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SERVICEMAN'S LOG If only the fault would show One of the truisms of service work is that you shouldn’t try to fix a fault that you can’t see. But we are forced to try sometimes, even if we don’t often win. My main story this month tells of the frustrations of trying to work this way. In order to set the scene for this story, I must reintroduce Murphy. Remember Murphy? He’s the pesky little leprechaun who sneaks around, shifting component values up and down, opening and closing dry joints, and generally contributing to erratic be­haviour in electronic devices. He’s not been around lately and I had hoped he had met with some calamity; like digging a hole for another pot of gold, digging it too deep, and pulling it in on himself. No such luck. More likely, he had been away at some lepre­chaun workshop, learning even more devious ways to create havoc. At least that’s how it appeared when he turned up on this partic­ular job. Not only did he waste a lot of time but I was left with a situation whereby the fault was cor- 50  Silicon Chip rected without being sure why things happened as they did. A simple beginning It all started out simply enough. It was a 68cm NEC colour set (model FS-6831S) that was only a few months old and still under warranty. The customer had purchased it over the counter, taken it home and connected it up himself, which is pretty much par for the course these days. And it had performed perfectly from the start. But now there was a problem, although it was straightfor­ward enough; it wouldn’t switch on. The only sign of life was the standby light but operating the “ON” button on either the set itself or the remote control unit had no effect. The failure of both controls was, in itself, not surpris­ ing, since both employ the same mechanism within the set. But it did rule out any fault in the remote control unit or the panel control. More than that I was not prepared to speculate on until I had my hands on the set. The size of the set could have been a problem. I try to avoid house calls as much as possible but transporting a 68cm set can be difficult for some customers and I have to make excep­ tions. Fortunately, this customer was cooperative and was both willing and able to bring the set to the workshop. So that solved that problem. This set is fitted with a fairly routine chassis and its on-off switching system is similar to that used on a number of sets. It employs relay contacts in the mains active and neutral lines, the relays being activated by the central processing unit (CPU). Most designs use a single relay with two sets of contacts but in this set, for some reason which escapes me, they have elected to use two separate relays – RL651 and RL652. Not that this matters a great deal; the relay coils are connected in parallel and driven from the collector of transistor Q651, a 2SC2002. And the base of Q651 goes to pin AA17 of CPU PW8. So, assuming an appropriate voltage appears at pin AA17, Q651 should turn on, activate the relays, and close the two mains leads. Finding the fault was no big deal. I checked whether the “ON” control produced a voltage on the base of Q651 and, yes, it did. So why wouldn’t the relays operate? Quite simple really; Q651 was open circuit. I didn’t have a 2SC2002 in stock but a look the specifications suggested that a BC639, which has somewhat higher ratings, should do the job. And it did. So the customer duly collected his set and went on his way. And that seemed like the end of the story. Which it was, for the next couple of months. Then the customer was back on the phone, explaining (almost apologetically) that he was having trouble playing his VCR through the set. He was unable to get a recognisable picture; just a mess of streaks and patches of colour, with only an occasional hint of a locked image. But he was quick to add that he didn’t think this was anything to do with the previous fault. Oh, for more like him! Tuning problem? And so began what was to be a long and tedious search for this new fault. My first query was whether the tuner programming had been upset in any way, such that channel 1, used by this VCR, was off frequency. As with most modern sets, this one has a search function which looks for each signal In short, I could find nothing wrong and the customer col­lected the set and took it home. But we had achieved nothing and I wasn’t really surprised when he was on the phone again with the same tale of woe. But he was a gluten for punishment. “Suppose I bring both the set and the VCR up to the shop?” Well, he’d forestalled my thinking there, except that I was envisaging having to make a house call. One way or another, I had to see the two units working together in order to see the actual fault, which Murphy had contrived to hide from me so far. Both units together in turn, adjusts the tuner, and allocates the channel to a selected button. If it was simply a tuning problem, it should be possible for the customer to correct the condition himself and avoid the expense of a service call. I therefore advised him on how to check this – simply run a pre-recorded tape in the VCR and ini­ tiate the search routine. The system would eventually detect the carrier from the VCR and treat it as any other RF signal. I left it with him but he was on the phone a couple of days later, to report no success. The system had apparently gone through its routine correctly but could not correct the signal from the VCR. After a few more probing questions, I began to feel that there must be a fault in the VCR. And so I suggested he drop this in for a check. This he did and at the first opportunity I connected it to one of my monitors, pushed in a test tape and set things in motion. Result: a prefect picture. I put it through all its paces and let it run for several hours after which it still gave a perfect picture. So that ruled that out. When the customer called to collect it, I demonstrated it to him, then went through the tuner programming routine with him again. It was the only explanation I could think of and I sug­gested he give it another try. But again, no joy. He was on the phone the next day com­plaining that everything was exactly the same. He then suggested that he should bring the set into the shop and, since it was his idea, I readily agreed. And so the set duly arrived but without the VCR. This seemed quite logical; I was convinced that the VCR was OK, which meant that it had to be the set. I set the colour bar generator for channel 1, fed it in, and put the set through its search and program routine. And it went exactly as it should. Granted, channel 1 was slightly out but that didn’t really surprise me. The colour bar generator is generally more accurate than the average VCR, the channel fre­quency generated by some being best described as “nominal”. Not that it really matters, as long the set is accurately tuned to whatever the VCR is delivering. And so I finally finished up with both units on the bench, with the customer standing by while I set things up. I put it through the search routine and, as I expected, the VCR output was slightly off-tune for channel 1 although that was easily allowed for. And so, at long last, I should now at least know what the problem looked like. But not a bit of it. Murphy saw to that. Would you believe that the whole setup worked perfectly? Because that is what happened and I was just as confused as the customer. So all I could suggest was that he take it all back home and try again. If I had inadvertently done something to cure the fault, then well and good. If not – well, I had a pretty good idea what the next step would have to be. No prizes for guessing. The customer was on the phone the next day and the problem was just the same as before. At that stage, I could only speculate that it was something peculiar to the house setup, or some weird local interference. In any case, it left no alternative; I had to visit the house, see the problem for myself, and take it from there. So an appointment was made and I faced up to the problem in the customer’s lounge room. He turned the system on and pushed in a tape. The result was pretty well as he had described it but the symptom which struck me most forcibly was that, on the few occasions when the picture tried to lock, it was pulling very badly. My impression was that either the RF out of the VCR was hopelessly unstable or that the TV set was being overloaded in some way. My first step was to put the set through the search program with the February 1994  51 antenna disconnected, so that all it had to search for was the VCR output. Well, it went through the motions but didn’t want to look at the VCR signal, simply skipping over it and going round again. I was still trying to make sense of all the symptoms, when the customer happened to mention that the lead between the VCR and the TV set was not the original (black) one supplied with the VCR. Apparently, the original had been temporarily mislaid at some time and this was a white “el cheapo” one from the local electronics store. Could it be the culprit? I didn’t think so but was prepared to clutch at any straw. I took a closer look and found that moving the cable near the socket on the TV set could cure the fault. In the meantime, the owner had fished out the original cable, so I substituted it. When I did, the system immediately came good. So what would you think? Faulty white lead. Of course. Except 52  Silicon Chip that I could find nothing wrong with it; both the active and braid circuits were continuous and appeared to be reliable. I then tried moving the black lead at the TV antenna socket and, wouldn’t you know it, the fault was back. Faulty socket? Yes, it was. The active (female) contact had spread and was not mating reliably with the male contact. But this fault was not working as one might reasonably imagine from the description. The poor contact in the socket was not causing the fault – it was curing it. An overload problem Remember my impression that the system was being overload­ed? Well, that idea was suddenly starting to make sense. I tight­ ened the sloppy socket contact and this restored the fault in all its glory, regardless as to whether the black lead or the white lead was used. I was beginning to sense victory now. From my kit I fished out a 20dB attenuator, one of several values which can be very useful in some sticky situations. I inserted this between the VCR and the TV set and bingo, we had a perfect picture. OK, so we had an overload problem. But why? My immediate reaction was a fault in the AGC circuit of the TV set. And the first thing to investigate was the preset AGC pot. It could be faulty (intermittent?) or it could be wrongly set, although this latter theory seemed unlikely. I removed the attenuator, then pulled the back off the set and tracked down the AGC pot. And one glance was enough to raise my eyebrows. One normally finds these pots set at around mid-travel but this one was almost fully clockwise. It is not often that this pot needs adjustment, as the factory setting should cope with 99.9% of conditions. But if it does need to be reset, the normal procedure is to first turn it fully anticlockwise, which produces maximum AGC voltage and a snowy picture, even on strong signals. The pot is then advanced until, usually quite suddenly, the snow vanishes and there is a clean picture. And normally, this setting will hold for a wide range of signal strengths. Which was what I did, using the signal from the VCR. And everything went according to the book, including a near mid-setting for a clean picture. I reconnected the antenna and re­ peated the check with off-air signals. Again, everything went according to Hoyle. So that was the answer; an incorrectly set AGC pot – a mistake which almost certainly occurred in the factory during final testing. It was all very gratifying except that I have no explana­tion for all the variations of behaviour. Why did the set work perfectly for the first eight or nine months of its life? Why did it work on my bench and not in the customer’s home? And so on. I’ve tried to work out the answers but I’m afraid they elude me. All I can do is take the easy way out and blame Murphy. The TV that flipped My next story is relatively simple. It did not involve any great detective work to solve but it was unusual. It involved a National colour set, model TC-2178, fitted with an M14 chassis, and about six or seven years old. The customer – the lady of the house in this case – rang to complain that “the set was doing funny things”. Not being quite sure what she meant, my imagination ran wild for few moments but I eventually pinned it down in more precise terms. This set uses a fairly simple channel selection system – two UP/DOWN pushbuttons on the front of the set which can select any one of 12 pretuned channels. There is also a remote control unit with a similar UP/DOWN facility plus a set of 12 buttons (one for each channel) and the usual volume and on/off controls. The problem was that the set had developed a penchant for position 11. It didn’t matter which position was initially se­lected; as soon as the UP or DOWN button was released, the set would immediately move to position 11, which was blank (only six positions were active – five for off-air signals and one for the VCR). In fact, the only way they could hold the set on a particu­lar channel was to select it via the remote control unit and then hold that button down; something which became a mite tiresome after a couple of hours. Well, it was new one on me and I could think of no explana­tion off the top of my head. So I could only advise them to bring it in so that I could see the effect. And so the lady and her husband turned up a couple days later with the set. Keen to see this strange phenomenon for myself, I turned the set on while they were there. And it promptly did all the right things; it brought up the channel the customer selected and it stayed there. No problems – except that, once again, I was stuck with a problem that I couldn’t see. So what if the channel had been selected via the remote control? They weren’t sure and they hadn’t brought the remote control with them. That meant the job had to be put on hold until they could drop it in. So I put the set in a corner and ran it for the rest of the day and for a couple hours the next morning, with no signs of trouble. When the customer came in with the remote control unit, it initially wouldn’t work at all. The reason was simple enough; the batteries were flat. So I fished out new batteries and fitted This view shows the innards of the remote control unit for the National TC-2178 TV receiver. At left is a rear view of the rubber pad & buttons, while at right is the PC board showing the button con­tact areas, the IC pins (bottom) & the IR LED at top. them, working at the bench with my back to the set. When I turned around, it had a white screen – it had moved to the blank position 11. At the customer’s suggestion, I pushed a button for one of the other channels and this appeared immediately. But then, as soon as I released it, we were back to position 11 – just as the customer had said. Well, at least I’d seen the fault and I sent the customers on their way while I thought things out. And it didn’t take much thinking to work out a likely theory; the set had behaved per­fectly until the remote control appeared on the scene. So the fault had to be in that, rather than in the set. It was easy enough to prove. I moved the remote control out of range and selected another channel at the set. It behaved perfectly. But as soon as I brought the remote control unit within range, we were back to position 11. So the control unit was transmitting a position 11 signal continuously – which also accounted for the flat batteries. Well, remote control faults are fairly common but this was a new one on me. Remotes are vulnerable Most remote control faults involve abuse of some kind. By their very nature, they are vulnerable devices. They are sat on, dropped, kicked, trodden on and generally bashed around. They are also soaked in various liquids – coffee, water, lemonade, beer and any other beverage you can think of. Liquid abuse, with the possible exception of water, means a write-off. And even water needs to be treated promptly, to ensure any chance of success. Otherwise, you drop it in the rubbish bin. But there ware no signs of abuse in this case. In fact, both the set and the remote control had been kept in immaculate condition. So my first guess was that button 11 was faulty, locking on in some way. February 1994  53 SERVICEMAN'S LOG – CTD By releasing one screw above the battery compartment and easing a knife blade between the two halves of the case, I was able to get the back off and lift out the PC board. Apart from the normal component connections, this carries copper contacts which sit behind the front panel buttons. The buttons have a conductive surface and are held in a rubber-like pad. When a button is pressed, it connects with its appropriate contact on the PC board. I could find nothing wrong here. The button was not jammed, and there was no foreign matter between the pad and its matching contacts. More importantly, the device was still generating a position 11 signal, even with the pad and PC board separated. It didn’t take much effort to narrow this fault to the IC, which is really the heart of the device. A new IC? No way; not available. That left a new remote control as the only option but it wasn’t a very satisfactory one; the price is around $150 when it is avail54  Silicon Chip able – which it wasn’t, stocks being on back order. In fact, the price structure on these devices is hard to understand. They are all basically the same – though seldom compatible – and yet prices range from around $50 to $150. Cold comfort And so it was all cold comfort for the customer. While I assumed they would accept the situation and pay up, I regretted that I could do nothing better. But then, while I was actually writing these notes, I sud­denly had a thought. Many months previously, I had been forced to write-off another National TV set as being uneconomical to re­pair. There had been a remote control unit with that, so what had happened to it? I moved immediately to my pre-loved, surplus equipment department and consulted the records (read: scrabbled through the junkbox). Sure enough, there it was. It had belonged to a more elabo­rate set, with many more remote control features (colour satura­tion, contrast, etc), but it also had the same basic functions as needed for this set. So would it work with it? I fitted a set of batteries and gave it a workout. Result – total compatibility with all the functions the set could provide. It would be a simple matter to ignore the other functions. In fact, this is not an unusual situation, even with new sets. In some cases sets are sold with a remote control having, say, Teletext controls, even though the set has no Teletext facility. It is obviously aimed at rationalising production and doesn’t seem to worry anyone. So I was able to offer the customer two options: a new control unit at around $150, or a secondhand one at a fraction of that figure. Of course, they jumped at the chance for the cheaper solution. So I scored a happy customer and made a small profit on a piece of surplus gear. It was smiles all round. Finally, a likely explanation for the failure. There had been a number of storms around his area recently and I have been involved in repairing some of the resultant damage, which has been quite extensive. TV sets, VCRs, microwave ovens, remote con­trolled roller doors, CD players, electric clocks and telephones have all suffered. When I mentioned this, the customer recalled that they had lost their roller door control, a clock and some other minor appliance during the storms but the TV set and VCR had not suf­fered. However, now that I had raised the point, he realised that the position 11 problem had appeared at about this time. It may have been pure coincidence, of course, but we do know that solid state devices are particularly vulnerable to lightning strikes – not so much direct strikes but to strikes in close proximity, which can produce magnetic fields to which these devices are sensitive. So there it is; no positive proof but SC a likely explanation.