV set. By 1984-85, the model was beginning to show its age and by 1990 most examples had succumbed to the years and were only to be found on the municipal tip. However, a few have survived and one of these came to my attention last week. I’ve been caring for this set for close to 15 years. I don’t know to what extent its longevity is related to my atten­ tions but I do know this – when I last worked on it about two years ago, it still produced a good picture. So when he called me last week and said that the picture had gone “...all purple” I wasn’t particularly worried. I was confident that it wasn’t the picture tube and fairly sure that it was going to be a simple electronic problem. I was even more convinced it was the latter when he said that thumping the cabi­net sometimes restored the picture to normal. A purple picture results from the loss of green content, so this problem had something to do with the green video output or the green gun. I firstguessed the former because, in the 3504, the video output load resistors are etched onto a ceramic sub­strate and I have found a number of these breaking down recently. In greater detail, the connecting pins break away from the ceramic where they are attached to the etched pattern. I’ve had no luck resoldering this connection; it is more practical to replace the printed resistors with discrete 10W units. Nowadays, I don’t have to resort to such subterfuge since I have a large collection of good boards taken from sets that have paid the supreme sacrifice. And so I decided it would be easier to do the job in the customer’s home, by simply replacing the suspect board with a known good one from this collection. And this did appear to be the answer; at least for a few moments after I had made the swap. But soon the green part of the picture disappeared April 1994  43 again and I had to admit that it was really a different fault. So where to start looking? I seem to have a habit of making the same mistake over and over again. The mistake this time was to forget to use my multi­meter. Circuit voltages are one of the best indicators of circuit performance, yet I always seem to make this my last test instead of the first! If I had made a quick check of the tube base board, I would have found that the fault lay with the screen (first anode) voltage, not with the cathode or grid voltages. So having done at last what I should have done at first, I set about trying to adjust the voltage on the green screen (pin 5). The result was uncertain – the green could be restored but it was erratic. It was hard to say exactly how it was varying, although the instability was seemingly related to the position of the screen potentiometer, R793. All of this made me think that we had a dirty screen pot. I’ve had these before and they usually respond to a 44  Silicon Chip squirt of contact cleaner. So, two or three squirts later, the picture came good and no amount of mechanical abuse would alter it. I let the set run for half an hour or so while I had a very welcome cuppa’ with the owner. The picture never varied and so, by mutual consent, we declared the job done. Pride commeth ... What is it that they say about pride coming before a fall? That night the owner rang to say that everything was back as it had been – no green, purple picture, and all! When I left the workshop next time, I made sure I had packed a complete convergence board. This panel carries not only the many convergence controls but also the three screen pots and their associated beam switches. My plan was to change over the whole board to make a quick and simple repair. What I had forgotten was that the “new” board had been set up for a different picture tube and would have to be completely readjusted. When I fitted it to the set, the picture came up with plenty of green but the convergence was grotesquely out of ad­ justment. I could see that it was going to be a long operation to do a complete convergence setup and I’d already spent as much time on the job as I could afford. So I decided to refit the original board and just change the doubtful pot for one of the good ones from the new board. And it was then that I found the true cause of the trouble. As I prepared to remove one of the screen pots from the new board, I noticed that the three beam switches were of two different types. Then I remembered! On a few occasions in the past, I have found that these beam switches develop internal shorts. It seems not to be a mechanical problem but an electrical one within the switch mate­rial itself. No amount of cleaning compound will cure the trouble – the only answer is to change the switch, something that I had obviously done on the new board at some time in the past. So instead of changing the screen pot, I changed the beam switch, S752. After a gray scale adjustment, we had as good a picture as any I’ve seen on a set this age. In fact, the colour, contrast and brightness were all excellent, as was the conver­gence. When I commented about the excellent picture on a set of such advanced age, the customer’s wife commented that they only used the set for a couple of hours at night. News and the early-evening soaps were all they ever watched and it was never on during the day. I made a quick calculation. Two hours a day, 365 days a year, for 18 years, makes over 13,000 hours! I seem to recall something about 10,000 hours being a reasonable life for a pic­ture tube. So this one is not only well past its presumed life­time but looks capable of going on for many hours yet! The only problem is that the set has only a VHF rotary tuner. However, the owner professes to have no interest in channels other than the ABC, so perhaps he really doesn’t miss the UHF facility. Thanks, J. L. – I hope you can keep the old clunker going for a few more years. As for the UHF channels, why not add a junked video recorder to the set to tune these stations; one in which the front end is still functioning? SC