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Five faults all at once! A heading like that is hard to ignore. Likewise the customer's claim as she went through the list of faults. Superficially, at least, it was a formidable list and I could only hope that it wasn't as bad as it sounded. The device in question was a Panasonic TC-2969 TV set, an upmarket unit which first appeared about four years ago although this particular set is only about three years old. It is a 73cm model featuring, among other things, an elaborate remote control unit. ·It has been a very popular set. My dealer colleagues, between them, have probably sold around 150 in this area but the number of faults I have encountered has been extremely small. This set belongs to one of my longstanding customers, an elderly widow, but most of my dealings were with her daughter and son-in-law, who were better able to transport the set and describe the various faults. I have had one previous call to this set but that was hardly the set's fault. FRACAR ·AVE ••• ... more than $10. from every UHF television antenna you buy. Fracarro's ====- 4PUN for better performance from weak UHF signals. Peter C. Lacey Services Pty. Ltd. P.O. Box 678 (74Fulton Rd.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 40 SILICON CHIP and enjoying a TV program when the screen suddenly went black. But it wasn't a shut-down as in fault number one because the sound was still running and there were traces of the brighter colours on the screen. This was something new and it fi~ nally convinced them to seek my help. So there we had it - five different intermittent faults. Naturally, there wasn't any point in trying to tackle a problem like that in a lounge room; it had to come into the workshop. Fortunately, the son-in-law had a ute and plenty of help on hand to load the set into it. And so it finished up on my counter. About 12 months ago, it was the victim of a lightning strike to the power lines which, as well as damaging the set, knocked out a microwave oven, the electric stove and the hot water system. In fact, about the only thing that escaped in the household was the video recorder - a typical Murphy touch. My first insight into the problem Related faults was a phone call from the owner's Now tracking down any one of daughter, who proceeded to list all those faults could be a nasty job. But the faults it was exhibiting. And, as was tracking down five going to be already intimated, it was a formida- any harder? Not necessarily. In fact, it ble list. didn't need Sherlock Holmes to deThe first fault appeared not long duce that all five faults were almost after I had serviced it and took the . certainly related; find the cause of form of occasional spontaneous shut- one and you'd probably find the cause down; ie, the set would simply turn of them all. itself off. But while this was mildly That was the bright side. The dark annoying, it wasn't regarded as very side was that it was still an intermitserious; it needed only a touch of the tent and likely to take a lot of time to remote on/ off control button to turn it track down. I started by simply turnback on. ing the set on as it came in and, sure Fault number two caused the set to enough, the number five fault just randomly change channels, again in- described was still in evidence; ie, termittently and also easily remedied normal sound but a dark screen with via the remote control. just a hint of colour. And, by using the Fault number three could be al- remote control unit, I could change most rated as two faults: random channels, as evident by the sound. changes of brightness, or the same It is necessary to digress here, so effect on colour saturation. that the reader can follow the next Fault number four was a fairly re- step. As is common with many upcent manifestation. It had happened market sets, this set's remote control on only a couple of occasions but was system offers a multitude of functions; somewhat more serious. As the daugh- far more than most people find necester described it, it was " ... a kind of sary and more than some people can blurting noise; frighteningly loud. handle. Nearly scared Mum out of her wits". In this case, the controls include And then she went on to describe the usual on/off button, 10 channel fault number five which had occurred selecting buttons, an up/down button the previous evening. She and her which takes the set through the chanhusband had been visiting her mother nels one-by-one, a button to allow a channel number - up to about 30 - to be keyed in, volume up/down, stereo sound enable/disable, video up/down and audio up/down. Suitably activated, the video up/ down provides a choice of brightness, contrast and colour saturation control. The audio up/down operates similarly, providing bass, treble and balance control. And any settings selected by these controls are held after the set is turned off, or even unplugged from the mains. There is also a TV /VCR switch which allows the control unit to be used on a Panasonic VCR. It provides all the usual VCR control functions: stop, rewind, fast forward, record, etc. Then there is a another group of five buttons. Button 1 switches from TV to video input; button 2 is a timer, to shut the set down after a selected time; button 3 displays the channel number in the top right corner of the screen; button 4 provides sound mute (now what would anyone want that for?); and button 5 is marked "N" (for normalising). Complicated? You bet it is. And many a viewer has got himself into a hopeless mess trying to cope with all those facilities. In particular, it is all too easy for the beginner to black out the screen or mute the sound, and then forget how to undo it and go into panic mode. Hence the above mentioned "N" button, which is almost an admission of the extreme complexity. It effectively cancels all the control instruc- TETIA TV TIP Samsung TV chassis No. PSOF & RM101 (used in Samsung, Tempest, Akai and Toshiba models) Symptom: picture size is small and pulsates, never quite filling the screen. The 125V rail is low but shows no sign of fluctuation. Reducing the picture contrast stops the pulsation but does nothing to increase the size. Cure: STR30125 voltage regulator IC defective. This is the main cause tions that have been fed in and takes everything - brightness, contrast, colour saturation, sound settings, etc back to their mid-position (or "normal") settings. From there, the user can start all over again. Back to the interruption That brings us back to where I interrupted myself; with the set on the bench with' normal sound but only a few spots of colour on a black screen. I hit the "N" button and up came the picture with normal brightness, contrast and colour. I then tried something else. Using the remote control, I activated the brightness function and wound it down to minimum, then did the same for the contrast. And that exactly duplicated the condition of the set when it came in. All this was quite significant. It sup- of the small picture - the pulsation is a secondary effect produced by the low rail voltage on the Automatic Beam Current Limiter. Resetting the contrast has the effect of reducing the beam current, so the pulsation stops. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. ported a theory I had been toying with from the start: that the fault was not, as might have at first been imagined, in the remote control system itself. Rather, something was "telling" the microprocessor to do these things - in this case to turn down the brightness and contrast. That "something" could be almost anywhere in the set. All I had to do was find it. I let the set run all day and every day for about a week. During that time, it exhibited almost all of the faults listed above; all, in fact, except fault number four (the distressing blurting noise). More importantly, I confirmed that these effects were totally random; they didn't appear to be related to temperature, period of operation, operation of other appliances on the same power circuit or, in fact, anything that I could pin down. But that very randomness created a gut feeling that it was due to a dry joint somewhere in set. The back was already off the set and so, armed with my favourite sledgehammer, I set to work. (The sledgehammer is really just a large screwdriver, the plastic handle of which is the right weight to deliver an appropriate blow to the circuit board). Looking into the back of the set, there is a large horizontal board, about 350 x 200mm, in the bottom of the cabinet. This carries the power supply and the horizontal and vertical output MAY 1992 41 SERVICEMAN'S LOG - CTD stages . On the lefthand side there is a similar size signal processing board which is mounted vertically. This carries the tuner, IF strip, stereo decoding, remote control processing (but not the receiver), and the audio/visual inputs and t;mtputs. On the righthand side there is a smaller board, about 150 x 100mm, which carries additional horizontal components and the pincushion correction components, etc. I tackled the horizontal board first, starting on the lefthand side, tapping moderately hard and watching the screen in the mirror. No joy. I made my way across to the righthand side , again without any result until I tapped the horizontal output transformer, which is at the right rear of the board. Suddenly, there was a reaction. I can't remember which fault it was, or whether there was more than one, except that it probably changed channels. Anyway, it was what I was after. Well, that was encouraging. The only snag was , I couldn't repeat it. No matter how hard I bashed after that, there was no response. I left it for a while , tried again, left it, tried again, and so on for more times that I can remember. The result was, at best, "iffy"; a couple of times I achieved a briefresponse but nothing consistent. Nevertheless, it did seem to suggest that the fault was not on the board, but somewhere nearby, and that the vibration I was imparting to the board was being transferred to it. So what was closest? There is an "L" shaped aluminium plate near the horizontal output transformer, best envisaged as lying on its side, with the longer section, about 200mm long and 80mm high, running from the back towards the front of the set. The shorter section, about 40mm long, runs parallel with the back of the cabinet and provides the support. One of this plate's functions is as a heatsink for the horizontal output transistor, which is mounted towards the AUSTRALIAN MADE TV TEST EQUIPMENT 10% discount on all test equipment. 15% discount on two or more items. SHORTED TURNS TESTER Built-in meter to check EHT transformers including spl it diode type, yokes and drive transformers. $78.00 + $4.00 p&p rear. It also carries a heavier, finned heatsink at the front for the power supply regulator IC and a couple of large resistors. And when I tapped this, ever so gently, the set "went mad", changing channels and varying brightness, contrast and colour. But then it, too, settled down and I went back to the main board again. But no joy here either. So over the next few hours, in between other jobs, I went through several tapping sessions. I achieved some briefresponses but, more importantly, I concluded that the aluminium bracket was the more sensitive of the two, although the point to be tapped was most critical. And it was while I was establishing this critical point that I encountered fault number four; the blurting sound. And it was only then that I fully appreciated why it had "nearly scared Mum out of her wits". With my head half inside the cabinet while I was probing, it must have hit my eardrums at about lOdB above the threshold of pain and it came close to having a more drastic effect on me than it had on "Mum". ANTRIM TOROIDAL TRANSFORMERS rllGH-VOLTAGE PROBE Built-in meter reads positive or negative voltages from 0-50kV. For checking EHT and other HT voltages. $98.00 + $5.00 p&p DEGAUSSING WAND Strong magnetic field . Double insulated for your safety with momentary switch operation. 240V AC 2.2 amps . As important as a soldering iron! $75.00 + $10.00 p&p REMOTE CONTROL TESTER Designed to test infrared or ultrasonic . control units. Supplied with extension ' infrared detector lead. Output is via a LED and piezo speaker. $84.00 + $4.00 p&p s: ---s ·s --.../'~ / ,__----~ --~ ~ TUNER REPAIRS ~ ~z ~ .,..'I From $19 exchange plus post & pack Cheque, Money Order, Visa, Bankcard or Mastercard 216 Canterbury Rd, I _- .!.. TUNERS , ':::..I Phone (02) 774 11 54 42 SILICON CHIP Revesby, NSW 2212, Australia. Fax (02) 774 1154 • • • • Now made in Australia Standard range 15VA to 2kVA Large ex-stock quantities available Competitive pricing available for OEMs & resellers • Simply the best transformers all round HARBUCH ELECTRONICS PTY LTD 90 George St, Hornsby, NSW 2077. Phone (02) 476 5854 Fax (02) 476 3231 What happened next was I ') more fortuitous than scientific. I,, The set was on a bench near one of the windows and, in the late afternoon, the sun ~ shines directly into my eyes. I have a blind fitted to take care of this situation and this darkened the area considerably but . .~ it was still light enough to see ,~-what I was tapping. And so I had another go at q~:==::.i--:;;-~·"V'·> .• the aluminium bracket. After -~~~ a couple of tries, I found the sensitive spot again and the set gave forth with another raspberry. This time, fortunately, I had moved back a little, so it wasn't quite so traumatic. oWl-,-H r,/\'-( 1-tG:Ai;> l~SI?~ "fl1E: But, more importantly, I INE:"i" J: E-Na::>ul\J"'t1::;..~e;D happened to be looking at the L.: i 1\-\~ 1)\-Sl'f<G:S'SlNG right spot at the right time. "BLURTING NOi The horizontal output transistor is in a TO-3 case, connected quick fix was out of the question. And to the main board via three flying SC5326, which uses the "Q" chassis. leads. The base and emitter leads are The "Q" chassis is a mite long in the so the customer had no option but to soldered directly to the appropriate tooth now; it first appeared at least 10 leave it with me. pins but the collector lead is con- years ago and remained current, with A little later, I turned the set on nected to the case via a solder lug minor variations, for several years. again and studied the problem more secured under one of the mounting This particular set has been through leisurely. I still couldn't make sense the workshop several times, mainly of the hold control behaviour, so I screws. At the instant the system took off, I for minor faults, but is still in good decided to disconnect the line to the was looking at this solder lug. And nick and has a few years of life left in sync separator and let the vertical circuit float. Normally, this would enthere it was - a tiny spark, clearly it yet. Initially, the owner contacted me able me to roll the picture slowly in visible in the subdued light. From there on it was routine. Closer exami- by phone and, from the symptoms either direction and even hold it nation confirmed that what was sup- described, it appeared that the pic- briefly (with a little care). posed to be a soldered connection · ture was rolling very rapidly. When I The vertical functions are provided between the lead and the solder lug suggested that he try adjusting the by IC401 (LA7800) and the video and was no such thing; it was a classic dry vertical hold control - on the front of sync pulses come in on pin 14 via the set - he replied that he had done number of components, including joint. And that was it; a few moments that, but that it didn't seem to make R402 (220Q) which was fairly easy to get at. It was also a convenient access work with some flux and a hot iron any difference. This clearly indicated that it was point for the CRO and this indicated produced a proper joint and all the problems were solved. It was almost not a simple case of maladjustment, that the waveform was correct. So I lifted the resistor and tried the hold an anti-climax, except that I had the so I said "bring it in". When it arrived, I switched it on for a preliminary check control again. satisfaction ofknowing that I had fiwhile the customer was still there. nally cracked it. The result was rather strange. IniThis confirmed the customer's obser- tially, the effect was much the same as But there was one puzzling aspect. vations and also seemed to confirm before, with the picture rolling rapI would normally expect such a fault, idly upwards. But it was now floating in that position, to take out the tran- my interpretation of them; that the sistor. So why had it survived? I dunno picture was rolling very rapidly up- and, by backing off the vertical hold please; ask Murphy. · wards. control, I was able to make it roll I tried the vertical hold control but quite smoothly and under perfect conConfusing symptoms it had only a very slight affect on the trol. Well, that was better than before. My next story involves a 53cm AWA upward movement in one direction. But it was when I tried to roll it colour TV set and some very confus- Rotating the vertical hold control in slowly upwards that it baulked. I could ing symptoms. Indeed, symptoms are the other direction then caused the hold the blanking bar at the bottom of not always what they appear to be at picture to suddenly reverse direction the screen and move it slowly upwards until it was about half way up first glance. And if you don't get the and roll rapidly downwards. Well, I hadn't seen anything like the screen. At that point, it started to symptoms right, well ... The set involved is an AWA model that before and any suggestion of a jitter, as though the bar was bouncing . ~ . .. . be ~ ~~ se:: a MAY 1992 43 • C4,Z 3.Cf 41,3 sov scv 3~ ~30_..u 040 ,.,_,z /2S0 /2SC20 66 o/2SC21 Fig.1: part of the vertical & horizontal output circuitry in the AWA SC5326. Transistors Q402 & Q403 form the vertical output stage & drive the vertical scan coils (at left) via capacitors C412 & C413. against some obstacle. And the further up the screen I forced it, the faster it jittered. At this point, I suddenly realised that I had been misleading myself. What I had imagined to be a fast roll was, in fact, a fast jitter. Of course, the difference was academic as far as the viewer was concerned but it was important diagnostically. In any case, I wasn't particularly worried. There isn't a great deal to suspect in this part of the circuit and I had already mentally cleared part of it anyway. Most of the circuitry is in IC401, which contains the sync separator, vertical oscillator, blanking circuit and the vertical drive. The latter comes out on pin 6 and goes to the vertical output pair, Q402 & Q403. These, in turn, drive the vertical scan coils. And it was everything from pin 6 to the scan coils that I had mentally cleared, on the simple basis that both the height and linearity, checked on a test pattern, were perfect. So was it the chip? I did a voltage check of all the pins and found nothing abnormal. I then checked the various minor components around the chip but again drew a blank. That threw suspicion on the chip and, since I had some in stock, it was easy enough to fit a new one. But, alas, still no joy. As far as I could see, that didn't leave anything else to check. It was 44 SILICON CHIP time to seek help. I rang one of the service technicians at AWA, described the symptoms and told him what I had already tried. He wasn't at all fazed by my story and immediately suggested two other components: capacitor C421 (220pF) across the scan coils and C588 (1000µF) across T573 in the pincushion circuit. He explained that both these components could give trouble and create symptoms similar to those I had described. So, full of hope, I changed both of them. But I drew another blank; it made absolutely no difference. And that really put the ball back in my court. I could hardly ask for more help; it was obvious that this was all that was known. But the advice had done some good. ... A CON\/G:t-.llli.N""i' AcCG:~':S ?DIN, FOR .,.-1-1e;. C. 'RO .. o It had broken through a mental barrier which had excluded anything in the output stage from my thoughts. If one component in this section could cause these symptoms, then why not another? And that brought me to capacitors C412 & C413 (330µF, 50V) in series between the output pair (Q402 & Q403) and the scan coils. I've often puzzled over this arrangement. Some models used only one capacitor, others the two-in-series arrangement (not back to back). Anyway, in desperation I pulled them both out and fitted two new ones. And bingo! That was it; everything was now back to normal. But the reason remains a mystery. I checked both capacitors on the capacitance meter and they both read full value. Nor did there appear to be any leakage, at least at the test voltage. Nevertheless, I suspect that leakage may be the answer - after all, what else can go wrong with an electrolytic? Or was one capacitor partially open circuit? And what was the reason for the two ·capacitors in series? Had leakage proved a problem in early versions, with only one capacitor, and had someone decided that two would reduce the risk? The reasoning, of course, would be that one could fail without creating a problem, while the chances of both failing would be quite small. SC