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H only I'd known all the symptoms My main story this month has two levels of interest; the purely technical one - because I have never seen a fault like this before - and the human interest one involving the customer's behaviour and reaction. The story concerns a National TC-2656 63cm colour set, fitted with the M11B chassis. The owner has been a customer for many years but this was the first time that I had serviced this particular set. The symptoms were straightforward enough; according to the owner the set was dead, having failed in the middle of a program. In view of what transpired, there were probably other symptoms as well but I can hardly blame him for failing to realise their importance. Well, he was right as far as it went an:d the cause was not hard to find. A couple of fuses had gone and this led me to the horizontal output transistor, Q551 (2SD951}, which had cashed in its chips. And in the process it had taken out the regulator transistor in the power supply, Q801 (2SD850}. r---, -+-----:-"-" L - - These two devices were replaced and this was enough to get the set working. However, one of my routine replacements in this series of chassis is C555. This is a 10µF 250V electrolytic smoothing capacitor for a 190V rail derived from pin 8 of the horizontal transformer, via diode D555. This rail supplies, among other things, the RGB driver stages. This capacitor is notoriously unreliable and can fail in a variety of ways, giving rise to an equal variety of symptoms, most of them quite puzzling unless one is a wakeup. The first one I encountered was a ringing pattern on the left hand side of the picture, which led me a merry dance for awhile. By contrast, I replaced one recently as a matter of routine, which was completely dried out with almost no r-- - - · • ,-v.,....-,,""' ...J Q501 8Vp-p 20/.ISOC . Fig.1: there's not much to go wrong in this part of the MllB circuit but the job was more costly than it should have been. 52 SILICON CHIP measureable capacitance, yet it appeared to have no adverse effect on the set's performance. But I always change it anyway. Having done all that, I switched the set on and everything performed normally. I did get the impression that the picture tube might be getting a bit long in the tooth but the picture was still quite reasonable. I ran the set for various periods over the next few days and all seemed well. So I returned it to the customer. It wasn't "orright" That was fine - for about a week. Then the customer was on the phone complaining that the set was not satisfactory. Apparently it was still going but his complaint was that the picture quality was poor and, more significantly, that the picture was creeping in from the sides. I don't muck about in cases like this. Any set that bounces within weeks - or longer for that matter - is cause for concern. It doesn't really matter whether it has anything to do with the original fault or not; the customer automatically assumes it is and a deal of diplomacy - and compromise - is often needed to preserve good customer relations. So I said, "Bring it in right away; let's have a look at it". "Yeah, orright." Only it wasn't "orright". He didn't bring it in that day, or the next, and by then I was involved in other jobs and lost track of the situation. In fact, it was three weeks before I heard from him again. Then he turned up with the set and the announcement that it was dead again. The reason for the delay wasn't clear and diplomacy dictated that I shouldn't pursue it. Well, it didn't take a genius to find the trouble; the horizontal out- •• .. "ti"" uou,o.,:• II d'_--.t, ----- , ~ :: ' \ . ~ ) {_/\J ~ ,..--..._____ ·-· l'J\"( S\G 0 .. ~~-;;,--.. -. GUN \JJ~'S P-. C'RO put transistor, Q551, had gone again, but this time it hadn't taken out anything else. So a new 2SD951 put the set back into operation. But there was obviously more to it than that. The fact that the first replacement had lasted no more than a couple of weeks was suspicious enough in itself; coupled with the additional symptom about the picture shrinking horizontally, it left no doubt that there was something more serious involved. There was no immediate indication of picture shrinkage, so I found a spot for the set, near the working area, where I could keep a close watch on its behaviour. If the shrinking picture was a prelude to the transistor failure as I suspected- I wanted to be there at the first sign; these transistors are not cheap. And, to make any such change easier to observe, I fed it with a test pattern. It took several hours far anything untoward to show, and then I detected a slight inwards creep at the edges of the picture; so slight that I would have missed it on program material. I switched off immediately and moved it onto the workbench to do battle. The big gun My big gun was a CRO - a rather special type of CRO which I acquired some time ago and which quickly proved invaluable once I learned to drive it. It is a digital storage CRO and its main feature is that waveforms can be stored indefinitely, then called up as required and superimposed on a later waveform for exact comparison. More specifically, it can store up to four waveforms and any one may be compared with either another stored waveform, or an active waveform; one running "live" on the screen. Unfortunately, instruments of this type are not cheap but with today's complex circuitry, and equally complex faults, they are becoming more of a necessity than a luxury. I connected one beam of the CRO to the collector of the output transistor and the other beam to the TETIA TV TIP Philips K9 chassis Symptom: No sound. In the K9 chassis this trouble is usually a faulty IC in U240 but not in this case. We found good sound returning from the volume control but nothing at the input to the first audio amplifier. Cure: D245, a BAW62 diode near Plug P1, short-circuit. This is one of two back-to-back diodes, presumably included in the circuit to limit the input to the audio preamplifier. TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania 7015. base. But by the time I had everything hooked up the set was back to normal, so I stored both waveforms. Then I let it run until the picture began to shrink again. I let this go on for as long as I dared, then stored both waveforms again. Then I brought up the previous waveforms and superimposed the current ones on them. As far as the collector patterns were concerned there was virtually no difference, but there was quite a significant difference between the two base patterns; enough to convince me that I was on the right track. Next I moved to the driver transistor, Q501 (2SC1573), and went through the same ritual. This produced the opposite result; the base pattern didn't change but the collector pattern did. So it appeared that the fault was somewhere between the base of Q501 and the base of Q551. This didn't involve much; just Q501, the driver transformer T501, and a few minor components which were quickly cleared. The best bet was Q501 and the best check was a shot of freezer. And that produced an instant cure. So out came the 2SC1573 and in went a replacement. It looked good too - for awhile. Then the fault was back exactly as before. That didn't leave much, except the transformer. But what kind of transformer fault would produce a gradual temperature fault like this? And why did freezing Q501 cure the fault? I had a feeling that I knew the answer to the second question; when I sprayed the transistor, some overspray probably reached the transformer. Fortunately, I had a spare one on hand and took only a few minutes to fit it. Reckoning up And that really did fix it. I ran the set for long periods over the next week and the pattern remained rock steady. Then I rang the owner and told him it was ready. He duly turned up to collect the set and was happy to see it working correctly. But he wasn't so happy when I presented him with another account. I had limited the charge to the OCT0BER1990 53 SERVICEMAN'S LOG -CTD ( 1989, when all these things were supposed to be in place. Not surprisingly, this has taken a good deal longer than bureaucracy had planned. And, while most of it has now been implemented, the Milton translator is still to come. In addition, the VHF channels, 4 and 5A, are still operating, the shutdown date having been extended several times. My colleague takes up the story. Colleagues's story cost of components - a little over $40 - but he obviously had imagined that there would be no further charge. So, as diplomatically as possible, I had to spell it out. The cost of the transformer was fully justified because it should have b<Jen replaced the first time around (had I been a little more astute and had he told me all the symptoms). I also pointed out that the cost of the new output transistor was also justified because had he returned the set when I asked him, it would not have been destroyed. Well, he mumbled and grumbled a bit, then finally took the set saying he would pay me later. I wasn't worried about that; he's perfectly honest but it was obvious he was taking a spot of insurance. Anyway, he came back a week later and settled the account by which time he was loud in his praise of the set. So it all ended happily. Don't you worry And now for a complete change of theme. In last month's notes, while introducing my colleague from the Illawarra area, I mentioned the change over from VHF to UHF TV in that region. It was, he said, a rather laborious process. Just how laborious became evident when we had a much longer 54 SILICON CHIP discussion. The upshot was a suggestion that he summarise the present situation and, as far as possible, predict the future. This he has done and we will get to that shortly. But first, for those not familiar with the area and its TV history, here is a brief recap of the story. The conversion to UHF is part of a broader plan to bring as many channels to country areas as are available to city dwellers. Also, in the Illawarra area, the aim is to eliminate the contentious VHF channels 4 and 5A. (Channel 4 occupies part of the established FM band, while channel 5A clashes with international satellite channels and the adjacent amateur 2-metre amateur band). The plan called for five UHF transmitters (the ABC, three commercials and SBS) at the original VHF site at Knight's Hill. To compensate for the reduced coverage at UHF, power was to be increased by a factor of 10 - from lO0kW to lO00kW (lMW), along with a taller antenna mast. In addition, three UHF translators were to be provided: one near Nowra, one near Bateman's Bay alongside VHF translators 9 and 11, and one inland from Milton. And the main VHF transmitters were due to be turned off in early At Knight's Hill, five UHF transmitters are operating - ABC, SBS, WIN, Prime, and Capital. All but WIN are on the taller mast but all with less than the planned lMW ERP - in fact, it's more like 600kW. At Cambewarra, near Nowra, four translators are operating: ABC and WIN translators with an ERP (effective radiated power) of 100W, and Capital and Prime at lkW. The ABC and WIN translators will eventually be upgraded to lkW, with the possibility of SBS at the same site. In the Bateman's Bay area, two •Wollongong Knight's1 Hin Kangaroo • •Kiama Valley:; Cambewarra • Translators OW is Mitton Translators• -a.Bateman's \ Bay • Bateman's Bay Translators Fig.2: this map shows the area from Sydney to Moruya on the NSW east coast, which is presently being converted to UHF TV. The Milton translator is yet to be built. 2kW UHF translators carrying the Capital and Prime programs have been installed alongside the VHF translators, 9 and 11, which carry the ABC and WIN programs. This mixture of VHF and UHF is to be permanent. This leaves the Milton translator still to come and this has become a major stumbling block. The planned site is about 5km west of Milton, on the edge of an escarpment some 400 metres above sea level. The plan calls for five translators, including SBS, each of 40kW. But the site is just inside the boundary of the Morton National Park, near the Little Forest trig site. This has created a conflict between the National Parks and Wildlife Service and the Department of Transport and Communications, which has been going on for nearly three years. It appears no nearer solution now than when it started. But even when - or if - the Milton translator eventuates, the overall result will be no more than a poor imitation of the coverage pro- vided on VHF. And that brings us to the main point of this discussion; by what stretch of the imagination was anyone so naive as to think that a VHF system could be replaced by a UHF system, on the simple basis of increasing the transmitter power by a factor of 10 - or any factor J or that matter. UHF signals simply do not work that way. Granted, the authorities made the gesture of prescribing a couple of extra translator sites. These will serve their immediate areas, but cannot fill in the gaps in between and there are plenty of those. Jn the meantime, the Milton translator's intended area is still dependent on the old VHF transmitters but, for WIN 4 particularly, transmission is becoming more and more precarious. This transmitter should have been replaced several years ago but has been flogged along from one postponed shutdown to another. I have monitored it on the same antenna for the past several years and it is now 12dB down on its original strength. I still get a good picture but plenty of IN ANYONE'S LANGUAGE' PROCON TECHNOLOGY LETS YOU TAKE CONTROL! We manufacture a wide range of 'real-world' digital 1/0 boards. Each board features: 8 opto-isolated inputs, 8 isolated outputs (relay or solid-state AC/DC), LED indication is provided on all I/O and IBM-PC Software is included. 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Although this is close to the Bateman's Bay translator site, there is severe shielding between it and the extensive built-up area around Bateman's Bay township. The translators provide good coverage to the south, including Moruya, but not into Bateman's Bay. As a result, most people have relied on VHF signals from WIN and ABC which provided some service, in spite of the distance. Retired signals But a loss of 12dB is intolerable in such circumstances. A retirement village in the area, built to cater for 150 occupants, requested a quote for a distribution system for the complex. I passed the preliminaries over to an antenna installation colleague, to find out just what signals were available. He reported that UHF signals were no better than 30dBu, or around 32µV, while the VHF signals TV TEST EQUIPMENT (AUSTRALIAN MADE) SHORTED TURNS TESTER Built in meter to check EHT transformers including split diode type, yokes and drive transformers. $78.00 + $3.00 p&p HI-VOLT AGE PROBE Built-in meter reads positive or negative 0-50kV. Can also be switched to check negative voltages. For checking EHT and Focus voltages as well as TVs and _Microwaves. Hi-tension voltages. $98.00 + $5.00 p&p LOW-VOLT AGE PROBE Ideal for checking microwave ovens and TVs. The ~ ranges are from Oto 5kV and from Oto 1 OkV. Double insulated for safety. $79.00 + $5.00 p&p D DEGAUSSING WAND Strong magnetic field, larger than usual coil with multicore centre. Double insulated for safety with momentary switch operation. 240VA/C 2.2 amps. As important as having a soldering iron! $75.00 + $10.00 p&p TUNER REPAIRS We repair most tuners on an exchange basis. From only $1 7 .00 + p&p Cheque, Money Order, Bankcard or MasterCard Y.V.TuNERs) 216 Canterbury Road, Revesby, NSW 2212 Phone (02) 77 4 1154 OCTOBER 1990 55 -- were only a couple of dB better. Such levels are quite useless. A generally accepted engineering standard for colour is lmV, although 500µ V is tolerable. Below that, performance falls off rapidly, particularly at UHF where receiver front-end noise tends to be higher. So the village management was told that nothing could be done in the conventional way. The only possibility is a satellite dish to provide SBS and ABC programs. But this would make a mess of at least $3000 - probably more - just to get a signal. The cost of distribution to the 150 occupants would be over and above this and we await ,their decision. Much the same problem applies to a lot of Bateman's Bay. Another problem area is Kangaroo Valley. This is a low lying area, surrounded by hills, roughly midway between Knight's Hill and Nowra. Because it is close to Knight's Hill, it has enjoyed some VHF reception, in spite of the hills. To a degree, the same applies at UHF, with quite strong signals in many places. The snag is multiple ghosting which makes reception quite useless. The situation is so bad that local antenna installation teams will not even bother to survey the area; they advise everyone that there is no 56 SILICON CHIP ' point in trying for UHF reception. Another problem - in all areas - is the way in which UHF signals vary from day to day, depending on weather and other conditions. Signal variations of ± 6dB are quite common. As a result, antenna teams will no longer guarantee the performance of any UHF installation. They will do a field strength survey, give the best advice they can, and install the best antenna the customer can afford. After that, the customer is on his own. Shut-down date The shut-down date for the VHF transmitters is now November 1990. Before that it was November 1989, with other extensions before that. So most of us down here are adopting a believe it when it happens attitude. But one thing is certain; when it does happen, it will come as a major shock to the general public. Sure, there has been plenty of publicity and the public are supposed to be informed, but much of it hasn't penetrated. Nor has this been helped by many conflicting announcements. A typical example was a recent announcement in a Milton/Ulladulla newspaper to the effect that all the problems associated with the Milton repeater had been solved; that everything would now go ahead as planned. The only snag was that when various colleagues and myself sought confirmation from DoTC, they knew nothing about it. Nor did WIN4 or radio station 2ST, in Nowra, both of whom have a vested interest in the situation. More particularly, they need to know when the translator will come on line and when WIN4 VHF is to close WIN4 for obvious reasons and 2ST because they have an FM licence which cannot be exercised until WIN4 VHF closes. Further confusion resulted from an article in June 1990 Broadcast Engineering. It says: "After considering the results of DoTC testing ... views of residents and ... bushwalking and parks organisations, the enquiry recommended that no action be taken to acquire land in Morton National Park, until investigations have been made into an alternative system." In view of all this, the local retail and servicing industry, which bears the brunt of the public's complaints, has reason to be very concerned about the future. We know that when the VHF system shuts down, the truth will hit home hard. There are going to be large areas of population with virtually no TV service at all. And we're not looking forward to that. Granted, those people in good locations have already benefitted. They have an extra two commercial channels, five channels in all, and will continue to enjoy this service. But the price is no TV at all for many people who now have it. In summary then, it has all the hallmarks of a good idea which has become an administrative nightmare and gone very wrong. Comment Well, that's my colleague's story. My reaction is to be devoutly thankful that I work in a predominantly VHF area. I also wonder how this conversion scheme is working out in other areas since, as I understand it, it is planned as a nationwide project. Let's hope that they are faring better than the 11lawarra area. ~