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It's an ill wind... , as they say "It's an ill wind that blows nobody any good", according to the popular saying. Well, this was a real wind that came close to qualifying. The only person to benefit was yours tr1:1ly and then mainly because it gave me a story to write. It happened in the early hours of a Saturday morning. It was a particularly violent local squall some suggest it was a willy-willy and it did a fair amount of minor damage to trees and houses. Its best - or worst - effort was to lift an iron roof off a house, then carry one large sheet of iron across a road and into an 1 lkV high tension line. That was bad enough - for the llkV line - but it made contact with the 240V line underneath it at the same time. It was probably only a fleeting contact because the sheet finished up further down the road, but that kind of mix-up doesn't have to last for long to have serious results. And this is the story of those results as I saw them. (Incidentally, the mention of brand names should not be taken as any indication of vulnerability, or otherwise, of that brand. There is no obvious pattern). The first I knew of the incident was when I opened the shop that Saturday morning. The phone rang and it was one of my regular customers, with the gist of what I have just related, plus a tale of wocl concerning his own appliances. It was quite a list. He has two t ~"'a-.... . ~- .. ~~ l..lGH1"Nl~G SlM?L-'( "DOESN'1'° K.NOvJ THE. 'f<UL..E:S 60 SILICON CHIP cc>o 6fO~- ,; video recorders (a National NV-370 and an AW A AV-14 ), two TV sets (a Sanyo CTP 5604 and an unknown brand), two clocks, a National microwave oven and two small motors driving fish-tank circulation pumps. Out of that lot, only one TV set had survived - the one whose brand I can't remember. Then to cap it all, the TV distribution amplifier in his home unit block had copped it. In greater detail, both video recorders had suffered open circuit power transformer primaries but no other damage. Repairs were more or less routine. The Sanyo TV set had suffered a similar fate, in this case to a small transformer used to provide 6.3V for the picture tube heater. This transformer is also used to pre-heat the picture tube when the set is turned off and so is connected permanently across the mains while the remainder of the set is disconnected. In the OFF condition, one set of switch contacts opens the 24ov line to the set proper, while the second set opens the heater circuit but are bridged with a power diode (a more effective device than a wasteful dropping resistor). The microwave oven had not suffered any really serious damage at all. This model houses its surge protection components - varistors, fuses, etc - in a single package and this had done its job well, sacrificing itself to protect the rest of the device. The replacement package is relatively inexpensive and so that problem was easily solved. As for the two clocks - well, the motor windings were open in both cases and they were a write off. The fish-tank motors were low voltage devices, run from a transformer, and this latter had lost its primary also. I referred him to the local pet shop to seek a replacement transformer. All of which added up to a pretty sordid story of destruction and financial loss. But it was only the beginning. Other customers from the home-unit block began arriving shortly after, with similar problems, along with others from the same area. And they kept coming; not just for that day but for days and, in some cases, weeks after. Apparently some had been on holidays; others simply hadn't had occasion to use a particular appliance. SILICON CHIP BINDERS * High quality * Hold up to 14 issues * 80mm internal width * Gold printed with SILICON CHIP logo on spine The mains supply All this aroused my curiosity to the point where I made some further enquiries about the area involved. As I hinted earlier, the violence was restricted to a very limited area; an area somewhat isolated from other housing. And it was isolated in another sense electrically. The whole area consists of two streets and a cul-de-sac, and this is serviced exclusively by one modest size substation. As a result, relatively few homes were affected; it could have been much worse. It could also have been worse had it happened during the day or evening, when many more appliances would have been on line. My final count, including the above, was eight ovens, six video recorders, one TV set, four electric clocks, a Philips battery/mains radio, the home-unit TV distribution amplifier and the fish-tank transformer. And since several colleagues also service this area, the total count would have been much higher. At first glance it may seem surprising that there were not more TV sets involved, but this is probably because they would be switched off at that time, and most sets feature a double pole mains switch which would provide a high degree of isolation. In this regard the situation would appear to differ from that of a lightning strike, where the voltage could be much higher high enough to jump across a set of switch contacts. The video recorders involved · were of three brands: National AVAILABLE NOW These beautifully made binders protect your copies of SILICON CHIP. In distinctive two-tone green high-quality vinyl specially selected for SILICON CHIP, and with heavy board covers, each binder holds a year's issues (the 14 issues of Vol.1 or the 12 issues of Vol.2). It will look great on your bookshelf. -------------------~-Yes! Please send me _ _ _ _ SILICON CHIP binder(s) at $A 11.95 pl us $A3 p&p each (NZ p&p $A6). Name _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ __ (PLEASE PRINT) Street_ _ __ _ __ _ __ _ __ _ _ __ __ _ _ __ Suburb/town _ _ _ _ _ _ __ _ _ _ _ Postcode _ _ _ _ __ Enclosed is my cheque/money order for $ _ _ _ _ or please debit my □ Bankcard □ Visa Card □ MasterCard Signature _ _ __ _ _ __ _ _ _ _ Card expiry date_ ~ -- ~-----------------------~ APRIL 1990 61 SERVICEMAN'S LOG -CTD Panasonic, AW A and Rank NEC. And they all suffered the same fate - an open circuit power transformer primary. But rather interestingly, in most cases the mains fuses were still intact. Also in most cases, that was the only damage. The exception was the National Panasonic NVG-7, in which two !Cs in the servo circuits had also been destroyed. Oven faults & repairs The ovens presented a more varied batch of faults. With the exception of the first customer's oven, already dealt with - and which used a mechanical timer - they all featured touch pad microprocessor control. Such units normally use two power transformers; a heavy duty one supplying the magnetron and associated circuitry, and a light duty one supplying the microprocessor and clock. In all cases the heavy duty transformer and associated components survived. Modern oven circuitry switches the magnetron on or off in the primary of the heavy duty transformer, so this would be open on at least one leg when not in use. On the other hand, only one of the light duty transformers survived. In addition, there were varying degrees of other damage. In two cases, the board had suffered minor damage in the form of vapourised copper tracks feeding the small transformer primary, plus complete destruction of the protective varistors and fuses across the mains. The copper tracks were very thin and I suspect that this was deliberate, as a form of protection. There was no other damage and it was a simple job to bridge these tracks and fit new varistors etc. Not so two other boards. Both had suffered damaged tracks in several places, with one suffering much more damage than the other. The varistors had been sacrificed in both cases but strangely, the transformer on the badly damaged board had survived. The price of replacement boards has been quite high in the past; so high that most customers elected to add a few 62 SILICON CHIP dollars to the repair price and buy a new oven! On this basis I decided to try a spot of cannibalising - taking the good transformer from the badly damaged board and fitting it to the less damaged one, in the hope that a few repairs to this might get it going again. It was a simple job but it didn't work out. I suspect that some of the !Cs had also been damaged. So the next step was to confirm the price of replacement boards and quote the customers accordingly. And this turned out to be the brightest spot of the whole operation. The price of boards had dropped dramatically; in some cases to one third of the previous figures. On this basis there was no point in trying to salvage anything; I simply ordered new boards, fitted them, and had two more (reasonably) satisfied customers. The remainder of the ovens fell into the simplest category; open circuit transformer primary and destruction of the varistor/fuse protection network. Straight out component replacement was all that was needed to get them back into operation. TETIA TV TIP Sony KV-1830AS (early version, before S/N 300,001) Symptom: No picture until set thoroughly warmed up (about 1 0 minutes). Luminance could be forced through by raising screen voltage, but there was no sign of colour. Cure: Fault turned out to be an inductor in the collector of the 7 .8kHz amplifier transistor, 0309. The coil was open circuit when cold but came good as it warmed up. (The fault explains the "no colour" but not the "no luminance", thus making a very confusing problem). 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. The distribution amplifier in the home unit had suffered a similar fate; open circuit transformer primary. Again a routine replacement was all that was needed. The Philips radio set was quite dead, and the transformer primary was open. Whether there had been any other damage was not immediately apparent but since it was also designed for battery operation, I was able to rustle up an appropriate voltage source and check it out that way. It worked perfectly, so only a transformer is needed. A proper replacement is not immediately available and I might have to improvise. The main problem is physical compatibility but I'm sure I'll find something. Vulnerable appliances Summing up, the first point to make is that the most vulnerable appliances are those which, by reason of their intended function, are connected directly across the mains at all times. Video recorders and microwave ovens fall into this category. TV sets, on the other hand, are relatively safe unless actually in use. And if that point seems so obvious that it didn't need to be made, I did so because I wanted to emphasise one I made earlier: the difference between a voltage surge of this kind and one due to a lightning strike. It would appear that, even at 1lkV, most appliance switches are still effective, at least in the short term, even though one would not deliberately invoke such a risk. On the other hand, long experience indicates that when lightning strikes, nothing is safe; lightning simply doesn't know the rules. To some extent, this point may seem academic but I have found that it is a good thought to keep in mind when the customer asks, "Why?", or "Why the video recorder and not the TV set?", and so on. The other point to discuss concerns the effectiveness, or otherwise, of the varistor type protective networks. Do they work, or are they just a gimmick? Based on the above incidents, I have no doubts that they are wor- thwhile. Granted, most appliances were damaged in spite of these precautions but I strongly suspect that the damage could have been a good deal worse in most cases had they not been present. But there is one important point to note; the varistor is no better than the fuse which goes with it. In fact, its job in life is to blow that fuse at the first hint of trouble, and it can only do that if the fuse rating has been carefully selected. Granted, fuse ratings are always a compromise but the varistor makes possible a much more realistic compromise. More to the point, if you have to replace such a fuse, make sure that it is exactly as the maker specified it, and not just in current rating. And so ends the saga of the ill wind, the roofing iron, and the 1 lk V power lines. Here's hoping they tie the roofs down tighter next time. Red in the face Now, for a change of pace, here's something quite different. It is an incident that happened to a friend who, while in no sense a professional, is a very keen TV hobbyist. Professionally, before retirement, he was a commercial airline captain and his main hobbies are amateur radio and TV. He takes his hobbies very seriously, taking several TV trade courses over the years, including one at the commencement of colour. And he has built himself a couple of colour sets, AUDIO TRANSFORMED based on commercial modules; no mean feat by any standards. The truth is that he would undoubtedly make a first class serviceman, yet he has never ventured into this field. It is purely a hobby and when neighbours approach him for help he invariably refers them to me. However, his family is a different matter and when his married daughter's set failed recently, a mayday call went out for "Dad" to come to the rescue. The set is a Kriesler 59-3 and while a little long in the tooth, has given good service over the years. Such faults as did occur were minor until about two years ago, when it was realised that the picture tube - an A66-500X - was on the way out. So my friend removed the tube, took it across town to a well known tube rebuilding firm, and purchased a replacement. Then he fitted the new tube, converged it, grey scaled it, and brought the set back to as-new performance. And this is no simple job; even removing and replacing the tube is a major 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-VOLTAGE PROBE Built in meter reads positive or negative 0-50kV. For checking TVs, microwave ovens. $84.00 + $5.00 p&p. j4' FOR PROFESSIONAL, OEM OR AMATEUR l:l roadcast qua lity a u d io transform e rs ex-stock More than 70 standard types ava il ab le Fast proto t y ping service for n o n-sta nd a rd t y pe s Co mpre hensive data av ailable o n requ es t Lo cally manufac tu red Compe titively priced HARBUCH ELECTRONICS PTY LTD 90 G eo rge S t.. HOR NS BY :--/SW 20 77 Pho ne <02 1476-585 4 j TELE-TEST TUBE TESTER & REJUVENATOR Removes cathode grid shorts & rejuvenates picture tube . Suitable for all colour tubes. 12 months warranty. $420.00 + p&p. NEW PRODUCT DEGAUSSING WAND: A must lor all workshops. Efficient design, strong magnetic field, low heat, double insulated with momentary on/off. $75.00 + $1 0 p&p. ALSO TUNER REPAIRS FROM $17.00. Cheque, Money Order, Bankcard or MasterCard Y.V.TuNERs) 216 Canterbury Road, Revesby, NSW 2212. Phone (02) 77 4 1154 APRIL 1990 63 SERVICEMAN'S LOG -CTD ''"' . . - •.. •. . .. ~ ~SSUME. -rH~T . 1H~ \NE.\J\1Aa~ V\~TtOI'\, -E.VE:.N ON GOOD 'ROADS_, WAS SUF~lCIE.NT"''TO 1)\SL.0~ 1\-\E. FO'RE-lG~ 'P~'RTIC.,<-€ FROM "ffiE: GUN o physical task, having to be done from the front of the cabinet. All went well until a few weeks ago when the screen suddenly went brilliant red, with no sign of a picture. That initiated the call for help. A check revealed that the set would come up normally when first switched on, run for a few minutes, and then the red gun would turn hard on, swamping the image completely. Initial checks My friend's immediate reaction was to suspect an intermittent breakdown in the red driver transistor (TR506}. These do break down and produce these symptoms, and not only in the red circuit, of course. One way to check this is by opening the plug and socket on the main board - PL506 and SK506 which disconnects all three cathodes from their driver transistors. All three collector voltages will rise but by approximately the same amount if the transistors are intact. If one is substantially lower, then breakdown or leakage would be suspected. My friend did this and all the in64 SILICON CHIP dications were that the transistor was not at fault. So what did that leave? A short in the picture tube red gun was the horrible thought that came to his mind, and he voiced this fear when he sought my advice. I had to agree that this was a very real possibility but cautioned him about being too hasty. The leads from PL506 run to the neck board and there are a couple of traps here. One is the protective spark gaps across the tube elements, which have been known to fail and create all the symptoms of a faulty tube. Another is the tube socket, which can also break down or leak and create similar symptoms. Thus encouraged, my friend returned to the fray but as I learned later, the news was not good. He pulled the neck board/socket combination, which brought the red collector voltage back to normal, leaving the picture tube as the only suspect. So he and the family resigned themselves to the cost of a new tube. At the same time, he was surpris- ed at this turn of events because he knew from me that the firm concerned has a very high reputation, their rebuilt tubes being virtually as good as new. But the fact had to be faced so he pulled the tube out, loaded it in his car and took it across town - a lengthy journey to the factory. The tube was out of warranty but was still valuable and, in any case, he wanted final confirmation that it was faulty. So imagine his mixed feelings when, after the most exhaustive testing, the technician could find no sign of a breakdown and in addition, pronounced the tube as having better than 90% emission . Good news & bad It was a good news/bad news situation; the good news was that there was no picture tube fault the bad news that there appeared to be a weird fault in the set which had beaten him. So, back at the ranch, he set to and refitted the tube. Then he switched on and prepared to do battle with the mystery fault. What battle? What fault? Nothing wrong with the set; working perfectly. And it has continued to work perfectly for the past several weeks, right up to the time of writing. The answer? It's only a guess but we both came up with the same idea. In short, the fault in the tube had been real enough but the journey to the factory had cleared it. And in support of this theory is the manner in which the tube was transported. Normally he would have used its original carton, with it sitting in the same position as in the set. But the carton had long since been recycled, so my friend used a large foam rubber cushion which he put in the boot of the car, then sat the tube on it face down, and packed it with blankets and other cushions. As I mentioned, it was a fairly long trip and we assume that the inevitable vibration of the vehicle was sufficient to dislodge the foreign particle from the gun. As I say, it's only a guess but what other explanation is there? Unless time reveals something, I'm afraid we're stuck with it. ~