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SERVICEMAN'S LOG The set that languished & died Some customers get rather attached to their TV sets, particularly if they’ve given years of trouble-free service. Fortunately, a full military service isn’t usually necessary. My main story this month concerns an NEC FS6325 63cm TV set. At first glance, this looks like a stereo TV set, with its twin speakers and left and right input sockets, but it doesn’t have a stereo decoder. If anyone wants the stereo feature, they would have to do what the Wilsons had done – purchase a hifi VCR and use the AV leads to get the full effect. However, the TV set had failed. It had apparently been “languishing” for some time before finally passing away completely during the night. Mr Wilson wanted to know whether it should be buried with full military honours because it was now getting on a bit, or could I perhaps “perform a Lazarus”? After all, it had been a good set. In the past, I have repaired several TV sets of this ser­ies. These are genuine NEC sets (ie, made by NEC) and, generally speaking, are very reliable. The genuine NEC sets are easily identifiable as they use a PWC number for each printed wiring board. In this case, the main board was PWC 3517. Most of the problems that do crop up are associated with dry joints to the power diodes on the secondaries of the horizon­tal output and chopper transformers. For this reason, I felt relatively confident that the set could be fixed on the spot and arranged to make a house call that afternoon. In due course, I settled myself behind the set and, with the help of an electric screwdriver (how did I manage before I acquired this?), made short work of releasing the back. Access to the underside of the main board is rather tricky until some of the wiring harness is unplugged. When I 56  Silicon Chip did this, I was relieved to see that my diagnosis was spot on and resoldered a very dry joint to D621 (which supplies the 130V rail). I also checked D522 in the 12V rail but it was OK. I was so confident that I had fixed the problem that I replaced the back and returned the set to its original position before switching it on. Unfortunately, my confidence was short-lived. It did come on for a few seconds but then, much to my disgust, it died again. Hoping that this was just a temporary aberration, I tried switching it off and on again. This time, the picture and sound came on for half a minute before going off. Was this what Mr Wilson meant by “languishing” before it died? “Well, sort of”, he replied. Apparently, they had been forced to switch it off and on a number of times before it would stay on. It’s problems like this that put a complete downer on your day. I hadn’t counted on this and I had other appointments to keep. My options were either to delve back into the set or take it to the workshop. One last effort I decided to remove the back again. I looked around for dry joints and resoldered a few suspects but nothing really caught my eye. In the fault condition, the multimeter indicated 130V on the collector of Q502 (the horizontal output transistor) and also on its driver transistor Q501. The 130V on Q502 was OK but not on Q501. If this was functioning normally and drawing normal cur­rent, its collector should have been around 54V. And that told me that the horizontal oscillator, embedded somewhere in IC701 and coming out on pin 6, was not functioning. By now, it was obvious that I was going to be late for my next appointment and so I quickly checked the other rails. The 28V rail for the sound was OK and so were the 17V and 5V rails. But that was as far as I could go for the time being; the set would have to go back to the workshop. I quickly cleaned up, cleared some space in the truck and carried the set out. Fortu­nately, this set only weighs about 30kg. The next day, I tackled the set again as soon as I had my compulsory coffee fix. I tried tapping the chassis, heating and freezing but it made no difference and I was now quite sure that this wasn’t a dry-joint fault. I followed the 12V rail via R599 to an 11V zener diode, then on to pin 8 of IC701 via D598. This is the soft start-up voltage, to fire the oscillator before it is taken over by the 12V rail via D599. The zener diode – ZD501 (13V) – checked out OK. It was time to review the situation. At the moment of switch-on, the entire set was apparently working OK. However, after a few seconds, something was shutting down the oscillator and the voltage on pin 8 of IC701 dropped dramatically. One possibility was that the microprocessor on the CPU board was at fault, as it supplied the sync input to pin 16 of IC701. Some sets have an arrangement whereby the set will switch off automatically after a few minutes when a TV station closes down at night. This is done by using a timer on the sync input to the jungle IC, which cuts off the horizontal oscillator. In this case, I felt that this was unlikely as the set rarely stayed on for more than a minute. It was only then that I noticed (and recognised from the old Rank Arena days) transistors Q2001 and Q2002 in an x-ray protection circuit. This circuit shuts off the drive to Q501’s varying, it settled down. Obviously here was the problem but was it the horizontal output transformer or a problem with the EHT regulation? I put the second channel of the CRO on the collector of Q502 (pin 10, T502) and noted that although the secondary waveform on pin 2, was varying, the primary on pin 10 wasn’t. This was all I needed to condemn the horizontal output transformer. I phoned Mr Wilson with the good news that I had found the fault. The bad news, of course, was the need to replace T502, its cost, and the time taken to order the replacement. He reluctantly accepted the reality of the situation and a new one was ordered. From then on, it was plain sailing. The transformer arrived in a few days, was duly fitted and the set returned. So far I haven’t heard any more from it or the Wilsons. The white line base if the pulses from the horizontal output transformer, T502 pin 2, go too high (Q501 is the horizontal driver transistor). And I remembered how much trouble this little circuit used to cause. The transistors became leaky, their gain was critical and there were modifications that had to be done to the early ver­sions. I shorted test point TP2001 to chassis to disable the protection circuit and the set stayed on indefinite­ly, so I was at least on the right track. Unfortunately, after spending over half an hour checking all the components in this safety circuit I couldn’t find anything wrong. Finally, I put the CRO onto pin 2 of the horizontal output transformer (T502) and checked the waveform. As luck would have it, the set now stayed on permanently with or without TP2001 connected to chassis. I left the set on test and went on with something else. Every so often on my way to the kettle for a slurp at my life support, I glanced at the set and the CRO but everything was still going fine. Eventually, I needed the CRO for another job and so the NEC was left alone, still switched on. Once or twice, I think I noticed the width vary momentarily but it may have been an optical illusion. Anyway, this went on for well over a week and a rather petulant Mr Wilson was now phoning quite frequently, wanting to know when Lazarus could come home. I told him the truth which was a mistake, as he was singularly unimpressed. Eventually, we finally agreed that I would deliver it if it was still working after one more week. The day before delivery, the weather turned damp and when I switched the set on that morning, it coughed and died. I’m afraid I called it a few nasty names but at least it had failed before I’d delivered it to the customer. I reconnected the CRO and this time I watched the waveform before it died and I noticed it was getting really large. With TP2001 shorted to chassis again, the set stayed on and though the waveform was initially large and M r s S i n c l a i r ’s To s h i b a 289X9M arrived unannounced while I was out, with a note attached describing the fault as a “white line across the screen; was intermittent, now permanent”. Interestingly, the set modestly advertises that it can handle 18 different TV systems. I didn’t even know there were that many in use. However, I suppose if one adds up all the small differences, combinations and permutations between each country it could be that many. The last list I saw included CCIR system M, which made 13 systems – obviously there must be at least five newer ones since then. Australia has the peculiar distinction of having two systems: CCIR B and G (one system for VHF and another for UHF). Anyway, I digress. I was hoping the fault might be attrib­utable to dry joints on the vertical output IC (IC303). Access to the chassis – especially the vertical timebase – was very poor. However, my diagnosis was correct. IC303 had several dry joints and I hoped that resoldering would be all that was necessary. Unfortunately, I was too late; the fault was now permanent, the set having been run in this condition for too long. February 1999  57 Serviceman’s Log – continued It didn’t take long to work out that there was no voltage reach­ing pin 7 of IC303 and this was due to R327 being open circuit. In fact, it was so badly burnt I couldn’t read its value and I didn’t have a circuit for this exact model. I did, however, have circuits for the 289X7M and 289X8M models but they each had a different value for this part, one indicating 8.2Ω and the other 4.7Ω. I chose a 10Ω resistor as, at the time, I didn’t have anything smaller. At switch-on, this component began to smoulder, indicating a probable short in IC303. I replaced the IC and at last had a picture and the resis­tor ran cool. The linearity was poor and this was attributable to two red electrolytic capacitors. Both C303 (1µF, 50V) and C317 (2.2µF, 50V) had spat the dummy and leaked onto the board. After cleaning up the corrosion and fitting new 105°C capacitors, the picture was at last perfect. I left it on soak test for a day or two before the lady picked it up. However, that wasn’t the end of the story. A week later it magically reappeared, with another note saying that there was a kink in the picture about two-thirds of the way up the screen. Disappointed, I rechecked and replaced everything I had done, just in case but it wasn’t until I replaced the previously re­placed R327, this time with a smaller value (4.7Ω), that the fault was finally cleared. I can only surmise that the 10Ω resis­tor I had fitted earlier had been weakened when 58  Silicon Chip it smouldered and subsequently had gradually increased in value. Anyway, I have my fingers crossed that this will be the last I see of this set for quite a while. I’m sure Mrs Sinclair feels the same. A write-off Mr Berry was very distressed; someone had broken into his house and tried to steal his TV set. I say tried because the thief found that the window was too small for the Philips 25GX1885 59cm model (Anubis BB chassis) that he was trying to steal. So in true caring style, the robber dropped the set about a metre from the window sill to the concrete floor and then made his escape. Amazingly, the set still worked but the case was cracked and the tube had a deep scratch in it. Fortunately, the set was insured so I checked the replace­ment prices: $185 for the cabinet and $1035 for the tube. The set only cost $999 new, complete in its box, and the insurance compa­ny took the logical option to replace it with a new set. So that let me off the hook. And, in any case, I wouldn’t want a scratched picture tube hanging around the shop until it had been let down to air. I have seen what an imploding tube can do when it goes off. Secondhand sets And now for a change of pace. Some time ago, I accumulated a number of working secondhand sets and decided to display them for sale in the shop. If nothing else, it would get them out of the way and bring in a few dollars. One of these was a secondhand Teac Televideo MV1440 which I switched on every day. Although I have an antenna distribution amplifier, there were too many of these sets and not enough antenna sockets for all of them, so some were connected to VCRs, some to the external antenna and some to indoor antennas. Recep­ tion from the external antenna is good but, as I am located in a valley, it is poor from an indoor antenna. Unfortunately, it’s not uncommon for someone to come in when I’m extremely busy and want to check out every – and I mean every – item on display. This bloke chose such a moment but wasn’t particularly interested in any of the sets that were switched on and running. Instead, he wanted to see a 34cm NEC that was tucked on a top shelf, in an inconvenient corner and, of course, attached to only an indoor antenna. I couldn’t persuade him that any of the others was a better buy; he was insistent that he should see this one work. I explained, “Yes, it works very well but as it’s only con­nected to an indoor antenna, there will be some ghosting”. The customer seemed very intent on this set so, after nearly killing myself, I climbed up through a precariously pre­sented display, found the power lead and plugged it in. The picture was bright and sharp but obviously ghosting and I could see that the customer’s eyes were glazing over and he had moved onto the Teac Televideo VCR which was playing tapes. “Well?” I asked him, “do you want the NEC”. “No”, he said; “I don’t want a TV set with ghosting”. I tried to explain that the ghosting was only due to the antenna but it was pointless; he had completely lost interest and was now intent on the Teac. Obviously, this bloke had the attention span of a gnat. I had to work fast. The Teac was easily accessible and I could swap an antenna lead with another set to demonstrate the off-air reception. Now this set had been in the window for months – for some reason it just hadn’t sold. Not that I had worried too much; I figured that it would sell sooner or later. Anyway, when I tried to demonstrate the off-air reception, the sound was OK but there was no picture. I Fig.1: the circuitry around the vertical output IC (IC303) in the Toshiba 289X9M. Dry joints on this IC sometimes cause problems but, in this case, R327 had also burnt out. didn’t panic imme­diately as I felt sure that it was the AV switch incorporated in the BNC socket that was sticking but after fiddling with it for five minutes, the customer said he would call back later when it was working. “Yeah and pigs might fly,” I thought. Embarrassed and feeling somewhat foolish, I picked up the offending Televideo VCR and took it into the workshop. I really couldn’t understand why it was working yesterday but not now but I suppose this is how everyone feels when something breaks down. I connected a signal generator into the AV BNC socket and the set gave a very clear picture. This could only mean that the video was being lost between the video detector and this socket. After removing the chassis, I followed the circuit back from the BNC socket switch until it disappeared underneath an electrolytic capacitor soldered onto the copper side of the board. This capacitor was anchored by a black substance, which on closer examination turned out to be the old brown corrosive glue we all like to curse. I removed the hardened black substance and located the track underneath it, which had corroded clean through. I then fitted a link across the gap and reassembled the TV set. It now worked perfectly and was back in the showroom window with a good antenna and running on Channel 9 for the cricket. Now I wonder – will that bloke ever come back? I thought it was just too bad that the glue had corroded right through the track in the last 12 hours – I deserve better! The snowy Philips Mr and Mrs Grogan own a Philips 28GR671 TV, which employs a G111-S chassis. They live in a nice spot on the side of a hill with magnificent views. However, because the VHF transmitters are on the other side of the hill, they were dependent on reception from a UHF translator. Because they were apparently not getting good reception, especially on SBS and Ch.2, they decided to subscribe to cable TV. However, when this was installed they were still getting snowy pictures, which indicated a problem with the TV set itself. In this case, the RF output of the set-top converter was connected to the antenna terminals of the TV set via a combiner (the external antenna fed the other input of the combiner). This meant that, as far as the TV set was concerned, the cable signals were just like an off-air UHF signal. Because everything was on UHF, I was surprised to see that the higher channels – 7, 9 and 10 on Band V – were giving good reception; it was just the lower ones on Band IV that were snowy. I checked the antenna installation out and everything seemed OK. I then connected the antenna to a portable loan set I had with me and there was no problem with that. At this stage, I decided to put the problem into the “too hard” basket and to take the set back to the workshop. At least, I would have time to think there and sort out this rather per­plexing problem. When I subsequently connected the set to my antenna, all the stations were perfect with no snow at all. Puzzled by this, I assumed that the set must have come good in the truck on the way back, and though I tried tapping, heating and cooling, I couldn’t fault the reception on any channel. In the end, I could only take the set back to the Grogans and make some rather weak excuses. When I finally got it back into its resting place (no mean feat, as it is a big and heavy set), I switched it on and was horrified to see that it was still snowy on the lower channel numbers. I just couldn’t believe it – what was I overlooking? It just didn’t make sense. I spent half an hour rechecking every­ thing before admitting defeat and taking it back to the workshop where, of course, the reception was still perfect. Eventually, I realised that it was possibly an AGC fault. However, when I adjusted the RF AGC control (VR­ 301212), I found that it was already set to its optimal position and could take the set from snow to signal overload as expected. Next I tried fitting a 6dB attenuator but this made no difference on my powerful antenna distribution system. However, when I got to 18dB attenuation, I finally managed to recreate the situation the Grogans were experiencing – the higher channels were better than the lower ones. I reached in to have another go at the AGC control when my hand brushed against the tuner and I noticed the snow momentarily clear up. Well, that was it. There was a bad connection between the tuner’s metal case and the main metal chassis frame. The problem was not that the tuner wasn’t earthed, rather that it wasn’t supplying a ground rail for other circuits in the small signal, IF and AGC areas. Anyway, that fixed up the fault even when it was back at the SC Grogans’ home. February 1999  59