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SERVICEMAN'S LOG Murphy has two bob each way Coincidence, chance, luck, law of probability, Murphy's law - call it what you will, it plays a large part in servicing. Sometimes it helps; sometimes it hinders. And when it hinders it has the potential to make an innocent serviceman look like a rogue. No, it didn't happen this time; but that was mainly because the customer has been dealing with me for nearly 20 years and he does trust me. Had it been a new customer, of the suspicious type, it could well have been a different story. In more detail, the customer is the proprietor of a local motel and the story concerns two colour TV sets from his units. They were identical sets - 48cm HMV Model B4803s - but with quite different faults. When he rang me, he explained 20 SILICON CHIP that the more serious fault was a complete picture failure, although there was still normal sound. The other set was still working but had a pronounced red cast. He suggested I take the more serious one first, since the second one was still usable to some extent. When customers say "no picture" it prompts a series of logical questions. Is there any light on the screen? If so, is it a steady raster or is it just a snowstorm? Provided the customer can give clear answers to these questions, one can usually get a pretty good idea of what it looks like and where, in broad terms, the fault is likely to be. In this case, the customer was able to give a clear answer; it was a steady raster, suggesting that the sync pulses were being processed but that the video was being lost somewhere down the track. But that was about as far as I could go at that stage and so I left it to him to bring the set in. I was familiar with the set in general terms and didn't anticipate any real problems. This model is about 12 years old and although carrying the HMV label, with the doggie on the front, it is actually a Japanese chassis, from JVC. And since I was very familiar with most JVC chassis and had a good stock of manuals, I reckoned it would be straightforward. When it turned up, I found it much as the customer had described; normal sound and a nor- mal raster. The only difference was that I detected a faint image and a few faint blobs of colour, particularly when I subsequently fed in a test card. But there was another effect which the customer hadn't mentioned; the raster on the screen was very bright and the brightness control had virtually no control over it. On the other hand, I noted that adjusting the colour control did have an effect on the brightness - not a very great effect mind you, but it was there. It also had a slight effect on the saturation of such colour blobs as were visible. A strange chassis I pulled the back off and took a closer look at the works. And that was the first setback. I thought I knew most of the JVC chassis but this was a strange one. More to the point, a thorough search through both my HMV and JVC manuals confirmed that I didn't have the correct manual, or even a circuit, for it. Tracking down a circuit diagram was obviously going to take some time and so I rang the customer to explain that there was going to be some delay. And that was the first bit of good news; he had kept the instruction books for each of the sets he had bought and these included a circuit. He had a stack of them and I was welcome to one for keeps. As he put it, they were double Dutch to him and so that problem was solved. He duly delivered one of the books and the relevant portion of the circuit is reproduced here (see Fig.1). On the left is the video sync separator, IC201, and to the right of it the video output transistor, X201, fed from pin 16 of IC201. Next on the right is the matrix/ colour IC, IC302. This delivers the RY, B-Y and G-Y signals from pins 6, 2 and 4 respectively to the CRT board, SU-3033A, at extreme right. Also, pin 16 of this IC takes the colour control voltage from the colour control pot (R13) which is on the control board, SU-4205A, at top right. (As an aside, note that the 7.2V on pins 2, 4 and 6 mysteriously becomes 7.4V on the bases of the blue, red and green output transistors, XlOl, 102 and 103. But contradictions like this are nothing unusual in modern circuits.) I checked the voltage on pin 16 of IC302 and this appeared to be responding normally to any adjustment of the colour control. From there, I moved to the three output transistors on the CRT board and checked the collector voltages. These were significantly lower - by about 20V - than the figures suggested on the circuit and enough to make them suspect. Similarly, the base voltages seemed to be marginally high, as one might expect, although it was difficult to be sure since it doesn't take much change here to upset the collector values. In any case, this all added up to a theory that all three guns were being turned hard on, effectively masking any video that might be present. And that was the next check; the luminance path. This starts at pin 16 of IC201 and goes to the base of the video output transistor, X201. The output appears at the emitter of this stage and is fed to pin 1 on the CRT board and thence to the emitter circuits of the RGB output stages. A voltage check around X201 produced figures very close to those on the circuit; nothing really suspicious there. The CRO also confirmed that there was normal video into and out of this transistor and from there into the emitters of the three output transistors-all of which further supported the idea that it was a hard turn-on problem. So what about the brightness control, R30, on the control board? Basically, this is between the +12V rail and chassis, with the moving arm coming out on pin 1 and connecting to pin 14 of IC201. However, the voltage range that this provides is nothing like 12V. Limiting resistors R31, R32, R33 and R34 restrict the range considerably. But by how much? I had no way of knowing but a measurement on pin 14 indicated from 9.5V to a little over 10V; only about half a volt. This seemed to be rather small but, on the other hand, seemed to tally with the .-.·~~-] - :~ r - ~ Wi I ~ , o :6a : [f ___ f]Oa- ~:'i 0 J ~? ;., ::~ - - l l - -- L-..J--------1-+------H----+--' I ' - - - - --- - - . - - ; , . _ . . . . - - - - l - + - - - - - ~ E: -----, ....__......._.. ug~ ]~ ~~ ,1 g~ ~ -· :~ - : pn-u : ~: f '-Y,••~ -- - -- -- - t - , ~ Fig.1: relevant section of the faulty HMV B4803. IC201 is the video sync separator and drives video output transistor X201 which in turn drives the emitter circuits of the RGB driver transistors (X101, X102 & X103). OCT0BER1991 21 SERVICEMAN'S LOG - CTD 1-\0W M\JC~ WOOL.'0 1\-\~ !V\\SSIJ'S &. l<\t)'S F6--rC-H ?.. -- "" ,,. . 1/ ' the almost unbelievable; two faulty ICs in the one set and both contributing to the same major symptom. See what I mean by Murphy having two bob each way! Of course, the prognosis still had to be confirmed. So another IC was ordered, delivered and fitted. And that really fixed everything. The set was given a routine grey scale adjustment, a soak test for a day or so and returned to the customer. And that was the end of that story. It had taken a lot longer and cost more than either I or the customer had anticipated. But he accepted it all philosophically, collected the set and delivered the second one; the one with the red cast. Too much red 9.6V shown on pin 14 ofIC201. So we put that one on hold for the moment. My thoughts went back to IC302 and the fact that adjusting the colour control not only affected what colour there was to be seen, but also the brightness which, in my book, was all wrong. A toss-up So what it all boiled down to was a toss-up between these two ICs. Was it IC201, as suggested by the seemingly small voltage change on pin 14, but about which I was not sure? Or was it IC302, as suggested by the interaction between the colour control and the brightness function and which I felt sure was incorrect? Not having either IC in stock, I had to make a choice and hope I ordered the right one. If Murphy had his way, of course, which ever one I ordered would be wrong, but I had to make a decision. After some thought I plumped for IC302 but, as it turned out, Murphy was having two bob each way. So the IC was ordered, was duly delivered and fitted. I then switched on and waited for the tube to warm 22 SILICON CHIP up. As it did, I was relieved to see a picture come up on the screen; it seemed I had made the right choice. But the joy was short lived. We had a picture all right and we had proper colour saturation control, but the picture was still too bright. It was not as bright as before but was still brighter than normal and, more importantly, there was insignificant control from the brightness pot, R30. So it was back to IC201 and the brightness control voltage. It appeared that my original doubts about the voltage range might be justified. In an effort to determine what this range should be, I disconnected this line from the main board, at pin 1 on the control board (this is actually a wirewrap connection). It was a different story now. With the line effectively disconnected from IC201, the voltage ranged from 9.5V to 11.5V; figures which made a lot more sense. But there was still about 9.5V on pin 14 and the picture was still too bright. I tried bridging pin 14 to chassis with some low value resistors, hoping to brute force the situation, but this had little effect. All of which forced me to accept Once again, his description of the fault was quite accurate; it was very red. Initially, I tried juggling the cutoff and drive presets, R102, R104, R106, R108 & Rl 10. However, it quickly became obvious that, while this produced some improvement, the error was far beyond such adjustments, even assuming that such an approach could be justified. And clearly, it wasn't. A voltage check around the red output stage confirmed what I now suspected; the stage was turned hard on. The voltages marked on the circuit suggested that this stage was normally turned on the least, but its collector voltage was now well below the other two. So, after a few more routine checks, I was forced once again to the conclusion that it was IC302. It was all getting rather monotonous but I ordered another one, which duly arrived and was fitted. And that fixed it. Or, rather, it did after a grey scale routine, my fiddling with the drives and cut-offs having put everything out of whack. So that finished that job and, after the customer had taken delivery, I mentally wrote both sets off. Murphy strikes But there was more to come. Some six weeks went by and then the customer was on the phone again. The first set was in trouble again and, judging by the customer's description, behaving just as it had the first time. I didn't trust myself to say more than, "Bring it in again". TETIA TV TIP Philips KT3A-2 chassis. Symptom: Bright red, blue or green screen, with similarly coloured retrace lines. The effect is exactly like a heater-cathode short in the picture tube but the heatercathode voltages remain near normal. The other cathode voltages can be greatly modified. Cure: Transistors 0241 (red), 0257 (green) or 0273 (blue) short circuited. The transistor is a PNP type BC558. On this occasion the fault was heat sensitive and was totally absent while the cabinet back was removed. is TETIA TV Tip supplied by the Tasmanian branch of the Electronic Technician's Institute ofAustralia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. Privately, I had a horrible fear that one of the ICs had shot its bolt again and that wasn't going to look very good, even to a trusting customer. And a quick check on the bench did little to reassure me; it was turned on just as hard as before, if not harder. The only bright spot was that I was now much more familiar with the circuit and layout. I went straight to the collectors of the three driver transistors, which all turned out to be well down; more so than before. This difference encouraged me a little; perhaps it wasn't an IC after all but something more fundamental. I went straight to pin 16 (video out) of IC201 and checked the voltage. It was close enough to normal. I moved on to the base of the video output transistor, X201. This was still normal, being virtually the same point, but the emitter and collector voltages told a different story. The emitter should have been at 6.5V but was actually closer to the +12V on the supply rail. And the collector, normally at 0.3V, was also way up, only slightly below the emitter voltage. Well, that was the answer, at least in broad terms. But why? My first impulse was to suspect the video output transistor and I went through the voltage check again, while I speculated. And as I prodded one of the legs , the set suddenly came good. And that was it. I had moved the collector resistor, R214, and further investigation revealed a subtle, but quite definite, dry joint between one of its pigtails and the copper pattern. A hot iron and some solder had it fixed in a jiffy. Well , that was a relief. But imagine the fuss a suspicious customer could have made. I can just hear the complaint. "Had the set for nearly a fortnight, charged me "x" dollars , then six weeks later it broke down again. Y'can't trust these blokes; leave themselves another job every time". Of course, nothing like that happened with this customer. In fact, we both had a bit of a laugh about it when I explained what had happened. And I didn't even charge him; it was on the house. But it can be a trap. Australian Made TEST EQUIPMENT YOU CAN TRUST SHORTED TURNS TESTER Built-in Meter to check EHT transformers including split diode type, yokes and drive transformers . $78.00 + $3.00 p&p DEGAUSSING WAND Strong magnetic field , larger than usual coil with multicore centre. Double insulated for your safety, also fitted with momentary on/off switch. 240V A/C 2.2 amps. Just about as important as having a soldering iron in your toolbox! $75.00 + $10.00 p&p HI-VOLTAGE PROBE Built-in meter reads positive or negative 0-50kV . For checking EHT and focus as well as any other Hi-tension voltages. Anew twist $98.00 + $5.00 p&p To change the subject, I have had several examples recently of a new twist to an old fault; a twist which fooled me the first time and cost me a couple of wasted hours. The sets in question were the old faithfuls, the Philips K9 and K11. I have had three of these in for repair recently, in as many weeks. And, yes, there is still a lot of them around, alive and kicking. The twist was that, while it was a fault with which I was very familiar, the symptoms were quite different. Add to that a sneaky red herring and one of them had me running around in circles. The K9 and Kl 1 use very similar circuits, the main difference being that the K11 uses a varicap pushbutton type tuner. However, the sections of interest in this story, the power supplies, are so similar as to be virtually interchangeable. The set that caused me all the problems was a Kl 1. It belonged to a long standing customer and I had serviced it on a number of occasions over the years. This time, it was suffering from a madly hiccuping power supply. It's not an unusual fault in these sets and, over the years, I have developed various approaches to it. Basically, one needs to determine whether the fault is in the power supply itself or whether it is a fault in the set which the power supply doesn't like. Often, one can get a clue by monitoring the main HT rail, which nor- REMOTE CONTROL TESTER (INFRARED OR ULTRASONIC) Designed to test lnfrared or Ultrasonic control units. With the extension lead you can also test infrared units which cannot be placed in front of the testing unit. Requires a 9V battery. Output is via the LED diode and piezo speaker. $85.00 + $4.00 p&p LASER DETECTOR PROBE A new addition to the remote control tester. Comparable with units costing $500 or more. You can test the laser pick-up in compact disc players. $27 .00 + $2.00 p&p NEW PRODUCT TEST TUNER Save time and money with this tester. Helps you determine if you have a tuner problem, an AGC problem or a fault in the IF stages. $250.00 (Tax inc.) TUNER REPAIRS from $17.00 exchange + p&p Cheque, Money Order, Bankcard or Mastercard ~v. TUNERS) · ·· · ·· · ······ · ••·.•-•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:-;-;,;,:-:•:•:-:-:•:•:•:•:::::::::f::;:::-:-· 216 Canterbury Road, Revesby, NSW, AUSTRALIA, 2212 'iEr (>~ (02) 7741154 (02) 774 1154 OCT0BER1991 23 SERVICEMAN'S LOG - CTD But replacing it, essential though it undoubtedly was, did nothing for the hiccups. By now there was only one component left; the chopper transistor, TSl 70. Initially, I simply disconnected the leads to it and made an in -situ test. This indicated that the transistor was perfect but, having been caught that way before, I decided to pull it out and replace it anyway. And what a mess I found. The insulating washer between the transistor and the heatsink had carbonised to the point where it fell to pieces when I tried to remove it. And that was it; I fitted a new washer and the set was up and running. Damaged T0-3 insulating washers are a common problem in the power supplies of old Philips K9 & Kll sets. A third washer completely disintegrated when it was removed. Why didn't I twig? mally sits at around 155V, and noting I had a chance to look at it again. By the value it reaches before it collapses. this time, however, the Variac was I do this by connecting the set to a tied up with another job, which I Variac and winding the supply voltdidn't want to disturb. age up until the set hiccups. In this I thus decided to apply full mains case, the system collapsed at around voltage and see what happened; 65V which is similar to the effect cre- maybe the faulty component would ated by a faulty tripler. I tended to show itself under pressure. Imagine doubt this possibility in this case, bemy surprise when the set produced a normal picture, without so much as cause a new tripler had been fitted only a few months previously. Still, any sign of a hiccup. stranger things have happened, so I An intermittent fault? There was disconnected the tripler. But it had no nothing for it but to pull the Variac effect. out of its current role and go through The next trick is to substitute a the previous test again. And, sure known good power supply. This is enough, at around 130V it hiccuped. somewhat easier with these sets than But when I notched the Variac up a with many others, for a couple ofrea- few more volts it stopped and I pressed on until I had the normal 155V HT. sons. One is that it is physically relatively simple and the other is that I And there it ran for the next half hour have several such units on hand, sal- and never missed a beat. I fished out a second spare board, vaged from other sets which have long fitted it and went through the whole gone to their last resting place. procedure again, with exactly the same It took only a few minutes to fit a spare, then I wound the supply ·volt- result. By now I realised I'd been had. I couldn't explain the spurious hicage up again, while monitoring the HT rail. This time, all went well as it cup, but spurious it certainly was; the went past the previous 65V limit and whole incident was a red herring. I was hoping for a clear run. But at It was all very frustrating but at around 130V the system began hic- . least I was now sure that the fault was on the set's own power board. So I cuping again. This different voltage level was a proceeded to go over the board, combit of a puzzle but I now felt sure that ponent by component. A frequent the power supply was not at fault . cause of this fault is the 225µF smoothHowever, after spending some coning capacitor, Cl 78a, following the siderable time checking out all the bridge rectifier. This sometimes has a other likely possibilities in the set, I dry joint to the board pattern and drew a complete blank. sometimes it simply dries out. Pressure of other work intervened In this case, the capacitor was .at this stage and, in any case, I needed quickly cleared. Next, I discovered time to think. So it was that the set sat that a 680kQ resistor, R171, was open on the bench for a couple days before circuit and I thought I'd struck oil. Now I know that someone is going to say that I should have twigged to this fault in the first place, since it is a regular problem in this set - and others. What's more , I have described this breakdown, in the K9 and Kl 1, in these notes in the past. Which is fair enough. Except that, in all previous cases, this fault has taken out the chopper transistor and the 2A fuse between it and the bridge rectifier. They are classic symptoms; never has it simply caused hiccuping. In fact, the first of the three sets exhibited those classic symptoms and I had it up and running in short order. And that's what threw me. That and the aforementioned red herring. Of course, I swore I'd know better next time. And I did too but the next time came sooner than I had expected; less than a week later, there was a K9 on the bench, hiccuping away merrily. I despatched it in short order. More to the point, I am wondering about that insulating washer. While we tend to refer to these as mica washers, they are not always mica and this one certainly wasn't. It was (blue) plastic. Now I've no doubt that, commercially, the plastic version has much to recommend it; it's probably cheaper, more robust and easier to store, to name just a few of the advantages that I can think of. But is it as good as mica? Is it, in the long term, chemically unstable with a couple of hundred volts across it? Some plastic capacitor dielectrics have exhibited characteristics along these general lines, limiting their use in certain applications. Well, it's just a thought. SC 24 SILICON CHIP