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It was the last thing I tried "It was the last thing I tried". How often have you heard a colleague say that? It's a pointless remark, unless the speaker is silly enough to go on looking for the fault after he has found it. What he means is that it is the last thing anyone would think of and that's the theme of this story. The story concerns a National portable (34cm) colour set, model TC1480A. This is a relatively recent model, released a couple of years ago, and the particular set had been in service for only about 6 months - which meant that it was still under warranty. In greater detail, it was one of a ~ batch sold by my dealer colleague ~· to a local motel. He dropped it into ,v.::::;.~~ ~ the shop with the explanation that {~ "It goes for a while and then it loses ~ the picture - the screen just goes black. But the sound is OK". That was a more detailed description than I get from most customers; in many cases, the -OF ~qO-\ \ ~U'R.~e:., ,-r w~s 11-\E: LAS, t"HING :C. TR\~D 46 SILICON CHIP symptoms have to be prised out of them. At the same time, I was a little surprised by it. Usually, a nopicture, no-raster situation suggests failure of the horizontal deflection system, from which most auxiliary voltages - including that for the sound channel - are derived. So I sensed that this was likely to be a "funny" one. Anyway, I set it up in a corner of the bench and let it run. It played perfectly at switch-on and for the next hour or so but then, just as my colleague had said, it switched to the no-picture, no-raster situation but with normal sound. At this stage I hadn't even fished out the service manual and my memory of the chassis - which is used in a number of models - was a bit vague. The last time I worked on one was some years previously, when the chassis first appeared. At this point, it seemed that the easiest thing to do was to take the back off and look for anything obvious. Fortunately, the back forms the larger part of the ea binet and when it is removed, most the boards are readily accessible. I made a careful visual inspection of the boards using a glass to check for dry joints or similar faults but there was nothing obvious. Then I switched it on and, again, there was a perfect picture. I went through the usual bash, wriggle and shake routine to see if I could provoke the fault but it just kept on working. In fact, it kept on working for the rest of the day. And all the next day. And the day after that. So had my prodding and bashing accidently cured the fault? It wasn't a very satisfactory solution if it had, but these things happen. If the worst came to the worst, I could return the set with instructions to bring it back when the fault recurred. I replaced the back and gave it another test run. That lasted about an hour, then it was up to its old tricks; just the sound with a black screen. Well, that changed the whole outlook. It was obviously a temperature sensitive fault which was critical enough to sense the difference between the "back-on" and "back-off" conditions. I put it through this cycle a couple of times and, in the process, learned something else. When it failed it needed to be switched off for only a minute or so for it to recover, after which it would run for another half hour or so before failing . This reminded me of an important feature of this chassis; a very effective protection circuit, designed to sense a number of abnormal conditions and shut the set down. And when this happens, the set will not function again, even if the fault is cleared, until the protection circuit is reset by switching the set off. (I mentioned a similar feature in these notes, involving a National TC2258, back in December 1987; how time flies!). Unfortunately, this can make servicing quite difficult in some circumstances. Is there a fault which is being sensed by the protection circuit? Or is it a typical intermittent, which commonly responds to the off-on cycle anyway? All I could do was press on and hope that more clues would emerge. Since it was necessary to leave the back off in order to work on the chassis, but also necessary to raise the temperature to initiate the fault, I fished out an old blanket I keep for such situations and draped it over the set. That worked fine; I was able to bring up the fault in about half an hour. That was a help but it served to reveal even more about the switch- TV TEST EQUIPMENT (AUSTRALIAN MADE) off/switch-on cycle; switching the set off for only a few seconds was enough to restore performance which only added to the confusion. Up until this point I had been paying more attention to the symptoms than to the manual and the circuit. Now, having established a behavioural pattern, it was time to study the circuit and try to relate the two. My first query was why the sound system continued working when, apparently, the horizontal deflection system was failing. The explanation was simple enough. The sound section, including the sound output IC, is fed from a 12V rail derived from the transformer in the switchmode power supply. This carries an additional winding which, after rectification and preliminary filtering , delivers a bout 17V. This is fed to a 3-terminal regulator which provides the 12V rail. So that was that - the sound system power supply was independent of the horizontal deflection system. That much clarified, it was time to tackle the fault directly. The first check was to determine whether PCB POWER TRANSFORMERS 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 O-5OkV. Can also be switched to check negative voltages. For checking EHT and Focus voltages as well as TVs arid Microwaves. Hi-tension voltages. $89.00 + $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. 24OVA/C 2.2 amps. As important as having a soldering iron! $75.00 + $10.00 p&p Proudly manufactured in Australia by TUNER REPAIRS We repair most tuners on an exchange basis. From only $17 .00 + p&p Cheque, Money Order, Bankcard or MasterCard ,, TUNERS'I V•____ • 216 Canterbury Road, Revesby, NSW 2212 Phone (02) 774 1154 HARBUCH ELECTRONICS Pty Ltd 90 George St., HORNSBY, N.S .W., 2077 Phone (02)476-5854, Fax (02)476-3231 AUGUST 1990 47 ::a ::t: z n n 0 ~ CJ) ~ ""' ~ ,,, 1vwr1 ~ - - ~cl:l! :).l)K. ~ . IS _£° 470 ~ MOAIVEI. 'l!.Q?-,, f~ '"°°" ~""" ~:21' ,.., ,-,o--v-·' - :~i :I[ - ;: ~ ~ ~ ~•\1::·1• U) J Lilli ISO £~~; cso, . ! SC92S- lo ""' c:YOR4M.n R413 1-'« . II 1 ' T;; t --1.'5!0 ._ , ., ~ - • . u ·· ~t -:::z cs.:s .:-sA~ ,. ''"""'!( ) I :-,, j . :81( ! ~~~ ~ I i . - - - -- 1 ; ou, • : j i I C50l ~ =k= ~ ·---=~. r~ ... '"' '·" R674 18.n: •. • r c- 140UUl/ZRIO ~ llo.)K ••• :_C-l401Al 11 ' It,~, ; ! ______, 00~~~!_ ; JS03' Uj ;>:zf ~ f : f',. ~V "~ I ·.;, "~ool " ii I ,~...,.,,. 4,'T,J,. ;U; .~ I ~ ·; . ~~ ~ ~ 2SAll2.l 0521 120 1.670 - ,."' I i;s ,,.. ,... ""' Jlf ! ""'! 1 I ~ I i ~ • I M.U, ~ r:::;' ;Y32\ mm "" E~[§~ 1 7' . ~ -- ID_5tj. IOO : ~ l ~ :1<at>l;~ ~ :, i :I ii ;j ·!j 6 r-- · '1 ,ir-- f~ ' 0521 ir } .,...., ~~ ! "\'t.'51 L:_;_ - .• <at>I :~ ..,,,.'-J" . '- i«~TD 6i _.7 . ~ i :c!,~'®! ';~ ®I - "" ~ , ,. .l. . ~ :o ~ I !~ ~ 470 16V •C6l'l i·z:t 25"13094 . n-,.. ::1 I M.Al61l 06)0 ~tls Fig.1: section of the National TC1480A circuit. The horizontal output stage is at bottom left, the protective circuit to the right of it, and the video/chroma IC at top centre. l pi; 1' I i b i ~~ ,.~KV ~ T i~&p ~e I ,g. ~ - R412 2-21( """'1.01"' ''·""""1 A .... -· l :"" "" 11~ t ~,~ "" ,;J.11 "'" !~ !W ~ ..Rl,,, 4Jf . -j---- -- - - - -- - -- --'~"':_f ...... :.-- , ~ E-BOARD TPN197059AB . .. FOR TC·1401AR TPN197059AA .. . FOR TC·1401MR TPN197059 .. . .. FOR TC·1401ZR VIOEOICHROMA JUNGLE Q630 there was any EHT. Well, there was some, but only a paltry 3kV rather than the 25kV I expected. This was the first real clue. From here I moved back to the collector of the horizontal output transistor, Q501 [lower left of circuit). This should have been showing an output of around 900V p-p . Instead, the CRO showed about 300V. That in itself was not surprising in view of the poor EHT figure, but there was something else wrong. The frequency of this waveform should, of course, be 15,625Hz. In fact, it was more like twice this frequency which was a good enough reason for the poor EHT performance. But why? Back tracking from Q501 takes us to Q502, the horizontal driver transistor, and from there to pin 41 of IC601, the video/chroma jungle chip [top centre of circuit). The waveform here should be a 5.6V p-p square wave at 15,625Hz. In fact, it was well down in amplitude, was nothing like the specified shape, and was also at twice the correct frequency. So, did we have a temperature sensitive fault in the chip? One way to prove this was to to spray it with freezer while in the fault condition. But this was easier said than done. Because of the aforementioned protection circuit, simply spraying the IC and waiting for a result could well be pointless. If the fault had tripped the protection circuit (which seemed likely since the horizontal output stage was involved), any change would not be apparent until the set was switched off and on again. But, as I said, this action might also cure the fault. Then, a possible way out of the impasse presented itself. Continued testing with the blanket in place revealed that, if the set was left on for some time after the fault occurred, so that the chassis became really hot, it might need up to 15 minutes in the off condition before the set would play again. So, after a good long heat soak, I gave the chip a generous spray of freezer, then switched the set off and on again, just long enough to reset the protection circuit had it been activated. Result - a virtually instant response as the tube warm- TETIA TV TIP Hitachi CWP139 (NP6C chassis) Symptom: Set apparently dead . Closer investigation shows that 11 0V rail is OK, as are the 800V screen supply, the 180V video output supply and the EHT. Cure: 1 2V rail missing . In this case there was an almost invisible dry joint at the anode of D705. The configuration of this supply is unusual in that the diode is placed between the earthy end of the supply winding and ground, rather than in the supply rail itself. 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. ed up. Well, it hadn't done that for the previous super heat cycles I had inflicted on it. I reckoned that was it. I had a spare chip on hand and 42 solder-sucked and resoldered pins later it was in place. Brimming with confidence, I then tossed the blanket back over the set and switched it on for a final test. My euphoria lasted exactly half an hour. Suddenly, the set was back in fault condition and I was back to square one. Brain in top gear So where to now? I suppose the shock of the set-back put my brain into top gear, or at least directed it along a different path. I began to think more and more about the protection circuit and the role it had played in that other incident back in December 1987. On that occasion it was the protection circuit itself that was at fault. So was that what was happening here? Had I been chasing a false alarm? One way to clarify this point would be to disable the protection circuit and see what happened. But first, a few points about the protection circuit. It involves four transistors, Q451, Q521, Q503 and Q504 (bottom left of the circuit), plus various diodes, zener diodes and resistors. Just how it senses all the parameters it does - or even how many it senses - is quite complex but the manner in which it shuts down the set is simply explained. The main voltage supply for the video/chroma chip is derived from the 110V HT rail via a 6.8k0 resistor (R519}. This applies 8.5V to pin 42. And pin 42 is also connected to the emitter of Q503 via a 560 resistor, R536. Q503 is normally turned off but when a fault is sensed, it turns on and this pulls down pin 42, reducing its voltage to a small fraction of its normal value. This shuts down the most vulnerable parts of the set. It was now fairly obvious that what I had taken to be a fault in the chip was really the result of the protection circuit doing its thing. To disable this circuit, I had only to lift the 560 resistor. But it wasn't quite as simple as that. If there really was a fault, there was a very real risk of an expensive component being destroyed before I could hit the power switch. But that was a risk I had to take. To reduce this risk as much as possible, I set up a meter to monitor the HT rail and the CRO to monitor the waveform out of the horizontal output transistor. Then I tossed the blanket over the set, switched it on, and waited. And I waited. After about two hours, with the set still playing, I was ready to believe that the protection circuit was the culprit. Then suddenly there was no picture. My gaze flashed first to the meter, then to the CRO. But neither had changed. And a further check confirmed that the EHT was normal. I checked the picture tube G2 voltage, the value of which is not given on the circuit. It was at something over 400V which I was happy to accept. I then checked the voltages on the collectors of the R, G, B driver transistors but all were normal. It was all becoming very disconcerting because I was now rapidly running out of ideas. As far as I could see, all the picture tube operating voltages were correct, yet the set just wouldn't work. It was almost as though the electrons had gone on strike. At a more rational level I reasonAUGUST 1990 49 SERVICEMAN'S LOG -CTD 1lrd £.1-E:C...--rR ONS ON ST'R \ Ke A, 1"H €" PI C..-TORE: "TV~E: H~-re::R c...tRC.U ed that there was only one thing I hadn't checked - the picture tube heater circuit (lower right of the circuit). There isn't much to this; the winding on the horizontal transformer, a 30 resistor (R523), a plug and socket, and the tube heater. I checked all these with the ohmmeter and all were OK. But there had to be something wrong and I wanted to make a visual check of the heater . Unfortunately, the physical layout makes this difficult unless one knows exactly where to look. I switched the set on again, switched off the lights, pulled down the blinds, and confirmed that the heater could be seen. But it could only just be seen and then at a fairly critical viewing angle. I restored normal lighting and let the set run. When it failed I darkened the room again and took a careful look. Result - no heater. The culprit - at last! So I was on the track at last. But was it the tube, or the resistor, or 50 SILICON CHIP l'r... the transformer? About the only way to determine this was to measure the heater voltage; something that needs a special meter because the supply is at horizontal deflection frequency. Fortunately, my Fluke 87 meter will handle this with ease and that was the answer; there was no heater voltage. Well, that cleared the tube which was a relief - and put suspicion squarely on the transformer. But I wanted to be sure before I pulled the transformer out and replaced it. What was needed was a continuity test but, from observations thus far, this would be valid only when the system had been heated to the point of failure - and it would have to be done smartly. I disconnected the lead from the 30 resistor and restored the connection with a clip lead. I set up a meter for resistance measurement, connected one lead to the other end of the winding (pin 8), switched the set on and let it run until it failed and then for a while longer to make sure it was really hot. Then I switched the set off and quickly changed the clip lead connection to put the meter across the winding. Result; open circuit. But I continued to check the meter and, within 20 seconds or so, it dropped to zero ohms. That clinched it of course but I had to order a replacement transformer. When this arrived, I fitted it and gave the set a thorough workout. It behaved perfectly, even with both the blanket and the cabinet back in place. OK, so why did the failure in the heater winding trigger the protection circuit? The answer is that this heater voltage, via diode D523 & filter capacitor C582, provides a DC voltage to the protection circuit. Presumably, this is . designed to monitor the whole horizontal output circuit but it was tricked by the winding failure. The protection circuit also monitors a 25V rail which is derived from pin 6 of the horizontal transformer. This rail also supplies the vertical output chip (IC401, pin 7) via a 1.50 resistor (R411), one end of which goes to the emitter of Q451. Apparently, this arrangement is designed to sense any excessive current drawn by IC401. The 110V HT rail is apparently monitored by Q521. But why did the set take so much longer to fail after the protection circuit was disabled? This is a real puzzle and the only theory I can advance is that the winding failure was not sudden. Instead, there was probably an increase in resistance initially. As a result, the heater voltage may have been reduced enough to trigger the protection circuit but not enough to seriously impair tube performance. But that is only a theory. And that's about as far as I have been able to work things out so far so it will have to suffice for the present. But about that heading; perhaps it should have read " ... the last thing left to try". Who done it? And now, for a complete change of pace, here is a short story without any serious technical aggravation. The main point of the story is: "Who done it?". It concerns a Sharp colour set, model 9C143, a set that has been around for about 10 years. The owner, a new customer, simply dumped it on the counter, said "It doesn't go", and wanted a quote to fix it. I'm not into quoting in the normal sense. It's a risky business. By the time you've found out what is needed, most of the hard work has been done, with the real risk that the customer will shake his head, say "Too dear" and walk out. But, not wishing to come on too strong with a new customer, and because it involved a total failure, I said I would have a quick look at it and try to get some idea of what was involved. If it was easy to diagnose that would be fine but beyond that I wouldn't commit myself. After a moment's hesitation, he accepted the offer. When I pulled the back off the set, the first thing I checked was the mains fuse. It was open circuit; not blackened or even visibly faulty, but open circuit nevertheless. Whenever I find an open circuit fuse, the next thing I suspect is the degauss circuit. A common fault is a short-circuited thermistor, although the fuse reaction is usually more violent. · So a typical routine is to unplug the degauss circuit, fit another fuse , switch on and watch what happens. If the fault is elswhere in the set, the fuse reaction can often reveal a lot about it. As an added precaution, I usually fit a lighter fuse than the one prescribed, since it no longer needs to survive the degauss surge. But in this case someone had been there before me; the degauss circuit was already unplugged and, what was more, the plug had been neatly taped to the winding, well out of sight. OK, why had the fuse failed? The prescibed fuse is a 2A delay type, so I fitted a 1.5A quick blow type and switched on. Nothing happened; nothing untoward, that is. The set came to life immediately and produced a first class picture. And strangely, there appeared to be no purity error. I let it run for a while, then checked the degauss thermistor. And, of course, it was shorted so that was easily fixed. I then refitted the degauss plug, changed the fuse to a 2A delay type as prescribed, and tried again. And again, the set worked perfectly. I let it run for the rest of the day but there was no sign of trouble. So why had the original fuse failed while the degauss circuit was unplugged? A closer look at it pro- vided the clue; it wasn't the prescibed type but a 1.5A quick blow type, similar to the one I had used experimentally. Well, I reckoned that was pushing things a bit. Such a rating is OK on a short term basis but is almost certainly too light for continuous use, even with the degauss circuit unplugged. On that basis, I felt confident that the set could be safely returned to the customer. But what was the history of all this and "Who did it"? I hate to say this but I suspect that it was a professional serviceman. Granted, it could have been a do-ityourself type but, if so, would they have thought to fit a lighter fuse? And would they have bothered to tape the plug out of sight, as if to hide what had been done? I doubt it. I fear it was one our own kind who simply couldn't be bothered doing the job properly and took the easy way out. If so, I would nominate him for the Striped Apron Award for the year. Of course, I quizzed the owner as discreetly as I could but it was obvious that he couldn't (or didn't want to) help. So I backed off and that means we shall never know what really happened. But we can do without "servicemen" like that. SILICON CHIP BINDERS These beautifully made binders will 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. * High quality * Hold up to 14 issues * 80mm internal width * Gold printed with SILICON CHIP logo on spine Price: $All.95 plus $3 p&p each (NZ p&p $A6). Send your order to SILICON CHIP Publications Pty Ltd, PO Box 139, Collaroy Beach, NSW 2097 (use the handy form on page 112). Fax 979 6503. AUGUST 1990 51