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SERVICEMAN'S LOG Just give it a flamin’ good thump Most jobs are fairly routine but these three jobs hardly fall into that category. Of course, the customer doesn’t always help and the one job that should have been straightforward was complicated by the customer’s rudeness and uncooperative attitude. A month ago, I was asked by Mrs Johnston to attend to her Sanyo TV. The problem didn’t sound like much in that most of the time the set worked perfectly. It was just that, every three days or so, it wouldn’t start although Mr Johnston could “fix” that by thumping the back. But it was an- 60  Silicon Chip noying and now it did it all the time. The set was a Sanyo CPP2601SV-00 employing an 84P-B26 chassis and is a large 63cm stereo remote control model. It all sounded so easy – just whip the back off, fix the obvious dry joint or loose plug, replace the back and Bob’s your uncle. A piece of cake really and what’s more, it should take no more than half an hour. I should have known better. I arrived at the appointed time full of optimism and had the back off, the soldering iron switched on and the meter ready before the tea arrived. The TV switched on perfectly and did so for the next 20 subsequent efforts. Mrs Johnston assured me that it always played up almost straight away but I know all about the contrariness of inanimate objects. I left it on while I finished my tea and then tried again. It still worked perfectly. I felt like Homer Simpson – TV goes on, TV goes off, TV goes on, etc. The time was slipping by, with no sign of a problem at the end of 30 minutes. It was time for a different approach. Using a strong flashlight, I examined every board very carefully but, as usual, access to the chassis was rather difficult. The leads to the front panel are far too short and the chassis could only be moved about 80mm out of the cabinet. Not only that, but there is a plastic support frame which obscures the main PC board. I decided to gently tap each board in turn but that made no difference. That was it – I couldn’t afford to waste any more time. I refitted the back, pushed the set back into its corner and switched it on for the last time. The set came on, to our mutual frustration. Mrs Johnston then had a go herself with the remote. She switched the set off and it wouldn’t come back on. How did she do that? I tried the remote and the main on/off switch – the set was completely dead. I was now too far committed to back away, so off came the back and we both tried switching the set on and off again. And would you believe it? – it now worked perfectly. I then replaced the back, carefully noting where it made contact with the chassis in case it was dislodging a plug or socket. This time, with the back on, Fig.1: the power supply circuit in the Sanyo CPP2601SV-00 63cm colour TV set. the set worked perfectly, no matter what either of us did. Three quarters of a hour had gone by now and we both had other things to do. Obviously, this was a job for the workshop and I advised the Johnstons accordingly. As it turned out, they were going on a fortnight’s holiday a few days later and so we organised for the set to be delivered to the workshop before they left. The fault appears Finally, the set was on the bench and the back came off yet again. Actually, the back wasn’t in very good condition. It is made of a very brittle hard plastic and had cracked in a few places where Mr Johnston had been hitting it. He must have been giving it a frightful whack on occasions! It took three days of continuous running before the fault finally appeared. And when it did, it didn’t take too much of a tap to make it work again for another three days. After about two weeks, I finally deduced that the problem was somewhere in the power supply although I still didn’t know the exact nature of the fault. The standby light could always be made to come on with the remote control, which meant that the +5V and +12V rails were fine from the Power Sub unit. And during one of the short periods while the set was not working, I measured approximately +325V on the collector of Q311 but found that there was no output at all on the five secondary power rails. Because it took so little vibration to make the set work again, all measurements had to be made with extreme delicacy. Eventually, I had precious multimeters and a CRO permanently hooked up all over the set, waiting for it to play up. This was a real nuis­ ance as it left me with only a limited amount of test equipment for fixing other things. My next step was to establish that the optocoupler (D311/TLP632) was working correctly. I found that there was +1V applied to pin 1 (on the LED side) when the set was off and 0V when the set was on. And there was 0V across the transistor on the secondary (hot side) of the optocoupler when the set was off and -20V across pins 4 & 5 when it was on. This implied that the optocoupler was working correctly. What’s more, leaving the meter connected between pins 4 & 5 of this device seemed to “correct” the fault because the set always started reliably while ever it remained connected. Obviously, the extra drain of the meter was having an effect on some part of the circuit. My two weeks were nearly up and something had to be done. I placed the set on its side for better access and shone a bright lamp onto the PC board. Freezing the various plug and socket contacts didn’t seem to make any difference and they all looked perfect. The components that did have a reaction to the freezer were the optocoupler (D311), chopper transistor Q331 and electro­lytic capacitors C327, C328 and C330. I changed them all and reworked the soldering on the entire board but, after three days, it played up again. Next, I replaced A301 (a JUO168 thick film IC which func­tioned as an error amplifier), along with resistors R306 and R307, the latter used to bias Q311. It made no difference and I was now only left with a few components that hadn’t been changed. I was fairly satisfied that it wasn’t a dry joint or a hairline fracture in the board and I was also fairly sure that the secondary (cold side) of the chopper transformer (T301) was OK. Of August 1997  61 Serviceman’s Log – continued course, I hadn’t checked the chopper transformer itself, mainly because I had regarded it as an unlikely culprit. However, as I was now running out of ideas, I removed it and carefully examined it, especially where the leads are wrapped around the posts. I could find nothing untoward, so I resoldered all the wire wraps and replaced the transformer. And that appears to have fixed the problem. Despite a further week of intensive testing, there was no sign of the fault and I can only conclude that one of the solder joints associated with the transformer must have been at fault, this despite the fact that they all appeared to be OK. The only other possible culprits are D332, D333 and C335 but none of these ever responded to tapping or freezing. I admit that it’s rather an unsatisfactory conclusion but I believe that the problem has either been permanently fixed or, at least, postponed for quite a while. Why can’t they make TV sets easier to service? Of course, the Johnstons will never 62  Silicon Chip appreciate how much effort I put in to overcome this obscure problem. But as Sanyo put it, that’s life! The crook VCR Mr Nasty brought his Samsung VCR in just after Christmas, complaining that it wouldn’t play. This was a VB-306 Winner mid-drive unit and when I removed the covers and inserted a cassette, it was pretty obvious where the fault lay. The loading arms wouldn’t move at all and it could only fast forward or rewind. Unfortunately, there is no access underneath the deck and the only way to get to the mechanism is to remove it, which is what I proceeded to do. This involves removing countless screws from the top and bottom, along with the front escutcheon, before the deck can be unplugged from the main PC board. Removing the loading motor assembly on the underside of the deck reveals a large rack gear which engages a master cam. And you didn’t need to be a genius to figure out what was wrong. The first tooth of the rack was missing and the teeth on the cam gear were all damaged. Obviously some force had been applied here to cause this. The gears were relatively cheap but, after adding in the freight and my labour to remove and replace them, the custom­er was looking at a bill of about $100. I may have thought that this was good value but not so Mr Nasty. Instead, he flatly contradicted me when I called him with the news and almost implied that I was being dishonest and at­tempting to overcharge him. I left the machine disassembled so that he could see the problem for himself when he called to pick it up. He was even more displeased with this and I received neither thanks for my diagnosis nor any payment for the time I had spent on the ma­chine. I was quite surprised at his rather disagreeable attitude and thought that that would be the end of it. I had more or less forgotten about the incident when suddenly, after about six months, his wife brought the machine in and asked for it to be repaired for the figure I had quoted. (Don’t you just love some of these guys? They back themselves into a corner by being obnox­ious and then hide behind their wives after they’ve thought better of it). Taken aback by this sudden about face, I carefully examined the parts to see if anyone else had had a go at the machine after it had left my shop. However, everything appeared to be as I’d left it and so I reluctantly agreed to take the job on. I don’t like dealing with customers who have been unreasonable in the past but I reasoned that it would be a straightforward job and I would be able to recoup my previous losses. Removing the old rack (or “slide main” as they call it) isn’t difficult but, when installing the new one, one has to align eight points simultaneously to ensure the correct timing. The two difficult ones are underneath near where it engages the gear cam drive (not shown in the service manual). The next point to watch for is the “Gear E/J Eject” drive which is loose and must be aligned so that slot #1 matches tooth #1 on the ejector rack at the top of the deck and tooth #1 on the gear master cam underneath. After that, it’s plain sailing and you simply reas­semble the parts in the reverse order that they were removed. Anyway, it all worked perfectly once it was all back together again. Obviously, the original gears had been damaged by someone forcing a tape in or out, though Mrs Nasty subsequently denied this when she called to pick up the unit. Instead, she was more interested in finding out what sort of guarantee I gave. I told her that I guaranteed the parts supplied and the work done for 90 days but only for the same fault and provided that the equipment was not abused, as was so obvious in this case. I don’t think that this advice sank in (or, more likely, she chose to ignore it) because she brought the machine back two weeks later, complaining that it didn’t work again. I stopped work, connected it up in front of her, inserted a tape and pressed play. There was sound but no picture; just snow. I ran a tape cleaner but it made no difference, so I removed the covers and gently wiped the heads using a lint-free cloth dipped in oil-free acetone. It left black marks on the cloth! I replayed the tape and the picture was now perfect. I reassembled it in front of Mrs Nasty and explained what had happened but, like her husband before her, she declined to pay for the work done to rectify their abuse of the machine and disappeared with it without so much as a word of thanks. I don’t need customers like that and I certainly won’t be doing any more work for them. A pig in a poke Steve runs a secondhand furniture shop not far away and one day he brought in this large Samsung stereo TV he’d bought at auction for $300. Of course, it wasn’t working and he wanted me to fix it for him. I know him well enough to tell him that he was mad to buy such a pig in a poke and gave him a quick rundown on some of the costs involved if certain parts like the picture tube were cactus (eg, anything up to $1000 for a large screen – and this was large). He didn’t turn a hair, being the eternal opti­mist he is, and agreed to pay to have it diagnosed and costed. When I removed the back and found a large plastic bag full of parts, I liked his chances even less. As I quickly discovered, this bag contained a number of parts that were missing from the deflection board. The flyback transformer had also been unsol­dered and was lying free inside the cabinet. Where was I to start? I removed the parts from the bag, sorted them out and found out where they had come from before performing some basic resistive checks with a multimeter. Most of the parts were either completely short or open circuit and had either come from the line output stage or the power supply. The line output transistor (2SC1880) was short circuit, as was chop­ per transistor Q801 (BUV48). Because the flyback transformer had been removed, I ini­tially suspected that it was also faulty. However, a few basic checks revealed that it was probably OK, so I replaced it. I also replaced the chopper transistor (Q801), the line output transistor (Q401) and any other parts that were faulty. Restoring the power supply was obviously the next objec­tive. A few checks revealed that fuse F801 and resistors R807 and R817 were all open circuit. The latter are designated on the circuit as 0.27W types but had been replaced with 0.22W 5W wire­ wound resistors. I quickly fixed that by fitting the correct fusible types. The power supply is a conventional switchmode type and is somewhat similar to the ones used in Akai and Nokia TVs. The difference is that it has two regulator circuits, with VR801 controlling the primary oscillator for +130V in the standby condition and VR802 controlling a secondary oscillator and feed­back circuit which ensures that the 130V rail remains constant in the power on condition. Both ICs in the power supply (IC801 and IC802) had pre­viously been replaced, along with a few other components. To be on the safe side, I replaced all the small capacitors (C817, C813, C814, C838 & C803) and then checked all the remaining diodes and transistors using a multimeter. I then removed both the deflection board and the small signal board from the August 1997  63 set to protect them from any further damage should something be badly amiss with the power supply voltages. Before applying power, I decided to take a few more precautions. First, I connected a dummy load consisting of a 100W 240V globe and a parallel voltmeter to the cathode of D814 (the +130V rail). Second, I shorted the base and emitter leads of the line output transistor to prevent the line output stage from firing up. And third, I connected the AC input via a Variac, with a 200W globe in series to limit the maximum current to a safe value. Now for the big test. I applied power and slowly advanced the Variac, all the time keeping my finger near the on/off switch. And at 130V AC, the oscillator fired up, the 100W dummy load began to glow and the meter on D814 read +130V. Delighted at this progress, I slowly wound the Variac up to the full 240V. Everything remained intact and so I switched off and removed the base-emitter short from the line output transis­tor after first confirming that there was 130V on its collector. Now for the acid test – would the line output stage fire up properly? I switched out the 200W globe and 64  Silicon Chip wound the Variac up again but nothing happened. The CRO revealed nothing on either the base or the collector of Q402 (the horizontal driver transistor), so I traced the circuit back from the collector through T401, R402 and D406, towards the +14V rail. According to the circuit I had, D406’s anode is connected directly to the +14V rail at the anode of D816. There is even a link position on the board, marked J109, to allow for this con­nection but there was no link in position. And, what’s more, this link had apparently never been fitted, so what was going on? In fact, it appears that the circuit is in error. In prac­tice, D406’s anode is linked via J430 and J813 to the emitter of Q805. And this transistor is controlled by Q803 which, in turn, is controlled by Q804. When Q804 turns on, Q803 also turns on and this does two things: first, it switches the +14V rail through to IC802 and second, it turns on Q805 which switches the +14V rail through to D406 and subsequently to the collector of Q402. So why wasn’t the +14V rail being switched through by tran­sistors Q803 and Q805. Answer – because unplugging the small-signal board had removed the base drive to Q804. This drive signal is normally supplied from the main board via pin 2 of connector CNP801. As a quick test, I switched my multimeter on the x1 ohms range and connected the red lead to the chassis and the black lead to the anode of D835 (in series with Q804’s base). The 2.4V across the test leads from the multimeter’s internal battery was more that enough to bias Q804 fully on and the EHT section burst into life. The meter on the 130V rail now showed that it was too high but readjusting VR802 soon corrected this. By now, I was optimis­tic that the set was a “goer” so I removed all the safety gear I had connected, reinstalled all the boards and switched on. Eure­ka! – up came the sound and we had a perfect picture. I rechecked and adjusted the two B+ pots before leaving the set to soak test. It was still going strong after about a week and I felt confident enough to ring Steve and tell him to collect it. So, in the end, Steve’s confidence was well founded and he had certainly got himself a bargain – this time! But I don’t generally advise people to acquire TV sets in this manner. SC