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SERVICEMAN'S LOG From soap to Teletext It takes all sorts of people and their problems to make a serviceman’s world. For one of my customers, a misbehaving VCR assumed the proportions of a major life threatening emergency. One won­ders how they would react to the real thing. Mrs Proby likes – no, that word is too soft – loves, even adores, her soaps (situation comedies for the uninitiated). She lives for them; is addicted to them. In between watching and recording them, she somehow makes time for her family. But, inevitably, disaster struck; the video recorder wouldn’t work. It was a real emergency and it had to be fixed immediately. The recorder was a Sony SLVX50AS and, on examination, the 30  Silicon Chip problem was worse than she could possibly have imagined – the recorder had had the audacity to actually chew up some of her beloved tapes and they were ruined! I did my best to calm her and tried to sound sincere as I pointed out that worse things can happen in life. But she was inconsolable. Eventually, I managed to convince her that I would do my best to fix it as soon as possible. Afraid of what might happen if I didn’t apply immediate first aid, I rushed it into the operating theatre and opened it. She had given me her most precious tape to examine but unfor­tunately it was in a real mess and beyond repair. Fortunately, I did have a manual for the machine and I have reproduced a couple of diagrams from it, particularly the section “VHS Mechanical Adjustment Manual ll”, which should help the reader follow the story. Seized bearing The reason for the damage became obvious when a dummy tape was run. The RVS guide (No.8) – the reverse search arm – had almost seized on its bearing and wouldn’t free the tape to re­tract into its housing on eject. I hoped I could free this with­ out removing it, as this would save time because it would not then require re- Fig.1: portion of the deck around the capstan/pinch roller area in the Sony SLV-X50AS VCR. The RSV guide is partially obscured and shown dotted. alignment. And I was in luck – after a squirt of CRC and some wiggling, the arm moved freely. After cleaning up any excess CRC with alcohol, the tape went in and out perfectly. I cleaned the heads and tape path and initially thought that that was the end of it; that I had scored an easy one. But I was wrong. On tape play there was considerable curl on the lower edge of the tape, just after the pinch roller, and the tape was being mutilated as it went into the cassette. I removed the pinch roller, cleaned and roughened the rub­ber, and tried again. It was still no good so I fitted a new pinch roller assembly. This improved the situation enormously but the tape was still curling a little on the RVS guide. There was nothing for it – I would have to remove, clean and realign this guide according to the instructions on page 25 of the manual. The gist of this adjustment is the need to move both the RVS guide (No.8) and guide No.7 up or down until the tape tracks with both guides and with the nearby ACE head assembly. This adjustment is by means of height adjuster nuts, one for each guide. The No.7 guide appeared to be correct, so I concentrated on the RVS guide. Unfortunately, the only adjustment setting which brought the alignment close to correct was to have the nut screwed hard down. This not only failed to cure the problem but it Fig.2: side view of the RVS (reverse search) arm, showing its relationship to guide roller No.7 in the Sony SLV-X50AS. Note the adjuster nuts. meant that, on eject, the arm jammed on a gear just below it. There was only one thing for it. I had to replace the arm – but where could I get one in a hurry? This part does not usually need to be replaced. After ringing around, I found a colleague who had one and would let me have it in view of the emergency. I dashed into the ambulance and shot around to pick up the donor organ. Secondhand shop My colleague, Jim, is the senior technician at a busy sec­ ondhand shop. He spends his days growing white hair from refur­bishing TV sets and video recorders, which they purchase from all and sundry. He was in a talkative mood when I arrived and was muttering rude words about two cranky TV sets which had defied all his efforts to repair them. It had got to the stage where he was fed up and was planning to send them off to auction. A cursory glance show­ ed them to be low-cost generic TV sets made in China but both were modern and looked good. I stu­pidly volunteered to have a look at them if he would care to stick them in the back of the “ambulance”. I then shot back to the operating theatre and transplanted the new part into the Sony vide recorder. This time, realigning the arm produced the desired effect, the tape now running through the guides perfectly. I carefully examined the old arm and pinch roller but could not detect any imperfections. The arm, which is a chrome-plated steel rod embed­ded in a diecast aluminium block, didn’t look the slightest degree bent or distorted. However, it only needs a minute change to cause problems like these. Anyway, I pronounced that the patient would live and so undoubtedly would Mrs Proby. But I did issue her with a prescrip­tion of sorts, advising her to purchase some new tapes and ditch the old ones (if she can bring November 1997  31 Serviceman’s Log – continued herself to do it). Anyway, she thanked me most profusely; I thought she was going to call me Doctor but she didn’t. Jim’s TV sets When things were quieter, I decided to have a go at Jim’s TV sets. The first was a 51cm Palsonic 5138 with remote control, twin speakers, and Fast Text – the Palsonic version of Teletext. The fault ticket read, “No picture, no sound”. Jim thought that the fault lay in the Teletext section but he didn’t have a circuit diagram. So my first step was to order a manual from N & G Enterprises. The second set was an Aiko Super­ vision VST 60, model 2801, as sold by Cumberland TV. This was also dead and so I ordered a manual for this as well. A week later I received photocopied circuits for both mod­els. The Palsonic circuit looked very similar to a Chinese chas­sis used in Teac and some other sets. At switch on, the LED on the front 32  Silicon Chip panel lit up and a telltale 15,625kHz whistle suggested that the line output stage was working. The remote control had no effect but turning up the screen control (G2) gave a full-screen faint blank raster. With no sound or picture, I started by measuring the vari­ous voltage rails. Most are not marked on the circuit but in any case, they seemed a little high. Shrunk heatshrink on an electro­lytic capacitor in the power supply (C514, 47µF/50V) pointed to the likely culprit and I immediate replaced it to prevent further damage. I was now getting 130V, 20V, 16V and 8V on the various rails which seemed reasonable but there was no 16V on pin 3 of connector XT01 on the main board, which mates with XT01 on the Teletext board. This feeds NT01, a 12V IC regulator on the Tele­text board. Following the 16V rail back from pin 1 of XT101 on the main board, I ran into a 10Ω 1W resistor (R536) which is not marked on the circuit diagram. This resistor was open circuit. Replacing it restored the sound but there was still no picture or onscreen displays. The voltages on the CRT socket didn’t provide much help, except that the cathodes were higher than expected. I reached for the CRO probe, planning to trace the lumi­nance (Y) signal through to the picture tube. However, this was not as straightforward as in most sets. Whereas most Teletext decoders are designed as plug-in optional extras and the set will work without them, this set diverts the colour difference and luminance signals through the decoder. Also, the circuit is somewhat misleading as to what plug fits into what socket. I was able to trace the Y signal into the Teletext card at connector XT03 (pin 1) but nothing seemed to be coming out on XT02; or rather, on a Y pin adjacent to XT02. From there, the path goes to X401 on the CRT board. This confirmed that the decoder could not be bypassed and so would have to be fixed. However, I didn’t fancy rushing in and replacing suspect ICs, particularly as they cost, on average, about $30 each. It was time for some help. I phoned N & G Enterprises and was informed that the decoder gave very few problems – the only thing they were aware of was the 27MHz crystal. I checked this and could find nothing wrong but I removed a lot of black gunk that had been coated over its pins. A red herring By now, this line of attack was looking like a red herring. I went back to the CRT socket and followed up on my earlier observation that the tube cathode voltages appeared to be high. It didn’t take long to discover that there was no voltage on the bases of video drive transistors V402, V405 and V409, although voltage was present on their emitters. The base bias is supplied via R403, a 120kΩ resistor connected to the 180V rail, and this resistor was very high. Replacing it restored the picture com­pletely. So problem solved but one part of this investigation has me puzzled. When checking with the CRO, I could swear I saw lumi­nance and colour difference signals going into the Teletext board but nothing coming out to the CRT board. But now there were Fig.3: the CRT board in the Palsonic 5138, showing the three video driver transistors, V402, V405 and V409. signals coming out of the board after I had replaced R403. I can see no correlation between the failure of R403 and the lack of signal into X401 from the decoder. I even removed R403 and re­checked with the CRO and there was still plenty of signal. And by what process had I concluded that it was the Tele­text card that was faulty? Well, in hindsight, I cannot explain it. Jim had voiced the thought that it might be in the Teletext section, so perhaps I had allowed this to lead me astray. The remote control needed a new rubber keypad and after alignment, the picture was good and all “Fast Text” functions checked out OK. Destructive overload Jim’s second set filled me with dread when I found that the power supply had suffered a severe destructive overload and a wide range of parts had been affected. Some parts, apparently damaged, had been replaced, some had been partially disconnected, and some were missing altogether. I refitted the disconnected parts and replaced all the missing ones. Then I disconnected it from the main chassis, substituted a 100W globe as a load on the 136V rail, and removed fuse FU502 in this line. I applied 240V AC power via a 200W globe and a Variac, connected meters everywhere and switched on. The 200W globe lit to full brilliance immediately, indicating a short circuit. Switching transistor V501 and diode VD504 in the bridge were both short circuit and were replaced. When I switched it on again, the power supply started to squeal as I turned the Variac up to around 100V and was obviously under stress. There was no output. I examined the circuit very closely. All the small electro­lytics had been replaced and so had R509, a 56kΩ resistor coming off the positive rail from the bridge. All the high value resis­ tors measured OK. The small transistors had all been replaced with substitutes but even the discovery that transistor V503, part of a Darlington pair, was fitted the wrong way around didn’t solve the problem. With the CRO connected, the waveform on the collector of V501 consisted of a series of positive spikes, suggesting that the mark space ratio was indicating a heavy load. I didn’t twig to the significance of this immediately and put it down to the low input (100V) to which I had set the Variac. To cut a long story short, I replaced V504, C519, C521, V505, VD513 and many more parts around this area before the penny dropped. There was no ringing in the waveform indicating, on the one hand, that there were no shorted turns in the chopper trans­ former (T503) but that there was a short circuit somewhere across the secondary. All the diodes measured OK out of circuit but there was something wrong around diode VD518 and it didn’t take long to find that C525, a 470µF 200V electrolytic across pins 19 & 21 of T503, was short circuit. Replacing this and turning up the Variac restored everything. Now for the acid test – I reconnected the main chassis to the power supply, replaced fuse FU502 and switched on. Unfortunately, it still wouldn’t work. A quick check soon showed that there was 136V on the collector of V303, the horizon­tal output transistor, but nothing on the collector of V302 which drives it. The reason for this wasn’t hard to find – the primary winding of transformer T301, which couples these together, was open, probably due to corrosion from the black gunk all around it. Ordering and fitting a replacement transformer fixed the final problem and the set burst into life at last. Jim was pleas­antly surprised and I think only he fully appreciated the time I spent going up so many blind alleys before tracking down the various SC faults. November 1997  33