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SERVICEMAN'S LOG How I ended up in the sync This month, I have a story about an apparently easy job that turned into a monster. I also have a follow up story on a Sharp VCR that I wrote about some months ago. My role as Dr Frankenstein concerns a General 48cm colour TV set, model GC-202. The owner brought it in early one morning, described the problem, and indicated that he needed it rather urgently. How soon could I fix it? In fact, I was snowed under and explained that I might not be able to tackle it for a few days. Although disappointed, he accepted this philosophically and decided to leave it anyway, so I said I'd tackle it earlier if I could. His description of the fault suggested picture pulling, as probably caused by hum in the horizontal system. On this basis, I assumed that it should be relatively simple and so I put it to one side without even bothering to switch it on. Now it so happened that I had a more than usually successful day. I FRACARRO SAVES ••. ... more than $10. from every UHF television antenna you buy. -~ i----:.c-------. • - - - -· --"-- - Fracarro's 4PUN for better performance from weak UHF signals. Peter C. Lacey Services Pty. Ltd. P.O. Bo x 678 (74Fulton Rd.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 40 STLICO N CH f P quickly knocked over several jobs which I had originally expected to be difficult and, by about 4 o'clock, had made quite an inroad into the backlog. It was still too early to knock off but too late to tackle anything nasty, so I decided to finish the day on something easy. And what better than the General which the owner needed urgently? Sure enough, when I switched it on, it came up with a classic hum pattern. It was pulling madly although it appeared to be normal in all other respects (brightness, contrast, colour and sync). I also checked the scan width, which can sometimes provide a further clue, but this was normal. The first setback My first bet was the main filter capacitor C408, a 330µF 200V electrolytic following the bridge rectifier, so I simply clipped another high value unit across it. And that was the first setback; it had no effect whatsoever. Oh well, it was worth a try. I moved on to the regulator transistor, TR701 , measuring voltages around it. The voltage from the main rectifier, applied to the collector, is shown as 146V, while the voltage at the emitter, which becomes the main HT rail, is shown as 128V. In fact, the collector voltage was closer to 156V but I wasn't worried by this small difference. But when I measured the emitter voltage, I knew that there was something wrong. It was also at 156V - an impossible situation if everything is working properly. But of course it wasn't; the regulator transistor had obviously gone short circuit and I really didn't have to test it to confirm this. At this stage, I needed to make another routine check. The focus control pots in these particular sets are rather notorious for failure. They can go open circuit, the moving contact can go high resistance to the track, or sometimes they can just fail mechanically. Sure enough, when I tried rotating the shaft, it simply spun in my fingers; it wasn't coupled to anything! Well, that looked like it - a new regulator transistor and a new focus control pot and we would be back in business. I had replacements for both on hand and fitting them was routine. It had been a simple job after all and I was well ahead on the day. It's not fixed Until I switched the set on, that is. Then I was in real trouble. I had sound and I had a raster but I had only a faint video image - just enough to tell me that I also had no sync. As anyone who has experienced this situation will testify, it is a most disturbing one. It is much worse than simply being confronted with a set with such symptoms. Quite apart from being totally unexpected, it immediately suggests that it is something careless one has done which has created a fault far more serious than the original. I tried to recall anything I might have done to create the problem but without success - at least then. Eventually, I did think of something but more of that later. Right now, I had to start troubleshooting all over again. A logical deduction from the symptoms was that the loss of video luminance and loss of sync were one and the same fault. There had to be a fault somewhere in the video IF or video chain which was wrecking the luminance signal. Unfortunately, the only circuit I have of this set is a much copied one which is extremely poor. I can follow it using a jeweller's loupe and a lot of imagination but it would be quite circuit is virtually identical with the GC-202 and the IF module is totally interchangeable. So what quicker and easier way to check the suspect module than by replacing it with known good one? If it worked, I could even leave it in place as a quick cure. Unfortunately, it wasn't going to be that easy. All I learned was that it wasn't the module, because the replacement made no difference. So what now? The tuner? This seemed to be a long shot, since I still had sound, but stranger things have happened. And how best to check it? I thought for a moment that I might substitute the tuner from the junked set but a closer look at the mechanics involved discouraged me - it would be a much bigger job than swapping the IF LOS module. 0 So I turned to a piece of test gear left over from the monochrome era but which I still use occasionally. It consists of a standard VHF tuner .' \ t?'. ·• .. mounted in a box with \. I.\ ·, ·. I\ d, ~ ,/ its own small power -~ h. • I supply, so that it can -s-r ·s-,E:? WPt~ be quickly substituted .,-o J:).??\..'< A 'ROU"'f\NE: for a suspect tuner. This \/O\.-"'f'AGS C.t-\e:C.'tC::oo~ ; ,; -'· was quickly hitched up and a test pattern fed pointless to reproduce it. So I will sync/video bOard. This provided a in. Alas, still no joy; the signal out have to describe the circuit as best I convenient access point for the CRO. was exactly as before. I made the connection, fed a colour can and hope that the reader can folThat really set me back. Why, when bar pattern into the antenna, and both the tuner and IF module were low it. My first step was a routine voltage checked the output. It's rather hard to known to be OK, could I not get a describe the resulting signal but it signal through them? check. The signal path supply rai1 sits at 18V and I traced this on the circuit wasn't good. There was very little inand the chassis, making sure that it dication of the luminance signal or of The next suspect was correct to all points. This was no normal sync pulses. The only place I could turn to for Anyway, the implication seemed inspiration was the next board in the simple task, because the circuit shows very few voltages. Having done that, I obvious - there was probably a fault chain (the sync/video board), the reaturned my attention to the video sig- somewhere ahead of this check point, soning being that something on it was nal path. either in the IF module or even in the loading the video circuit. This board The main video chain is contained tuner. Of the two, I plumped for the IF carries the 2nd and 3rd video amplifiin one module; a single board in a module. ers, the sync separator, the noise gate, metal box. This board carries the 1st At this point, I need to digress and the red, green and blue output video IF amplifier transistor (TRl); briefly. In these notes, back in January stages. It also carries the brightness the 2nd video IF amplifier and AGC 1989, I described how a General GC- and contrast control circuitry. gate (ICl); the 3rd video IF amplifier 181 had suffered a picture tube imOnce _again, I decided to substitute stage (transistor TR2); the video de- plosion, incurring mechanical dam- the board from the junked set but tector; the 1st video amplifier transis- age to the cabinet and chassis in the halfway through the exercise, I realtor (TR3); and, finally, IC3, which is process. And the economics of the ised that it was not totally compatthe sound IF amplifier and detector. situation were such that the owner ible. The suspect board used a 3-pin So where could I check for video elected to write it off, giving it to me plug (P201) to connect to the brightsignal? The video signal comes out of to salvage anything I wanted. ness control (J201), while the contrast this module via pin 7 of a plug and It was too good to send to the tip, so control was mounted on the board socket assembly, J412, and from there it has been languishing in my junk and protruded from the rear of the goes to pin 3 of plug P407 on the pile ever since. More to the point, the cabinet. \ \.I!~\,;,::~- ~ I ... ~li1 /,~ :,·. A UGUST 1992 41 now and I was running late for tea. Oh well, Mrs Serviceman is used to this kind of thing. SERVICEMAN'S LOG - CTD Who dunnit? .- Qt.-1> ,s"i-U• •• V'-K ISPO"-A o. ""'«..\. -(. e;T"c.. r ----·· s . -:~-"' ) i -;;;---(' ~ -·-a..· ~ ? .. <... 'RE::S \'DUE: \>J~~ 'P~E.-n""( N\UC.K'-f A.T ~\~'S-r... i\.-'\S The substitute board used a 5-pin plug, to allow the contrast control to be mounted on the front panel. After some head scratching, I lashed up a set of patch leads which allowed the substitute board to work, albeit with maximum brightness. Progress at last As it turned out, the lack of brightness control didn't matter because I now had a normal picture - the fault was on the original sync/video board. This was confirmed by the CRO pattern, which was now showing normal staircase and sync pulses. And so I went back to the faulty sync/video board. The video signal enters on pin 3 and goes in two directions. One path is to the 2nd video amplifier (TR201), while the other is to the noise gate stage (TR208) and thence to the sync separator stage (TR209). I tried the video amplifier 42 SILICON CHIP first, pulling TR201 out and checking it. It checked OK but, more to the point, when I tried the set with this transistor removed, the fault was still evident on pin 7 of the video IF board. I replaced TR201 and set off in the other direction, to TR208. I pulled this transistor out and, immediately, the waveform at pin 7 jumped back to normal. When I checked TR208, it showed a short from collector to base. I fitted a new transistor and tried again. Problem solved? Not really; half solved would be more like it. The CRO now indicated a normal luminance signal but still no sync pulses. I could hardly wait to pull out TR209 and test it. It was open circuit, both base to collector and base to emitter. A new transistor brought everything back to normal and restored a first-class picture. But the job had taken much longer than I had expected. It was dark by But why did it happen? Thinking it over, I can recall only one possible cause. It was when I was checking the regulator transistor and found the same 156V on both collector and emitter. Access to the emitter connection is awkward and, although the probe on the meter lead is insulated as close as possible to the tip, I did observe a small spark as I manoeuvred the probe. And it could only have been from the emitter, at 156V, to chassis. So was that it? It seems a long shot but it's the only explanation I can offer. Anyway it was all a nasty experience. The only bright spot was first thing next morning when the owner turned up unexpectedly. There was a special program he wanted to watch and he had decided to take the set, fault and all, and bring it back later. Imagine his surprise when I told him that it was fixed. So he went on his way, happy as Larry, without any idea of how I finished up in the sync. (The price I paid for being late for tea was that I found myself stuck with the washing up. I will refrain from labouring the point about sinks). The Sharp VCR And now to the VCR story. Last March, I described a fault in a Sharp VC-9300X, involving a capstan shaft which had seized in its bearing due to some "gummy" substance. I cleaned the shaft and bearing and the machine went back to the customer. However, a couple of months later, it was back on the bench with the same problem. This time, the diagnosis was easy but I was surprised that my previous effort had failed and, in fact, that there had ever been oil in the system. I was under the impression that these machines used socalled oiless phosphor bearings. Anyway, I tackled the problem with more vigour this time. With the capstan shaft removed I set the machine up so that I could spray cleaning fluid into the bearing every few hours. I did this over the next couple of days. The cleaning fluid was Caltex Aqua Tee, described as "a drying and penetrating spray", aimed mainly at the marine market, but which I use a lot in the workshop. So I kept spraying it into the bearing, with a folded tissue underneath to catch the residue. This residue was pretty murky at first but eventually came clean. I then relubricated the bearing with CRCZ-26 and put everything back together. As before, everything worked fine but, having been caught once, I wasn't going to make a song and dance about it until it had stood the test of time. Well, now I am cautiously optimistic because when the machine came back recently for another fault, all was well with the capstan. But that is all by the way. The real story concerns the reason it was back in the shop. The owner's complaint was that recording was unreliable although playback of previously recorded tapes was OK. More specifically, it would sometimes produce a good result while at other times it appeared as if the heads were very dirty. In fact, he had brought in a new tape with a sample of the fault. To my mind, it looked exactly like a fouled h ead problem. I put the machine through a series of recording and playback functions, TETIA TV TIP Sharp DV4884 TV set Symptom: no on-screen display. The set works perfectly but the on-screen display which advises channel number or control settings cannot be called. Cure: the vertical sync pulse into the character generator chip is missing. This could be. a loose plug and socket or a dry joint. H and V sync pulses are necessary to allow the chip to output coherent signals. 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. using my own tapes, but couldn't fault it. Eventually, for want of a better idea, I gave it a clean-up, put it through another set of record/playback cycles (without result), and gave it back to the customer. But I wasn't very confi- 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 split diode type, yokes and drive transformers. $78.00 + $4.00 p&p dent an d warned him accordingly. Sure enough, he was back in a few days and I tried another tack. How old were his tapes? Most of them pretty old, he admitted. I suggested he bring some in for me to check an d the next day he brought in four tapes, one relatively new, the other three quite old. I rejected tw o immediately, put one in the too-hard basket, and used the fourth one for further tests. Meanwh ile the custom er went off and bought a new set of tapes. The risk with worn tapes is that they not only foul the heads on a more or less permanent basis, necessitating professional cleaning, but that they can also cause short term fai lure. A scrap of oxide can foul a head for anything fro m a few secon ds to several minutes, then be swept clear and let everything return to normal. Initially, I suspected som ething like this , since the symptoms can be similar to an intermittent electrical fault. But subsequent testing an d observation convinced me that, if it was, it was not the only problem ; there just had to be an electrical fault. For one thing, the fault was now ANTRIM TOROIDAL TRANSFORMERS HIGH-VOLTAGE PROBE Built-in meter reads positive or nega- _,,. live voltages from 0-50kV. For check- ' ing EHT and other HT voltages. $98.00 + $5.00 p&p a 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 ~-~~~ ~ 2~ TUNER REPAIRS ~ From $19 exchange plus post & pack Cheque, Money Order, Visa, Bankcard or Mastercard 216 Canterbury Rd, Revesby, NSW 2212, Australia. Phone (02) 774 1154 Fax (02) 774 1154 • • • • Now made in Austral ia Standard range 15VA to 2kVA Large ex-stock· quantities available Competitive pricing available for OEMs & resellers • Simply the best transformers all round HARBUCH ELECTRONICS PTV LTD 90 George St, Hornsby, NSW 2077. Phone (02) 476 5854 Fax (02) 476 3231 AUGUST 1992 43 ' VIDEO joints and hope for the best. I hooked up the CRO to monitor pin 11, pushed in a 3-hour tape, and set the machine running. Granted, it was hardly practical for me to sit with my eyes glued to the CRO for hours at a time but at least I could keep a close watch on it while I did other jobs. After I been through this ritual several times, I was convinced that the signal was reaching pin 11 at all times. Several breaks had occurred on the tapes but I had not observed one failure on the CRO, even though some breaks had been 15 minutes long. The next logical step was to fire up the CRO's second channel and monitor both pin 11 and pin 8 together. And I was particularly careful to clip the CRO leads directly onto the IC legs rather than to nearby circuitry. This produced a real breakthrough; it wasn't long before I glanced at the CRO and found that there was no signal out on pin 8, although it was still solid on pin 11. H£A0 Missing voltages Fig.1: the relevant circuitry in the Sharp VC-9300. IC201 is at the top, Q205 to its right and IC301 beneath it. The video head plug is at bottom left. Note the heavier lines which run to the plug - these were colour coded on the original circuit to indicate playback & record paths. occurring relatively frequently on known good tapes. More importantly, it was occurring only in the record mode. On the law of probabilities, this didn't add up. I had played one ofmy own 3-hour test tapes right through several times and it never faltered. But when I took a known good tape and made a 3-hour recording, the fault showed up in several spots. I noted these positions on the counter, then put the tape in another machine and made new recordings over these spots. These recordings were perfect, so it wasn't damaged tape. The signal path At this stage, I turned to the manual to trace the record signal path and check likely trouble spots. The relevant part of the circuit is shown in Fig.1 and the main components are IC201 at the top and IC301 below it. To the left of IC301 is a 7-pin socket which connects to the lead from the video heads. In the original circuit, some paths 44 SILICON CHIP around IC301 are colour coded blue for playback and orange for record. These cannot be reproduced here but I think the reader will be able to follow my description. Among other things, IC201 generates the necessary FM signal for the video record function. This signal emerges from pin 9 of IC201 and is fed via filter unit FL201, transistor Q205 and level control pot R310 to pin 11 ofIC301. It then passes through the recording amplifier in IC301, comes out on pin 8, and goes to the aforementioned 7-pin socket. The rest of the IC performs the switching functions. I decided that this was as good a place as any to monitor the record signal. In all such cases, one has to start somewhere in order to determine in which direction the fault lies. And there was another reason. I had checked the soldered joints to the IC pins and was somewhat suspicious of pins 11 & 12 on IC301. Butl couldn't be sure and I wanted prove it if I could, rather than simply remake the Why? It could be the IC of course but what about my suspicions regarding the joints to pins 11 & 12. And what about the voltages on the IC pins? The circuit shows two sets of voltages: an upper set for the playback mode and a lower set (enclosed in brackets) for the record mode. And, in the record mode, pin 8 is marked 5.6V, pin 11 as 4V, and pin 12 as 11.2V. This latter is the supply voltage for the IC. These three voltages were now missing. I measured 0V on pin 8, 0V on pin 11 and ov,on pin 12, even though I traced the 11.2V right up to the pin 12 connection. But there was nothing on this pin itself - nothing, that is, until I pressed the probe hard against the pin and pushed it into the solder connection, whereupon everything came good. So that was it; a dry joint to the IC pin. It was easyily fixed, of course, and I remade all 12 pin connections while I was about it. No sense in tempting fate or giving Murphy an even break. And that wrapped it up. It is now nearly two months since the machine went back to its owner and a recent check confirmed that there has been no further sign of trouble. So it looks as though I have finally cured the capstan fault and fixed a nasty intermittent into the bargain. SC