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SERVICEMAN'S LOG The dangers of a quick fix A quick turnaround is something every serviceman aims for. It minimises the number of devices cluttering up the shop, it helps cash flow, and .it provides a certain sense of satisfaction. What's more, the customer expects it. But it isn't always the best approach, particularly with older sets. IC provides a number of functions, including the horizontal oscillator, sync separator, etc. I quickly found that R458 needed only a minor adjustment for the system to jump back into lock, after which the set produced a good picture. In fact, on a quick turnaround basis, I could probably have regarded the set as fixed. But a couple of things made me suspicious. One was the owner's attempt to describe the fault which, garbled though it was, hinted at more than one symptom. The other was that, now that the picture was locked, it was obviously overscanning horizontally. Not grossly - and the customer might well have accepted it - but enough to warrant investigation. That little bit of philosophy was prompted by a recent experience involving a Kriesler 59-4 colour set. This model is now about 12 years old and is about the last of the "true" Kriesler sets; ie, before they became a Philips chassis in a Kriesler cabinet. And of course, at that age there is always the question as to whether a lot of expense can be justified. In this case, the Kriesler was a "second" set, having been replaced by a more recent model a few years ago. But it still had its place in the household and the owner was keen to keep it going. Unfortunately, he was unable to provide more than a very garbled description of the trouble which, boiled down, was best summed up in his own words, "It doesn't go properly." Fortunately, he and his family were going on holidays for several weeks, so there was no rush for at least that period, or even longer if necessary. He just wanted to be sure it was fixed. TETIA TV TIP Philips 14CT2006 etc (CTXA-1 S chassis) Symptom: No sound . Voltages around output IC 7181 (TDA 2611 AO) are all wrong but some No sync When I eventually switched it on, one fault was immediately obvious; the horizontal system was out of lock. This could mean either a loss of horizontal sync pulses or serious horizontal oscillator drift, such that the sync system could no longer control it. Removing the back of the set, I sought out the horizontal hold preset pot (R458), a 47k0 trimpot connected via R459 to pin 15 of IC450 (TDA2590) - see Fig.1. This 40 SILICON CHIP are more wrong than others. Audio output on pin 8 of SIF chip 7164 seems to be OK but nothing reaches the power chip. Cure: C2177, a 22nF 50V ceramic capacitor, has gone leaky and shorts the signal to ground. i 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 width control, R687, 10k0 (lower right of Fig.1 ), had no effect, suggesting that the overscan was more than a maladjustment. In fact, I decided to bypass that problem for the moment and simply let the set run while I attended to more urgent jobs. In particular, I was anxious to make sure that the horizontal hold problem had really been solved and was not intermittent. In fact, the set ran perfectly for the rest of the day. But next morning there was a new fault. As the set warmed up, all it presented was a 50mm high band of picture across the centre of the screen; ie, partial frame collapse. And I had a feeling that this was only the beginning. On the off chance that this might be a faulty height control pot, R637, lkO (lower left of Fig.1), I tried adjusting this but it had only a minor effect. There is a waveform (64) given for the signal feeding this pot, indicating a p-p value of 3V. But a check with the CRO showed only a fraction of this value. From there it was a fairly simple step to transistor TR636 which feeds the pot. This transistor had dropped its gain and replacing it brought the scan back to normal. . So back to the width problem. This turned out to be due to R688 (47k0) which connects to the moving arm of the width pot. The resistor was found to be open circuit, so that problem was easily fixed. More trouble By now the set was beginning to look pretty good. I tidied it up a bit, then put it to one side where I could let it run in case anything else decided to show up. For the next three days it went like a bought one but then the picture started to tear, as though it was about to lose horizontal lock. It didn't actually lose lock but the 1sae1 .. u I ~£ ~~ To R-425. R.416. L501 . UU : IT. R47S . SK2:42 Pin 6, I SK211 Pin I . I , :::, I : Lrn , C4U ~ ' - - - - - - - - - -1 - lDo_ - .3;- - - - :~v FRAME S FRAME FLVBACK PULS 46 J; -~:::1 JOV DEFLECTION SOARD to-1U55 CIS1 'r-{Il]---.-- -,-~22~~v R&St ;;J; 35V 7TR65i 80213 cna 1:: RH5 TCH3 n'n 1Hn rlHF'-tl~l1?,'z51. fRAMf OUTPUT RH4 OEFUCT10N YoKE - - FRAME OSCII LATOH , J Fig. 1: section of the Kriesler 59-4 circuit. IC450 (TDA2590) is at the top, the vertical oscillator stage at bottom left, and the width control at bottom right. tearing action was accompanied by loss of colour. My first reaction was to suspect a fault in either the sync circuitry or the horizontal oscillator. And I assumed that the loss of colour was probably due to misplacement of the burst gate and its effect on the colour killer. In any case, either fault would be in the TDA2590 IC. So the only real test was to replace the IC. I was lucky in this regard. I had such an IC on hand and the IC in the set was mounted in a socket. So it was only a few moments work to make the change. And that was it; the picture came up perfectly stable, with steady colour. So, again, the set went back into "soak corner" where it ran perfectly for several more days. Then, one morning, it came up with the picture rolling. I let it run in this mode just to see what would happen and after about half an hour, when the roll rate had slowed appeciably, it suddenly locked. And it remained that way for the rest of the day. It rolled again the next morning at switch-on and I immediately reset the vertical preset pot (R622) until the system locked. Of course, I knew what would happen; after about half an hour the picture started rolling. So it seemed like a thermally sensitive fault in the vertical oscillator circuit. I let it cool and tried again. Again the picture was stable at first, then began rolling as it warmed up. I reached for the can of freezer. The vertical oscillator consists of two transistors: TR621 (BC548) and TR622 (BC558). I froze each of them, one at a time, but this had no effect. I sprayed the surrounding components and even the sync amplifier transistor, TR615 (a BC548), all to no effect. But I still suspected one or both of those two oscillator transistors. So out they came and in went two new ones. And I was right; that fixed it. After that, the set ran for several more weeks before the customer called for it. In the meantime, I checked. the two transistors. Both seemed OK on a rough "continuity" test, even when frozen or heated. But it was a different story when I tried them on a curve tracer. I could find nothing wrong with the BC558 but the gain of the BC548 was all over the place. But strangely, heating or freezing had no effect, one way or the other. Naturally, all this had chalked up a fairly substantial account and I was a little apprehensive as to how this would be received. After all, it was an old set. But the owner didn't quibble; in fact, I think the list of faults convinced him that I had done a thorough job. This all happened several months ago, and a recent check confirmed that all was well. So everyboby's happy. But it was lucky that I had plenty of time to run the set. Had it been a rush job, the set would have bounced more times than a rubber ball. A thorny Thorn And still on the theme of not-soyoung sets but from a slightly difJUL Y 1990 41 apart from the condition of the picture tube. So I wasn't too happy when this set turned up. The owner said that the picture had gone but the sound was OK and the screen lit up. That absolved the power supply and made the job a little less discouraging. So I agreed to take it on. The first thing was to test the picture tube emission. This was a real surprise. Most of these tubes now test in the 10% to 30% range well into the red on the meter. This one read 105%, only a little below full scale, and the same on all three guns. It would be a shame to junk such a set, so it seemed to be worth a couple of hours of labour and a moderate figure for parts. The owner set $100 on the repair and I imagined that I could do it for that much. So I set to work. Blue screen ferent angle, here is a story from my Tasmanian colleague, J.L. Just how much is an old TV set worth? It's one of the more unpleasant duties of a television serviceman to have to tell a customer that his TV set is too old to be economically repaired. He might have paid nearly $ 1000 for it 15 years ago and finds it hard to accept that his investment has gone sour. Sets of that age are usually worthless but there are exceptions. A Thorn 9104 came in recently and at first glance I didn't want to have anything to do with it. Although it was still in excellent mechanical condition, I had no doubts about what it would be like to service. Over the years I have repaired many of these sets and have also 42 SILICON CHIP written off just as many with worn out picture tubes. Most of the repairs involved the switchmode power supply and so long as this could be fixed easily, the repair was worth doing. Unfortunately, this chassis was built in the UK, to the usual UK standards of complexity and inconvenience. The power supply is extremely awkward to work on, being a three layer assembly with very short leads linking the various parts. The supply cannot be operated while it is open for service so one is continually unsoldering and resoldering leads if chasing a difficult fault. This alone is enough to turn me off the model but other parts of the set are just as confusing and any obscure fault is all it takes to write off the set, quite When I switched the set on, it came up with a blue screen, retrace lines and a faint trace of picture. It seemed at first like a screen (G2) supply problem and I checked the screen voltages on the tube base. This didn't help because they were all about normal and the same on each gun. Next, I tried the grid voltages and again, all were normal and equal. Finally, I measured the cathode voltages and found only the red cathode to be normal, with the other two only about half of what they should have been. I removed the video output board and began a systematic check of the transistors in the blue and green channels. I could find no fault there so I began a check of the diodes, inductors and finally resistors in both channels. In fact, I found the fault among the latter components without having to measure them. I happened to touch the heatsinks on all three output transistors and only the red one was warm. The blue and green heatsinks were stone cold. This could only mean that the transistors had not been turned on, or were only just turned on. And, if so, then it could only be due to a high resistance collector load. And so it turned out to be. The special thick-film network carrying the load resistors for all three transistors had gone funny and the two transistors were trying to work with loads six times higher than normal. Suddenly, something occurred to me. " Whoa there! Back up a moment!" I said. " If the green and blue channels are turned off, how come we have a blue screen?" I went back to the screen controls and gun switches on the convergence board. Switching off the green and blue guns made not the slightest difference, while switching off the red gun removed the blue screen! So the main problem seemed to be gross impurity and I'd have to get that right before I went any further. And this is where my problems really began. I started the process by manually degaussing the tube. This removed the nice even blue screen and produced a varied mass of uneven colour similar to that which usually follows the next procedure - moving the yoke to the end of its travel. At this point I wondered if that was all that the trouble was - just the yoke out of position. But try as I might, I could not find any combination of yoke and purity ring positioning that would give an all over red screen. The best I could get was reasonable red down the centre half of the screen, with a wide band of blue on one side, and the same of green on the other. For the moment, I returned to the video output board. I patched a couple of 12k0 1W resistors across the faulty thick-films, replaced the board, and restored the three gun switches. When I turned the set back on, the nice, pure centre screen that I'd had earlier had become all blotchy again. I degaussed the screen again and this restored a monochrome picture down the centre, with the green and blue bands down the sides. In fact, this procedure was repeated three more times and on each occasion the tube somehow altered its purity each time it was allowed to cool down. It was very puzzling, and I had almost come to the conclusion that the tube had an internal fault. A loose shadow mask was about the only thing I could think of and I began to wonder if we would have to write it off after all. Degaussing circuit Then I realised that whenever I degaussed the tube, the central red band became a little wider. If I kept this up, then I might have a good red screen by tomorrow morning! And this led to the realisation that in all probability, the automatic degaussing circuit was not working. One of the idiosyncrasies of English design is that nothing is where you'd expect it to be. The auto degaussing circuit is not on the power supply board, or the vertical output board, or even the convergence board. It's on the video IF board! Fortunately, it is easy to get at and I soon determined that the coils were OK. I wasn't so sure about the rest of the circuitry because they gave some funny readings. The cir- TV TEST EQUIPMENT PCB POWER TRANSFORMERS (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. $89.00 + $5.00 p&p LOW-VOLTAGE 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 DEGAUSSING WAND • Proudly manufactured in Australia by HARBUCH ELECTRONICS Pty Ltd 90 George St. , HORNSBY, N.S.W. , 2077 Phone (02)476-5854, Fax (02)476-3231 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 TUNER REPAIRS . We repair most tuners on an exchange basis. From only $17 .00 + p&p . ... , . . . . Cheque, Money Order, Bankcard or MasterCard Y.V.TuNERs) 216 Canterbury Road, Revesby, NSW 2212 Phone (02) 77 4 1154 JULY 1990 43 WOOD FOR CHIPS ... WOOD FOR CH IPS .. WOOD FOR CH IPS ... WOOD FOR CHIPS . . WOOD FOR CHIPS .. WOOD FOR CHIPS ... WOOD FOF · Emtek E-301 Logic Probe • • • • • • . Level Detector Pulse Detector Pulse Memory HVLow/Pulse Leds Suits TTUCMOS/HTL etc Operates from 4.5 to 20Vdc Specifications Max Input Frequency Min Pulse Width Input Impedance ,3!> - . Over-Voltage Protection ~ . ·:, Pulse LED flash time 17MHz 30nS 1Mn 220Vac 500ms ": . • $31.50 ~,,~ :~~si=~~~:!~s NEW Digital Engine Analyser ~ ::r: (.J cc a lJ.. Cl a ~ ~ ::r: (.J cc The APPA23 gives you 8 functions dwell, tacho, duty cycle, continuity, volts, amps, ohms and diode testing. Huge 20mm high contrast 3 1/2 digit LCD readout. 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Box 671, Lane Cove N.S.W. 2066 Telephone: (02) 428 4111 Fax: (02) 428 5198. 8.30am to 5.00pm Monday to Friday, 8.30am to 12 noon Saturday. Man Orders add S_5 00 to cover postal charges. Next day delivery in Sydney add $5.00. c Cl .,., a ::0 ~ "":"' 1 . ro GLADESVU.E Tax exemption certificates accepted if line value exceeds $10.00. BANKCARD, MASTERCARD, VISA, CHEQUES OR CASH CHEERFULLY ACCEPTED "'ii::r: V) SERVICEMAN'S LOG -CTD cuit consists of just three components. Only one has a logical part number - R199, a 6800 5W wirewound resistor. The other parts are X199, the usual PTC thermistor, and Z199, a device which looks like a 40mm diameter disc ceramic capacitor. This latter device was in series with the degaussing coils, and had a resistance of several megohms. This seemed to be all wrong and for the moment, I couldn't identify its symbol on the circuit diagram. So I had to find out what it was, and if it was indeed faulty. In some respects, I'm glad I'm a hoarder. Although it is hard to live among all the junk, there are times when it becomes invaluable. Like when I want an old Thorn IF board complete with degaussing components. In fact, I found three such boards and all three gave readings similar to the suspect one, which seemed to clear Z199. I still wasn't sure what Zl 99 was but was beginning to recall that the symbol could mean a voltage dependent resistor, which would have a high resistance at low voltage - including the voltage ap- plied by the ohmmeter. On this basis, a theory as to the circuit's purpose and operation began to evolve. It seemed most likely that its purpose was to reduce the residual current in the degaussing coils to a minimum, following the degauss function. This theory was eventually proved correct but more on that later. Anyway, since the circuitry seemed to be OK, the next test was X199 PTC THERMISTOR 200V'--.\oliliill.J.-----, Fig.2: the degaussing circuit used in the Thorn 1904. The VDR (Z199) and the 6800 resistor (R199) network was an early refinement to the conventional degaussing system. to see if it actually passed any current. For this I used an AC clamp meter. I hooked the clamp around one of the AC leads and switched on. The needle didn't move. This meant that there was either a break in the circuit that I hadn't found, or no voltage was being applied to it. A check across the degauss network showed only BOV, whereas the circuit diagram indicated there should be at least 200V. There were no less than three plug and socket connections between the degaussing circuit and the power transformer. None of these was easy to test but I soon found myself at the relevant terminals on the transformer. And there was the trouble - the winding was open circuit. This was quite a puzzle because, according to my circuit , the degaussing system is fed from the same winding that supplies the HT to the chopper transistor. The chopper was working quite normally, so I had to assume that there was a second secondary, so to speak. Then my magpie tendencies came to my salvation - again! Just outside the workshop was the latest of these Thorn sets to be junked. It was only a few minutes work to remove its power transformer and fit it in place of the faulty one. At the next switch-on, the degaussing coils gave out a good solid "boing" and the last of the impurity disappeared. The next thing this set needed was a thorough convergence setup. These old delta gun sets never had enough convergence adjustments; it always seemed that the one you wanted was on a different brand of set. This one needed at least four controls which were not provided. But I perservered and eventually produced a reasonably well converged picture. But it was still only in monochrome. By this time I had spent much more than the two hours I had budgeted for the job. I had replaced the load resistors on the video board and the power transformer. I had no idea what would be involvcontinued on page 99 JULY1990 45 Dual power supply: ctd from page 83 hence reliability. A larger heatsink may also be necessary. Performance This unit can be thought of as two power supplies in one. As a dual voltage variable supply it is capable of delivering up to 150mA at ± 18V and in excess of 250mA at ± 15V or less at very low ripple, and with excellent line and load regulation. The Serviceman's Log: ctd from page 45 ranges from 4k0 to 400k0, ± 1.2% for the 4MO range and ± 3.0% for the 20MO range. The accuracy for frequency measurements is ± 2 % on the three ranges from 4kHz to 400kHz. The unit also has a continuity and diode test range with audible alarm and a data-hold feature which stores the last measurement. Priced at $169.50, the Metex M-818 is available from all Jaycar Electronics stores. 48V switchmode supplies from Amtex ed in restoring the colour, and was beginning to regret ever having started the job. Then I thought, "Damn it! I'm never likely to need the chroma board out of this junked set. Why not swap it for the one in this set?" And it was no sooner said than done. That fixed it and the old Thorn went home with a new lease of life. Its picture tube is likely to last another 15 years but whether the circuit boards will last that long is another matter. Then again, there are likely to be more of these old Thorn sets coming in with dud tubes, so perhaps I can keep this one going beyond my retiring age! How it works This new range of switchmode supplies provide an output of 48 volts DC with output currents ranging from 1A to 10A. Each unit has short circuit and output overload protection as well as switchable 110/230 VAC inputs. Also included is soft start power up circuitry and input surge protection. For further information, contact Amtex Electronics Pty Ltd, 13 Avon Road, North Ryde, NSW 2113. Phone (02) 805 0844. With time to think, I evolved the following theory about the workings of the degaussing circuit. At the moment of switch-on, the PTC thermistor, X199, is cold and its resistance is low. This allows almost the full voltage to be applied to the degaussing coils and Z199 which, at the high voltage, offers minimum resistance. In its fixed voltage guise, it can provide in excess of 550mA at 5V with short circuit protection provided by R7 and internal foldback current limiting. As a combination unit, with both supplies in use, the 5V specifications still stand but, as you might suspect, the 18V output may not be usable depending on how much current is drawn from the fixed supply. The lower voltage supplies are unlikely to be affected if current demands a re reasonable. ~ As Xl 99 heats up and its resistance rises, the voltage applied to Z199 and the coils will fall and the reduced voltage across Zl 99 causes its resistance to rise. But this would allow X199 to cool and increase the current through the coils. So the function of R199 is to maintain sufficient current through Xl 99 to keep it warm and its resistance relatively high. At the same time, most of the current it does pass will flow through R199, rather than the coils. Subsequently, a colleague came up with two text books which confirmed this theory: "Colour Television;' by R. N. Patchett, and "PAL Colour Television for Servicemen" by W. C. Cook. That leaves one point still unexplained; the 80V I measured across the degaussing circuit, connected to an open circuit transformer winding. How come? It was another of those (expletive) digital multimeter misreadings. Apparently there was enough capacitance, or leakage, across the break in the transformer winding to develop 80V at the picoamps needed to drive the DMM. Later, using a 100,0000/V analog meter, I could not detect any voltage. ~ Is Your Product Getting The Exposure It Deserves? Consumers need to see your product if you want them to buy it Contact Paul To Reserve This Space - (02) 979 5644 ]UL Y 1990 99