Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG Life’s tough without TimTams I must be getting old because a couple of jobs really had me on the go this month. Fortunately, persistence won the day and I had a really good win over a recalcitrant VCR. If only I’d stocked up on TimTams . . . Panasonic are up to sneaky things – they seem to think that people cannot remember anything that’s older than 10 years and so now they are recycling their television model numbers (of course, in those days they were called National so it doesn’t really matter). This is extremely confusing for old codgers like myself who can still actually remember the original model. In this particu­lar case, it was a model TC1401. The original cost over $550 and was an extremely heavy, 14-inch, portable TV set with a white cabinet and rotary tuning (VHF only). By contrast, the beast on my bench, also a model TC1401, was a dual-speaker 34cm set with remote control and a black cabinet. It weighs just 12kg and is so light that the owner has to be careful they don’t lose their grip if they are carrying it in a heavy wind! The complaint with this one was “no memory” – I’m glad I am not the only thing that suffers from this fault from time to time! When the set was switched on, the only channel available was 1 and on opening the “Preset” menu all the channels were designated as “skipped”. They could be retuned in any con­figura­tion you wanted, with the correct TV channel numbers, but when the Preset button was pushed again all the information was lost. So the fault description at least was accurate and seeing as there is an IC (IC1104, MN12C25D) marked “MEM­ ORY”, the obvious thing to do was to replace it. I ordered in the chip, fitted it and switched on. Initially, this seemed to have fixed the problem because the set worked but then, after about five minutes or so, it failed again. Curses – it wasn’t going to be that easy. Next, I checked all the voltage rails, especially the 5V and 30V rails, but they were all OK. I even checked them for ripple using an oscilloscope but there was none. I also checked all the other voltage rails before going on to measure the voltages on all the pins of IC1104 and IC1102. There was nothing was untoward – not even a dry joint. By now thoroughly frustrated, I reordered another MN12C25D IC plus an MN151142TEA (IC1102), if only to make sure that it really wasn’t one of these chips. When they arrived, I couldn’t wait to fit them one at a time. IC1102 was a bit tricky as it is a 42-pin high-density chip but eventually they were fitted prop­erly and this time the symptoms were . . . exactly the same as before! This was definitely not the result I was looking for! What next? By now, I was totally perplexed by all this and was contem­ plating abandoning the repair. Maybe a cup of coffee would get the ol’ brain cells working again? Well, maybe it did Sets Covered This Month • • • • • Panasonic TC1401 TV set Teac CTM-143 TV set NEC FS-6325 TV set AWA CT-1447AM TV set Mitsubishi HS-338A VCR help be­cause I was closely examining the main board (rather hatefully) when I noticed that the small glass diodes fitted to it were completely different to the diodes fitted in modern TVs (like 1N4148). And as I quickly discovered from the circuit, that was because they were completely different. Surprisingly, they were germanium OA90s, a diode rarely seen these days except in the odd discriminator. Yet here was a fairly modern set using lots of them and – hey-ho – they were predominantly in and around the memory circuit I was working on. Well, this was all academic and of purely historical inter­est except, of course, one of the main reasons for the shift from germanium to silicon was reliability. Silicon diodes are much more reliable than their germanium counterparts and have better (read “less”) leakage. The only drawback with silicon is that it requires 0.6V to bias the junction, as opposed to 0.2V for ger­manium diodes. Well, I didn’t have any better ideas, so I began testing each diode in circuit on the x10 ohm range of my multimeter. The reverse leakage varied a lot between them but at least there was a difference between the forward and reverse directions until I got to D1129. This diode connects pin 5 of IC1104 to pin 20 of IC1102, the microprocessor. D1129 measured nearly open circuit in both directions and it just had to be the culprit. I rummaged around an old miscellaneous diode box and found an OA90 and fitted it. And that solved the problem – everything was now properly stored and remained that way even after the set was switched off. Without the benefit of a block diagram, I cannot tell the precise function of D1129 except to say it con­nects C2 with P32-C2 (I’m sure you are all the wiser for that bit of priceless information). I have to admit this repair was pretty fluky and I’m now off to buy MAY 1999  29 a lottery ticket. Maybe I’ll crack the jackpot? Unhappy customer Mr Burton wasn’t too happy about his Teac CTM-143 TV set. It still had the same fault as when I’d fixed it last time, or so he claimed. Well, he may have genuinely believed this but it was 1996 when I fixed it last and I don’t give 3-year warranties. And as it turned out, it wasn’t the same fault as last time. On this occasion, the set was intermittently not coming on and it appeared again to be a problem with the line drive stage in the 34cm Goldstar PC04A chassis. The previous fault allowed the set to start but it would then “go off” after a short period of time. This was caused by D402 in the 27V rail being open circuit. This allowed the driver stage power to start via D401 (18V) but because D402 was open circuit, the stage would then shut down. This time, the voltage on the collector of Q401 (KTC2230A) was 20V. Sometimes there was a kind of square wave on this col­lector, while at other 30  Silicon Chip times the waveform collapsed into a re­duced waveform with large negative spikes, which in turn produced a waveform on the secondary of T401. This was insufficient to turn on Q402 (KDS1555), the line output transistor. This was baf­fling because the square waveform on the base of Q401 seemed adequate to turn the stage on and, of course, it would have to be intermittent, just to complicate matters. To eliminate any traces of the problem I had addressed last time, I connected an external variable power supply to the junc­tion of C404 and T401 and pumped in 18-28V. It made no dif­ ference, thus eliminating the power supplies. The square waveform WF2 was correct at all times. I replaced Q401 and Q402 and the fault went away for one week but it was back again just after I had confidently given Mr Burton a quote for the fault. Next, I checked all the components in the collector circuit of Q401 and in the base circuit of Q402, to no avail. So what was I overlooking? Basically Q401 is biased and switched on by the square wave arriving via C401 (which had also tested OK) but the waveform became distorted on its collector. Why? It’s always the way; the simpler the circuit, the harder it is to find what’s wrong. The vital clue came when I monitored the waveforms with an oscilloscope. This showed that the amplitude of the waveform was much greater before R402 than after it. Certainly, the difference was much greater than I expected, considering that R402 is nomi­nally only 560Ω. When I removed R402 from the board and measured it, I found that it’s value was actually 750Ω, an increase of almost 50%. Replacing this resistor increased the waveform amplitude at the base of Q401 and the set now remained on. I soak tested it for a week and crossed my fingers when Mr Burton collected it. No TimTams It was a hot day and I was praying that Mrs Norris’ NEC FS-6325 TV set was going to be straightforward. I was running late because the previous job had taken far too long, due mainly to the client’s addiction to talking – she could talk the hind leg off a donkey! Serviceman’s Log – continued Anyway, if this next job was easy, I could still make it back to the workshop in time for a leisurely cup of coffee and a couple of “TimTams” before knocking off for the day. When I arrived, I quickly unscrewed the back and it was easy to see why the set was dead – F601, a 2A mains fuse, was as black as the ace of spades. I unplugged the degaussing coils and measured the resistance across the bridge rectifier – it was still nearly a complete short circuit. By following the path from the bridge rectifier, I soon established that IC601 (STR­ 41090) was short circuit and hopefully the cause was due to the obvious dry joints on C609, the main tuning capacitor. As luck would have it, I had a new STR41090 in the van. I quickly ran out, found it, shot back into the house and replaced it before you could say “Micky Finn”. I then switched the set on and prayed hard but nothing happened. It looked as though I was snookered. Suitably chastened, I turned the set off and measured the main HT voltage across C609. It was still 340V which told me that the circuit was stable DC wise but wasn’t starting up. I desperately looked around and saw a 1MΩ resistor (R607). Hoping that this was the critical startup bias resistor, I replaced it and switched on again. Much to my frustration, there was still no response – my TimTams were melting away as in a mirage. That was when I spotted that R610 had a little chip missing from its body. There was enough of it left to determine that it was once a 1Ω resistor. I measured Q601 (sandwiched between R610 and R607) to find that it was short circuit as well. I tore back out to the van, rummaged through the mess in the back, found the parts I needed (miracles do happen) and rushed back inside and fitted them. This time, the set fired up and the picture and sound 32  Silicon Chip were good. Thank you God, thank you. I quickly scribbled out the bill, put the back on the set, hopped in the van and shot back to the workshop. I went straight for the percolator the minute I got back. Great; there was still some coffee left but what – NO MORE TIM TAMS. Life lost all its meaning! Predictable trouble Mike Tester’s (the name is changed to protect the guilty) AWA CT-1447AM was always going to be trouble. You see, Mike always fancied himself as a technician and he also lived near the sea. This combination meant his set was always breaking down and he was the kind of guy who liked to have a go. Well, this time there was no picture but the sound was good and there was a raster with the on-screen display working OK. Anyway, it looked as though he had lost only the video between the IF detector and chrominance/luminance decoder IC. As he is a good friend of mine – despite his foibles – I tried to help him over the phone but I really didn’t have a clue as to the exact cause of the problem. Initially, I told him to check all the voltage rails, especially those feeding the signal circuits. This he dutifully did but everything measured fine, so I told him that the only course of action was to feed in a signal from a colour bar generator and trace it through with an oscilloscope. After a bit of coercion, he finally agreed and dropped the set off at my workshop. When I removed the back, I could see how rusty the whole set was from the salt air. I started by confirming everything he did by checking all the voltage rails. These all proved OK, so I hooked up the oscilloscope and followed the video from the video detector (pin 10 of IC101, MS51496P) through to Q1A0 TP12 (wave­form 1), thence to LC201 DL/BPF and finally to pin 18 of IC201 (M51412SP). After that the scent became very cold. I then spent an inordinate amount of time examining the contrast control circuits but got nowhere. By now I was beginning to think that the fault was somewhere in the beam limiting cir­cuit. I started at pin 8 of the flyback transformer and traced the circuit until I got to the two beam limiting test points designated PT1 and PT2. It was then that I noticed that R555 and R556 were badly corroded. I de­ soldered them and measured them to find that they were both nearly open circuit. Replacing them fixed the problem completely but I had to warn Mike to keep the set dry, otherwise it wouldn’t last very long. Unfortunately, he didn’t listen too well and within another three months the set was worse than before and he was forced to bin it. A Heath Robinson job Mrs Daniels, a widower living in a housing commission flat, was a very keen soap watcher and loved to record her serials every day on her beloved Mitsubishi HS-338A video. She first brought it in complaining of poor fast forward and rewind, which just turned out to be belts and tyres, but a month later it was back. This time, the complaint was “snowy pictures”. At the time, I felt sure it was just dirty heads but after cleaning them vigorously I came to the conclusion that the heads were worn out, especially as (with the same tapes, at least) I was getting almost clear pictures with Pause/Freeze Frame/Still. I removed the heads and checked them on my tester to find that they were indeed low – enough, I thought, to be causing the problem. When faced with the news, Mrs Daniels was very stoic, accepting that as it was in use every day, the heads were bound to wear out eventually. And although she could hardly afford new heads, she would find a way to come up with the money as it really was her main source of entertainment. I ordered in the new heads, fitted them and confidently switched the machine on. To my horror, I found that the problem was just as bad as before, although the picture was still OK when paused. It was obvious that I had misdiagnosed the fault. I got the CRO out and examined the FM envelope at TP-2A to find half of it missing. It was unlikely to be the new heads but it could be the head amplifier IC, the switching pulse or worse still, the toroidal transformer inside the drum itself. Using the second channel of the CRO, I quickly established that the switching pulse (FF or flipflop on pin 2 of IC201 M51473P) was exactly in phase with the FM envelope switching. From there, it didn’t take long to find that the toroidal trans­former primary measured 100kΩ between pins 1 and 2 of plug SB. I removed the entire drum assembly, then removed the drum motor and upper transformer to reveal the lower primary coils glued to the bottom with – yes, you’ve guessed it – the notorious brown glue. There was nothing that could be done to fix it as the coils were only accessible on the underside and the transformer was glued too tightly to the base. Unfortunately, the trade cost of a complete drum assembly was a prohibitive $416.18 (if indeed it was available), so I tried to obtain a junked machine from one of my colleagues in the trade. Two heads or three When I enquired, one young technician asked me whether it was the 3-head version (which it is) or the earlier 2-head HS337A. At first, I didn’t quite realise the significance of his question but he went on to suggest that I substitute the pause head and Fig.1: this diagram shows how the connections to the transformer windings inside the drum were modified. Fig.1(a) is the original circuit, while Fig.1(b) is the modified circuit. Fig.1(b) also shows how the leads to the heads were modified on the top of the drum. Fig.2: here’s how the connections to the head terminals on the top of the drum were modified. The pins were desoldered from the PC board at all points marked A and B and the two pins at A then connected to pins C using short lengths of insulated wire. This photograph shows the modified drum assembly. Amazingly, it worked and produced quite a good picture. its winding for the open circuit winding. At first I thought that this was an absurd idea, knowing the tolerances these heads are made to, but having had no success in obtaining a second­ hand drum assembly, I decided to at least give it a try. First, I completely reassembled the drum, refitting the old heads in the process. I then fitted two jumpers across the toroidal transformer primary windings, connecting the pause head on wind­ing (L1) in parallel with the open circuit winding (R). There was an added complication in that the open circuit winding (R) shared the centre tap with the good winding (L). I now had to guess which head was which on the upper drum, as they are not marked anywhere. The pause head is L1 and I reasoned that this would be mounted close to play head L, which would be diagonally opposite head R. I then unsol­dered head R, fitted two links to the pause head winding (L1) and tried it out. It made no difference, which meant that I probably had the L and R heads mixed up. Next, I assumed that the heads were arranged as shown in Fig.2, with the R head adjacent to the pause head (L1). I then rearranged the leads as shown so that the active heads were L and R but I still didn’t really expect it to work. However, I was thrilled to see that it actually did work and what’s more, the pictures were pretty good. Even more surprisingly, the pause mode wasn’t bad either. I then made a recording and played it back and the picture was still quite acceptable. Frankly, I was amazed that this had worked at all and I am full of praise for my friend. Obviously, it isn’t perfect but Mrs Daniels thought it was acceptable under the circumstances, especially as I didn’t charge her for the new heads and put them SC back into stock. MAY 1999  33