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SERVICEMAN'S LOG A clear case of sabotage As I have shown very often in these notes, servicing involves much more than merely finding and fixing faults. All too unwittingly, one can easily become involved in a domestic battle of wills; a situation which can call for the diplomacy of an ambassador. One tends to relax more on Saturday mornings, knowing that the weekend starts when I close the shop in a couple of hours and that I can leave my worries behind. This morning was particularly glorious; it was a beautiful day, I was relaxing with a cup of coffee which tasted really good, and I had only a couple of routine jobs to finish off. So I didn’t really notice Mr Roberts carry in his huge TV set – a Mitsubishi C6343 – all by himself. I must have been half asleep because you really can’t miss Mr Roberts; he is a giant of a man with muscles everywhere. His presence brought me back to earth with a bump. I booked the set in, noting that the complaint was that it was dead and that it was needed urgently. I went back to my coffee but somehow it didn’t taste quite the same. The lovely morning had been spoilt. There was nothing for it but to attack this huge TV set. I removed the back and checked the mains fuse – it was OK. I plugged it in and checked if the HT rail was OK – it wasn’t. Being fairly familiar with this model, I knew that the grey power switch gives a lot of trouble and decided to check it. This wasn’t really necessary as it had already been replaced but I checked it anyway. It was OK. The multimeter indicated that there was no AC input and I traced the fault back to the AC power lead, which was open cir­cuit. I noticed someone had wound some insulating tape along the first 7cm of the lead, next to the moulded mains plug. I cut off about 10cm and checked continuity – it was now OK. I fitted a new mains plug, plugged it to the power socket and switched on. The set burst into life and both picture and sound checked out OK. A case of sabotage Intrigued as to why the mains lead had failed, I removed the insulating tape. To my amazement, I realised immediately that the set had been sabotaged; the blue lead had been neatly cut and bent back. But why? Had someone set up a bodgie fault, designed to trap some unsuspecting serviceman – muggins in this case – into making an embarrassing diagnosis? Perish the thought. There was nothing for it but to confront Mr Roberts and find out what was going on. When the phone was answered, I in­formed the listener who I was and asked could I speak to Mr Roberts. A juvenile voice replied “I am Mr Roberts’ son, Roger, and I know all about the TV set. Have you fixed the lead OK?” “How do you know about that?” I asked. 40  Silicon Chip “Well, it’s like this. My dad watches TV all night and none of us can get any sleep, and we don’t dare tell him to turn it down or off. My HSC exams are due and I need all my sleep, so I cut the wire last night before he came in, intending to reconnect it first thing this morning. When I got up, the set had already gone.” “Well, you realise you will be up for the minimum service charge and the plug”. “Yes, yes, that will be all right but please, please, don’t tell Dad. Just write on the invoice that you repaired the power supply and don’t mention the cut lead.” Apparently, Mr Roberts senior was not a new age bloke and definitely would not understand the situation. I took pity on Roger and was suitably vague with the report’s repair details. Mr Roberts paid up without comment and left with the set under his arm – figuratively speaking, that is! And I hope Roger’s HSC exam went off well. Fig.1: Akai VS-303EA. The review guide (A) mounts on the review lever arm which is pivoted at (C). The pinch roller mounts on the play arm and this pivots on the top hole at below right. Akai video recorder My next story is about an Akai VS303EA video recorder. This is getting a bit long in the tooth now but, in its day, has proved to be a good and reliable performer. The owner carefully reiterated what “the boss” (his wife) had told him. He cheerfully confessed he knew nothing about it but the message was that there was a cassette stuck inside it and it wouldn’t eject or play. I checked it out and it behaved exactly as he said it would. I also discovered that it could still fast forward and rewind. Removing the covers and checking the controls, I noticed that the loading motor made no attempt to turn in either direc­tion. By disconnecting one lead and applying an external DC supply, I could make it eject the cassette. So the motor was OK but there was no drive to it. When the cassette ejected, I noticed that the tape hadn’t retracted fully back into it. I didn’t pay much attention to this at first, as the reel motor wasn’t being told that I was artifi­cially operating the loading motor. It was when I put the cassette back in that I noticed that the cue/review guide pin had not moved back into its correct position, although the loading guides were correct. Fig.1 will make this easier for the reader to follow. Fig.2: Akai VS-303EA. This mechanism is on the opposite side of deck to the parts shown in Fig.1. The play arm is shown dotted and is engaged by the pinch roller link. This in turn engages the cam plate and is moved by cams in the mode selector switch. The guide, indicated by arrow “A”, is mounted on the review lever arm (partly ob­ scured), which pivots on nut “C”. Well it should but it didn’t – pivot, that is. Instead, it had seized solid. A little lubrication and the arm soon came free and the tape now played and ejected OK. Unfortunately, the story didn’t end there. A day later, while soak testing it, it became erratic and intermittent in playing and ejecting. I returned it to the workbench and removed the covers again. After much erratic behaviour and red herrings, I eventually concluded that there was a problem involving the mode selector switch/ cam assembly and the pinch roller. In broad terms, the pinch roller wasn’t being moved far enough. This takes us back to Fig.1. The pinch roller is mounted on the larger plate – sometimes called the “play arm” – and this is shown with two circles at its larger end, at extreme right. The upper circle indicates a pivot point, while the lower circle indicates the point where the plate is engaged by a lever. This lever is part of the “pinch roller link”, on the other side of the deck, and is shown in Fig.2 (the play arm is shown dotted at the left of the diagram). This link in turn engages the cam plate; a triangular plate which pivots near its apex. The axle on which this pivots was also suspect but more of that in a moment. The cam plate engages cams in the mode selector switch. So the cam action is transferred, via the cam plate January 1998  41 and the pinch roller link, to the play arm carrying the pinch roller. So, why wasn’t the pinch roller moving through its full range. Basically, the problem was due to wear between the cam plate and the pinch roller link. I consulted Akai and learned that the pinch roller link has been modified, with extra metal reinforcement where it moved against the cam plate. Ordering and replacing this part helped considerably but still left something to be desired. The problem now involved the support for the axle which supported the cam plate. This was slightly loose, although this movement could be felt rather than seen. As far as I could ascer­tain, the axle and its associated support was fastened to the deck simply as a push-fit arrangement. I couldn’t do much with this other than treat it with Loc-Tite. It wasn’t the most elegant solution perhaps but should prove to be both adequate and economical. 42  Silicon Chip Anyway, that seemed to provide the final answer, the ma­chine functioning perfectly during a prolonged soak test. More to come But it wasn’t really the end of the story and, as it turned out, in more senses than one. There is one part of this story which I bypassed initially, for the sake of clarity. But it needs to be told. Not all the erratic behaviour I en­ coun­ tered after the initial simple lubricating exercise were due to the mechanical problems already described. And the reason wasn’t very clear initially, until I re­ alised that this model recorder can be fitted with two different types of ejector assembly. The first type is a conventional one, as shown in the service manual, and uses a 7-pin plug (P134/132). The second type (not mentioned anywhere) has a 9-pin plug to accommodate an optical slack tape sensor. This consists of an infrared LED sender and an optocoupler receiver, which creates a monitoring light path. And if the tape isn’t fully withdrawn into the cassette and clear of the infrared light path – or if any­thing else temporarily obstructs this path – it will prevent the system from playing or ejecting. At the time, neither this mechanism nor its function were obvious. Consequently, I wasted a lot of time following red herrings, especially considering the fault’s intermittent nature. But there it is; one to make a note of in your own records. The machine bounces Anyway, I was relieved when every­ thing was finally fixed and the machine returned to the owner. I thought no more of it until a few weeks later when he returned with it, complaining of exactly the same fault. After spending so much time and repairing three faults for the same symptom, I was shocked – not to mention embarrassed – to find that the machine had bounced. I checked it again in front of him and he was quite correct. I apologised and assured him that I would get back onto it straight away. Fortunately, he is a very pleasant, easy- going fellow and remained unfazed. I pushed aside the work I was doing and went straight onto the recall. Looking straight at the top of the ejector, I could­n’t see anything wrong with the optical slack sensor but the cassette was in the way. I removed it by disconnecting and power­ ing the loading motor from an external source, as before. With the cassette removed, I had a clear view of the in­frared diode sender and optocoupler receiver and could see imme­diately what had happened. A small sticky label belonging to the tape cassette had come away and become stuck over the receiver sensor, causing the same symptoms as before. I was quite relieved that it wasn’t due to any carelessness on my part and the owner accepted that it was just one of those one-in-a-million chances. So it all ended happily. stock of the kit. No matter; Bob just ordered the IC separately from another supplier. He had a resistor in stock and plenty of heatsink com­pound left over from previous kits. When it arrived, he fitted the IC and resistor just as he had done on dozens of previous occasions. However, when he switched the set on, absolutely nothing happened. He had subse­ quently spent hours investigating why and had run out of inspira­tion, before finally asking if I would take a look at it for him. As he had often done the same for me, I was happy to return the favour and in due course he dropped the set around, along with a list of all the things he had tried. The only problem with a list is, of course, the possibility of missing something some­where. I would have to recheck his list as well as add my own checklist. Measuring the set’s vital signs, I established that 330V was present on pin 3 of IC801 but there was only 40V at pin 5 instead of the 113V shown on the circuit. My checklist included T801, R803, Q833, IC601, Q500, T500, Q451, T501, Q504, D502, R511, C836 C808 and all stops along the way but I wasn’t getting anywhere either. As a precaution, Bob had ordered two STR50113 ICs and had tried them both to no effect. Fortunately, I had a scrapped set (broken tube) and was confident that it had worked correctly. This meant that I could check suspect components by substitution with the knowledge that they should work. At this stage, more in desperation than anything else, I decided to swap IC801 from my set into Bob’s. And would you believe it? – it worked perfectly and con­tinued to do so even after I replaced all the original parts. So why did Bob’s two brand new STR50113s not work in this set? It’s interesting to note that the part number specified in the circuit and the parts list is STR50113-M but the M isn’t marked on either the original or any of the substitute. The internal circuit of the IC (see Fig.3) is shown as consisting of two NPN transistors and one resistor but this may be only a simplistic block diagram. In the end, the only conclusion I could come to was that the two ICs Bob fitted were cheap clones that weren’t up to the job in the Panasonic circuit. Significantly, neither the A friend in need Bob is a colleague who works for an opposition service organisation and has been in the game for as long as I have - too long perhaps, or so it seems at times. Anyway, we help each other out on odd difficult problems. It’s good to be able to discuss and think through a problem, or maybe get a different perspective – a spot of lateral thinking and all that. So it was with a National TC2038 TV set with which he had been lumbered. This set uses an M14 chassis which has been highly reliable over the years, making it a good rental set. Most prob­lems are well known and understood and this one should have been the same. Its symptom was simply that it was dead and he had discov­ered that resistor R841 (4.7Ω, 7W) was open circuit and that the switchmode IC (IC801, STR50113) was short circuit. The usual reason why this switchmode IC fails is poor heat transference. Normally, one purchases this IC as part of a small kit that includes a new insulating washer and thermal grease. However, on this occasion, our mutual suppliers were temporarily out of LOUDSPEAKER SALE - Limited stocks Prices include sales tax Australian Audio Consultants PO Box 11 Stockport SA 5410 Phone or Fax 08 85 282 201 Fs Sens Qts Vas Size Vifa D25AG-35-06 $69.00 $45.00 850 89 Metal dome tweeter Vifa D19SD-05-08 $49.00 $34.00 1700 89.5 Shielded tweeter Vifa D26SG-05-06 $62.00 $49.00 1450 92 Shielded tweeter Dynaudio D21/2 $185.40 $99.00 1300 89 Dome tweeter SEAS ExcelT25-001 $194.00 $179.00 750 90 5mm soft dome tweeter. Scanspeak D2905/9300 $160.00 $90.00 600 90 28mm soft dome tweeter Vifa M11WG-09-08 $99.00 $79.00 68 0.46 4.3 125 woofer Vifa P13WH-00-08 $99.00 $75.00 60 0.33 10.0 140 woofer Vifa P17WJ-00-08 $99.00 $75.00 37 0.35 34.7 170 woofer Vifa P17SJ-00-08 $109.00 $75.00 41 0.35 33 170 shielded P17WJ Vifa M18WO-08-08 $145.00 $90.00 33 0.34 34 180 woofer Vifa M18WN-19-04 $149.00 $90.00 63 0.66 10 180 Car woofer Vifa M22WR-09-06 $195.00 $110.00 30 0.33 55 225 sub woofer Vifa M22WR-19-04 $195.00 $120.00 48 0.70 20 225 car sub woofer Vifa M22WR-29-04 DVC $210.00 $125.00 47 0.66 20 225 car sub woofer Vifa M26WR-09-08 $229.00 $135.00 26 0.32 130 271 sub woofer Vifa M26WR-19-04 $229.00 $150.00 44 0.69 53 271 car sub woofer Vifa M26WR-29-04 DVC $249.00 $159.00 40 0.67 53 271 car sub woofer Dynaudio 24W100 $399.00 $199.00 32 0.35 62 2 40 woofer Scanspeak D3806/8200 $181.00 $90.00 38mm soft dome midrange 1000-13500Hz Call for full specs. Many other drivers available at special prices. January 1998  43 Serviceman’s Log – continued suppli­ers nor anyone else have been able to come with another theory. The end of the weekend After a fairly lax weekend, I arrived on a Monday to find three jobs awaiting me. One was a large stereo Teac CT-M631S TV set which was described as “dead and burning”. Well, fortunately, the latter symptom had long since stopped but the acrid smell of burnt plastic was obvious. As soon as I removed the back, I expected the cause would be obvious and straightforward – for example, burnt horizontal output transformer insulation. I was surprised, therefore, to find that the AC power line input filter coil (L901) had burnt and taken fuse F901 with it. The adjacent 0.47µF capacitor across the AC mains (C901) was also badly melted. So had the coil shorted and melted the capacitor or vice versa? When I measured the capacitor it still read OK but I have had cases in the past where capacitors like this develop and clear their own shorts, blowing the fuses on a random basis. However, I don’t think that this was the case this time as the heating process must have lasted for quite some time. But that wasn’t the end of it. In the course of cleaning and resoldering the board, I noticed that the main electrolytic (C905, 220µF 400V) appeared to be loose. I unsoldered one leg and the whole capacitor came away in my hand. The other terminal had been corroded by the electrolyte. I replaced this along with C908 and Fig.3: National TC-2038. The circuit shows that IC801 (STR50113-M) consists of just two NPN transistors and a resistor. Note the suffix “M” which is not marked on either the original or any of the replacement units. C910 which can also cause the HT rail to fail. On reassembling and powering up, everything was now OK; the sound and picture were perfect. But it left a niggling thought in my mind as to what was the original fault and what caused what to fail in what order. The second set was an NEC N-3419 with a Daewoo C-43M chassis. The fault description on the job sheet stated that the picture height decreased after about half an hour, so I left it to run Silicon Chip Binders ★  Heavy board covers with 2-tone green vinyl covering ★  Each binder holds up to 14 issues ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A12.95 plus $A5 p&p each (Australia only; not available elsewhere) Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 44  Silicon Chip REAL VALUE AT $12.95 PLUS P &P while I dealt with the third set. This set was a Teac Televideo MV-1440 combination TV/VCR and it was dead. Removing the covers revealed a chassis which is quite difficult to work on, as access to the PC board is restricted by the short connecting leads to the video recorder beneath it. I measured the main HT rail, which should be at 112V, and there was nothing. Nor was there any voltage on the 12V secondary rail. Next, I checked the 320V rail to IC1501 (STK7348) and again there was nothing. I had to remove the chassis altogether to get the ohmmeter to finally confirm that the IC was short circuit and that R1501, an 8.2Ω 5W anti-surge resistor, had gone open cir­cuit. I replaced the IC (along with fresh heatsink compound) and replaced C1507 (2.2µF, 50V) and R1501 for good measure. I also found that D1506 was short circuit and replaced that as well. This restored both the 112V and 12V rails but the set was still dead. I quickly established that there was no voltage on the collector of Q1401, the horizontal output transistor, but it was there on R1407, the supply resistor. When I subsequently removed the horizontal output trans­ former (T1401), I found that its primary was open circuit due to a corroded lead to one leg. After mucking around with extra wire splints and microsurgery, I eventually managed to repair the coil and reassemble the set again. And that was the end of it. The set now worked fine and it must surely be bad luck to find so much wrong all at once. Shrunken picture In the meantime, the NEC TV set had been doing its thing and the picture had indeed shrunk from top and the bottom. I measured the main HT rail and found that it was 125V instead of 103V. Freezing IC Q801 (STR5412) caused the HT rail to drop and the height to increase. I replaced the IC, applying fresh heatsink compound in the process, and changed C806 (100µF). I also re­moved some brown goo from around IC 1502 (the 12V regulator) and reworked a few potential dry joints before trying it out. The set now worked perfectly and it was still going strong after a 3-hour SC soak test.