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SERVICE 'SLOG Confession is good for the soul Confession is good for the soul, they say. And this story is basically a confession, so let's hope that my (technical) soul will benefit appropriately if I lay bare my sins. No doubt, someone will suggest a suitable penance. The set involved was a Panasonic 68cm colour TV set, model TC-2969 , · and the complaint was a double barrelled one; a "wishy washy" picture and a tendency for the picture to roll. It belonged to one of my regular customers but the call actually came via my antenna installation colleague. He had installed the customer's antenna and the customer had called him in, believing that it was an antenna problem. My colleague quickly cleared the antenna and passed the problem on to me. Because the customer was in an excellent location for TV signals, I realised that it had to be an internal fault. I decided to visit the customer's home, check the performance there, then bring the set back to the workshop. My antenna colleague went FRACARRO DELIVERS..• ... best television antenna performance for less cost. ~ Fracarro 10BL4, 10BL5, 10BL45, 10 element yagrs 1hat <at>filly work Peter C. Lacey Services Pty. Ltd. P.O. Box 678 (74Fulton Rd.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 42 SILICON CHIP along to give me hand to load it into the van. Checked in situ, there was no doubt about the rolling problem; it was quite bad and nothing as simple as a hold control setting. The "wishy washy" complaint wasn't quite so clear cut. I had to admit that there was something strange about the picture but I couldn't pinpoint it. The closest description would be that of a weak picture tube but that didn't seem likely, or tie in with the rolling. (With hindsight there may be a better explanation). Its performance was much the same on the workbench and I gave some thought as to how best to tackle the problem. The picture quality was a rather nebulous symptom to work on; I was convinced that it was real enough but without some idea of how it was wrong, it was hard to know what to look for. The rolling problem, on the other hand , was much more tangible. Along with the poor picture quality, it suggested some form of distortion of the video signal, involving the sync pulses. This should be easy to track down. I fished out the manual and started tracing the video path. This proved rather awkward because the circuit is spread over two sheets, each folding out to about 75 x 50cm, and the particular path one wishes to follow often shuttles back and forth between them. This also makes it difficult to illustrate these notes with anything more than a small section. Colleagues who have a circuit should be able to pinpoint the components I nominate. I started at IC101 . This is described as a Phased Locked Loop, Split Vision IF & Sound IF IC. Video comes out on pin 3, goes to emitter follower Q130, then passes through resistors R152 and R150 to pin 3 of IC2501. This is described as a TV/ AV Control IC and is mounted on the B board. I fed in a standard staircase pattern and checked it at pin 3 of IC101, at test point B15 (ie, the emitter ofQ130), and at pin 3 of IC2501. And everything appeared to be normal. As will be evident from the IC2501 diagram (Fig.1), this is basically a switching IC. This type of device is now quite common in video recorders and is also finding increasing use in TV sets as they become more elaborate and offer more features. Switching function In this case, there is a switching function between pin 3 and pin 5, with pin 5 connected to a video-in terminal on the back of the set. But, regardless of the signal source, the switch sends the video signal out on pin 12. I check this point and everything still seemed to be normal. Unfortunately, there are no oscillograms in the manual for this part of the circuit but I was seeing a standard staircase pattern, with no sign of sync pulse compression or other obvious distortion. From pin 12, the signal goes directly to pin 2 of plug/socket combination B2 on the extreme right of the diagram. The signal is then taken across to the E board, where it goes to emitter follower Q302 and then to pin 15 of the Video/Chroma Jungle IC (IC601): Unfortunately, after tracing the signal through this circuitous path, I was no closer to a solution. As far as I could see, the signal pattern held up all the way. So what now? At this point, I came up with what I thought was a smart idea. Why not feed a video signal from the colour bar generator directly into the system ci'..'2'Bll - - - T - - - - - - - - - --- - - - ' ,i,, 1 '- - - - - - - F_=I=.- - - - - - - "j 1 I 02206 UNl212,,.,. UN4212 I 1 AUOI) CX/r-L AIJOD CXlr-F. : I I I I 1 I I I : TJ8ffllG 1 ONLY :-------' I AIJOll IN-I. I 1 FOR CA-2111111 I I I - AIJOll IN-R I i : : ~I I NILL I 1 1 ~-------,--~------, ONLY Fa! CA-21611 - :J20 : 1 I , .✓• L.. - ,1 rJ!OOOT ; -----~ l. lll!O IN 8~ 1. R2511 -T ~; l~~aJARO k1I= 1 J.,l,;;_ _,_, VID!O I I. ·. 'l, •m1 NP ,__j......,;..;._ _ _ _ _e-:,;;;...-----:--"lf/'r--a-""--' C2522 sov I I I I I What did it mean? So what did it all mean? The best construction I could put on it was that it was somewhere on the B board, u~1=t H7~ ; ~u~.1 4 - . , VIDl!ON : ONLY ~OR ONLY F"OR CA-21611 -N21• _____ -~~:~I :__ ~ -=--_-_ ~ __ ~1)STANMY Fig.1: this diagram shows the relevant part of the video chain in the National TC-2969 TV set. The video line comes in at lower left & goes to pin 3 ofIC2501. It then comes out on pin 12 & goes to pin 2 of the B2 socket at right. via the appropriate video-in terminal on the B board and thus into pin 5 of IC2501? Unfortunately, the result of this was inconclusive. On the positive side, it had cured the rolling fault - no doubt about that at all. But the picture quality was another matter. It seemed to be better but it can be very difficult to make a precise subjective assessment in this type of situation. At best, it seemed to be better but it still wasn't right. I decided to try another tack. This time, I decided to take the video signal out of the set via the video output socket (effectively the signal out of pin 12 of IC2501 via Q2504) and feed it into the video-in terminal of another receiver. This time the result was more tangible; it was virtually identical with that of the set in its original condition - except that, if anything, the rolling was marginally worse. r-,-- -- - . around IC2501. But I was all out of bright ideas. It was time to ask for help, so I rang my contact at Panasonic. Having established that I had the circuit in front of me, he directed me immediately to IC2501. He then proceeded to draw my attention to the signal levels at pins 3 and 12, pointing out that the signal at pin 12 should be 6dB up on the level on pin 3. And suddenly I realised how I had slipped up ; the IC diagram contains a small amplifi er symbol , marked 6dB, which I had completely overlooked. And I had made a further mistake by not comparing the two signal patterns closely enough. Thinking in terms of unity gain, I had "assumed" that the pattern at pin 12 was close enough to the one I had seen at pin ·3 - particularly as there was no obvious distortion - and that this was all I should expect. In fact, subsequent measurements showed that it was about 3dB down on the pin 3 level. Again I should have noted this, but 3dB means a voltage ratio of only 1.4 to 1, and I took less notice of it than I should have. CA-216Q A loss of 3dB instead a gain of 6dB meant that the circuitry beyond pin 12 of the IC was being fed with 9dB less than it needed. And, in simple terms, the system didn't like it. In fact, that probably oversimplifies the situation and I'll expand on that in a moment. The cure, of course, was simple. I ordered a replacement IC, fitted it, and everything came good - including the "wishy washy" picture. Significantly, it was only when it did come good that I appreciated just how bad it had been. So what was happening to the picture? I'm still not sure; all I can suggest is that the amplifier failure in the IC involved more than a simple loss of gain. Almost certainly there was also a serious degree of non-linearity, producing an odd scale of video tonal values. This probably involved compression of the highlights, thus robbing the picture of its "sparkle". And another question. Why did my attempt to feed a video signal into pin 5 of IC2501 cure the rolling problem? The answer, I feel, is almost certainly that I fed in a somewhat higher amplitude signal from the generator than was normally available from off-air signals. This lifted the sync pulse amplitude but could do nothing about OCT0 BER1992 43 the picture, looked wise, shook his head, and pronounced the problem as being due to the "poor signal conditions". Nobody was really satisfied with this excuse and after tolerating it for a few more weeks, they called the dealer in again and applied a little more pressure. Only then did he concede that all was not well but he then opted out again on the basis that it wasn't his problem and that they should consult the local serviceman: - ie, yours truly. And so it was that I received a call from the daughter, who outlined the above events, and tried to give me some idea of the problem. This proved difficult initially but, by careful questioning , I gained the impression that there were coloured patches on the screen, a suggestion which she latched onto immediately. She also added that they felt that the fault had been there from day one, at least in some degree. Naturally, it sounded like a classic purity error and I quizzed her about the possibility of any magnetic devices, such as hifi loudspeakers, being located near the screen at any time. But no; she was quite adamant that there was no possibility of this. At this point, I suggested that they bring the set in and, if the fault was as I expected, I could fix it on the spot. And so the set duly turned up in the shop . ~~-- . CONF'~S'SI0N \S GOOD Ft::>~ 11-(~ SOUL-, ·nus:.y SA.Yuo any non-linearity in the IC amplifier. And that's the nearest I can get to explaining what happened and why. But an explanation is not an excuse. The simple fact is I goofed and I've learned a lesson from it. A very rare fault My next story is about another National Panasonic colour set, a model TC-48R60 with a very rare fault. (Incidentally, the "48 " in that type number indicates 48cm , the makers having finally converted to the metric system). The job involved one ofmy regular customers, or at least indirectly, in that the set belonged to her mother. The set was only a few months old and, of course, still under warranty. 44 SILICON CHIP The mother lives alone, a few kilometres from her daughter, and in a rather poor reception area. At this location, TV signals are weak and somewhat ghost ridden. This situation caused some initial confusion, frustration, and delay. The story as told to me was that, shortly after the set had been installed, the owner felt that there was something not quite right about the picture, although she couldn't really pinpoint it. She raised the matter with her daughter and son-in-law and they both agreed that there was something "funny" about it but, similarly, they were at a loss to pin it down. Eventually, they approached the dealer from whom the set had been purchased. He came out, examined Red raster, blue patches I set it up immediately and fed in a blank red raster. Sure enough, it displayed a classic purity error. There was a big patch of blue on the left hand side, about half way up the screen, plus a few lesser patches on the right and along the bottom. All in all, it was quite severe and I wondered how the set had ever been delivered to the customer in that condition. Anyway, I reached for the degaussing wand, plugged it in, and waved it across the face of the tube. This will normally have some effect on even the most serious cases, even iffollowup is needed at the back of the tube. But this time it had no effect whatsoever. Nor did additional passes with the wand around the back of the set. This was a setback but I was not all that worried. I envisaged that I would need to go through a full purity and convergence routine and explained that the set had better be left with me but that I should have it ready later in the day. Ian J. Ttuscott's ELECTRONIC WORLD Problem No.2 30 Lacey Street, Croydon, 3136 Phone (03) 723 3344 Fax: (03) 725 9443 Mail & Credit Card Orders Welcome The daughter accepte d this readily enough but, before she left, she raised another matter; apparently the set also had a nasty h abit of switching itself off, for no apparent reason. Typically, it would do this a couple of times a night but could be turned on again quite readily. I made a mental note to follow this up after the main problem had been fixed . I moved the set from the shop counter to the workbench, pulled the cabinet back off, and switched it on again. And the first thing I noticed was that the pattern of the purity error had changed. The main patch was now a different shape and a slightly different colour. I didn't pay much attention to this. I had turned the set through 90 degrees (ie, from east-west to northsouth) when I moved it and this can sometimes have an effect on purity problems, though seldom on the finished job. TETIA TV TIP Akai CT-K209 TV set Symptom: no picture, no snow; just an even grey screen. Channel selection was normal, as was the sound. The CAO showed that video was going into the video processing chip but nothing was coming out. Cure: R501, the "matrix adjust" trimpot, needed adjustment. This pot adjusts the balance of chroma signals in the delay line and somehow it caused blanking of the luminance part of the video. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute ofAustralia. Contact Jim Lawler, 16 Adina St, Geilston Bay 7015. **** NOW ALSO AT COLLINGWOOD **** ** 200 Smith St (near PO) ** ** Phone: (03) 419 8208 ** We stock an extensive range of semiconductors, FETs & Mosfets, power & zener diodes, SCRs, Triacs, Linear ICs, 74LS/HC & 4000 series IC's, high-intensity LEDs and displays. Accessories inc. pots, trimpots, RF chokes, voltage regulators, relays, fuses, globes & heatsinks, as well as a comprehensive range of many popular switches, knobs, plugs & sockets. We offer a huge range of resistors & capacitors and popular parts at competitive prices. · More importantly, I have had problems with the metal frame ofmy workbench and make it a point to degauss this before attemting any serious purity or convergence adjustments. I went through this routine again, then attacked the set with the degaussing wand again. Result- no improvement in any shape or form. So it looked like the full treatment. This involves moving the deflection yoke back and resetting it for purity on a green raster (as used for in-line gun tubes , where green is the centre gun) , then going through the full convergence check. All this was a bit of a bind and, although I knew it had to be done, I decided to leave it until after lunch so as to have a clear run at it. When I came back and switched the set on again, I was sure that the purity error had changed yet again. This simply didn't make sense and I was becoming very suspicious. Horizontal streaks And there was another effect; one that I had noticed before but which had then vanished and come back again. This was a series of black horizontal streaks flitting through the raster. It looked for all the world like external electrical interference but this was hardly likely in a bench generated raster pattern. In the light of all this, I decided to have a bit of a poke around the deflection components before going any further. So, armed with my favourite sledge- Rechargeable batteries, both regular, tabbed, rapid charge and back-up types are available ex stock. Ring for a price on our sealed lead-acid, video, cellular phone and watch batteries. We now offer a speedy, courier delivered, mail order service. HI-FI & PA SPEAKERS Huge variety of all common hr-fi replacement speakers, crossovers, speaker cloth & grilles. Call for a price on our great range of speakers, microphones, cables, audio leads & adaptors - we'll send you a catalogue. 2-way crossovers (pair) from $9 3-way crossovers (pair) from $16 SWF12 12" 100 watt RMS woofer $75 SWF8 8" 60 watt RMS woofer $39 Redford PECC 6.5" 60 watt woofer $38 Redford PECC 12" 160 watt woofer $114 DM40 4" 50 watt RMS midrange . $16 DT30 1" 35 watt RMS dome tweeter $11 Ferro Fluid 30 watt (60 max) tweeter $13 Piezo tweeters from $16 Speaker wall mounts from $39 2-W'f,Y speaker switch boxes $18 3-way speaker switch box (impedance matching) $49 Don't forget our disposals store at Sth. Croydon. There's plenty of hard-to-find parts and help you won't get elsewhere! Phone 723 2699 (ask for Mai) OCT0 BER1992 45 SERVICEMAN'S LOG - CTD C )j~•" 1"'\-\6- 0\..-1> SL,.6:96,E.H~W\~6:'R ~ \ \<.., •• hammer (the plastic butt end of a screwdriver), I began tapping any likely components, while watching the purity pattern in a mirror. This had no effect until, more or less in desperation, I gave a light tap - a very light tap - on the back of the picture tube. And bingo! The purity error and the black streaks vanished completely, just like that. I tapped the tube again; and the purity error was back - in a different form - and I had a pattern of black streaks far more prolific than before. I tapped several more times and every time I tapped there was a different purity error. It was sometimes better than before, sometimes worse, and there were similar variations in the black streak pattern. I gave it away then. There was little doubt in my mind what the problem was. The shadow mask was adrift inside the tube, probably because one of the supporting welds had failed . From there on it was largely routine, although there was still a lot to be done. First, the owner's daughter was contacted and the situation explained. She was rather shocked to learn that they had a crook picture tube but I quickly put her mind at rest by explaining that the set was still under warranty and would cost them nothing. I then rang Panasonic and ordered a new picture tube. It arrived in a couple of days and I wasted no time fitting it. This is relatively easy - the single board on the bottom of the cabinet is slipped out; the neck board, convergence yoke, and deflection yoke are removed; and the four corner nuts holding the tube are unscrewed. Fitting the new tube is essentially the reverse procedure, followed by the setting-up routine for purity and convergence. This has been simplified by means of a technical informa- 56mm 29.7mm 26mm Convergence Yoke (TLC2047) 46 SILICON CHIP Fig.2: the convergence yoke assembly for the National TC-48R60 TV set. It shows the suggested spacings for the deflection & convergence yokes for minimum setting-up adjustments. It turned out to be almost spot on. tion sheet setting out typical spacing dimensions for the two yokes. As a precaution, I had supplemented this information by measuring these distances on the old tube with a pair of callipers before I pulled it out. They were within a millimetre or so of the given dimensions. I re-assembled everything, switched on, and was presented with a near perfect picture. Only a few minutes ' work was needed to touch it up. I then let it run for several days, just to make sure. It ran perfectly with no purity problems or black str'e-aks and, significantly, no tendency to spontaneously switch-off. In fact, I am now convinced that all the problems were related. The faulty weld in the tube was allowing two metal surfaces to rub together and, due to the high voltage field around the tube, this caused sparks. This in turn produced the streaks on the screen, along with interference pulses that caused the remote control microprocessor to shut the set down. Crystal frequency To finish off, I'd like to acknowledge a letter from a Mr P. Maljevac, an engineer with OTC in Sydney. He has very kindly supplied the answer to a technical point I raised in these notes for June 1992. While looking for a colour fault, I suspected (correctly) that the reference oscillator crystal had failed and replaced it with one from stock. However, the faulty crystal was marked 8.86MHz instead of the more usual 4.43MHz and I wondered why. Well, Mr Maljevac provides the answer and his letter was reproduced in Mailbag for the August issue (p.5). If you haven't already seen it, I suggest you make a point of referring to it because it gives a very succinct explanation. How had I missed out on that development? Well, it's hard to keep up with every advance these days and, in the service scene, one tends to ignore those parts of a set that don't normally cause problems. It's only when a section fails that one starts asking questions about it. And, unfortunatelv, I don't encounter many Philips sets; they don't seem to break down very often in my area. Anyway, thanks to Mr Maljevac, I now know the answer to my query.