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SERVICEMAN'S LOG The rich tapestry of servicing What makes a non-technical person fiddle with his VCR when something goes wrong? And why do young children like “post­ing” money into the cassette wells of VCRs? It’s all part of the rich tapestry of servicing. The tall, distinguished looking gentleman who wandered into the shop clutching his prized video to his bosom didn’t look the type to have a go – he looked more like a lawyer than a service­man. Anyway, I wasn’t presumptuous enough to enquire about his profession; instead, I politely asked him for his particulars and asked what was wrong. He explained that the tape would go in and down and wrap around the drum motor but it wouldn’t play, fast forward or rewind. He freely admitted that he had had the covers off and so I decided to carry out a few preliminary checks while he was there. I connected the machine – a Teac MV505 –to the power and pushed in a tape to confirm what he had said. His description of the problem was spot on but that wasn’t all. I also found that the tape wouldn’t eject because the cas­sette flap wouldn’t open, which meant that he had also removed the front escutcheon and not replaced it properly. This he sheep­ishly admitted was the case. After he left, I removed the covers and the front panel and re-engaged the door flap lifter so that the tape would now eject properly. Anyway, that was only a minor detail; I now had to track down the main fault. Preliminary checks The deck, surprisingly, was a Mitsubishi Fo swift mechanism and I could see that the tape was not lacing up fully. The drum motor was spinning but there was no sign of life from the capstan motor. My preliminary diagnosis was that something was wrong with the loading mechanism. But what? Was it jamming? Were the gears out of alignment? Was it the timing? Or was it a faulty mode select switch? I began by inserting a tape and when the loading motor stopped with the tape 3/4 laced up I continued to rotate it by hand, consciously feeling for any resistance. I couldn’t feel any and I so I continued to turn the motor until the arms were almost completely laced, at which point it would unload itself. Because the loading motor turned a squirrel gear, I con­cluded that this test may be misleading. Because of the gear ratio, I would not necessarily feel any resistance at my fingertips. My next step was to check if there was anything preventing the arms from completing their travel to the end stops. They seemed quite free and loose and so I concluded that either a 52  Silicon Chip gear had jumped a tooth in the loading gear chain or the mode select switch was at fault. Unfortunately, as I discovered when I removed the bottom cover, it’s not easy to check the gear alignment as there is a printed circuit board covering the master cam, along with several sliding plates. However, the mode select switch is easy to access and so I decided to check that first. This switch is soldered to the PC board via five connections and there is an alignment point which must marry up in the eject position. I replaced the switch but there was no improvement in the loading sequence. Regretfully, it looked like major surgery was required and I was extremely grateful that I had a full set of instructions for this deck, even though these were for a Mitsu­bishi VCR. It is hard to summarise the next hour of invective and bad language. The instructions make it all sound so easy and I sup­pose it is if you work on this deck all day every day. If you don’t, then it’s not quite so straightforward. Anyway, I removed the reel belt, capstan brake spring, cam plate B, three gears, the loading gear arm and five screws, before desoldering the leads to the full erase head. At this point, the deck PC board is ready to be prised off – at least in theory. However, on this model, the lower moulding that supports the deck is somewhat generous and the PC board won’t come out unless the whole deck, including the ejector, is removed. A closer inspection revealed that it would be necessary to remove about 6mm from either side of the moulding to free the PC board. As a result, I decided in the interests of time that an Australian modification was required and so I used a soldering iron to melt away the plastic so that the board could be removed (this didn’t alter the strength of the structure in any way). Finally, I had access to the main cams (1 and 2) and, after wiping away the excessive grey grease, I could check the align­ment hole. Would you believe that all was correct? The shafts and levers were all in the right places. I removed the cams and carefully examined them on both sides for broken or bent galler­ies but all were perfect. Even their teeth were straight. Worse, naturally, was to come. Any damn fool can take things to pieces – it’s getting them back together properly that sorts us out. Inevitably, I fell for all the traps, in particular the record safety lever which should be held back whilst insert­ing the board, not to mention pin “e” getting in the wrong track of the cam slide plate B. On the third attempt, it all finally came together and we were back to square one with the original fault. It was now that I had a little bit of well-deserved luck. Whilst cogitating menacingly over this vile mechanism, I noticed that it had been fitted with a new green pinch roller. Now the original pinch roller arm mechanism was made of white plastic and it is common for one of the arms that guides it down the squirrel gear cam to break. This is replaced by the green type which you can either purchase as a single part or as part of what is called “Abrasion Part Kit for Fo DECK (Rubber)”, whatever that means (the part number is 789­C007020). This kit comprises the arm, the reel belt, the circlip, a sachet of grease and a comprehensive instruction booklet. However, if a serviceman doesn’t March 1997  53 Serviceman’s Log – continued know about this kit (it isn’t mentioned in any service manual) and only fits the new pinch roller, he usually also neglects to clean and lubricate the shaft it slides up and down on. And that’s precisely what had happened in this case. In operation, the pinch roller started to slide down the shaft but it was too slow because of the friction and it was jamming the roller against the top of the capstan shaft housing in a way that wasn’t obvious to the eye. Cleaning and lubricating it with grease fixed the problem completely. Anyway, when our lawyer (?) friend arrived to pick it up, I asked him about it and he confirmed that the pinch roller had indeed been replaced just over a year previously. What a palaver over what, in hindsight, should have been a straightforward simple repair. Christmas treat After a long cup of coffee, I tackled the next job, praying it would be easy. Oh the joys of Christmas – the presents, the new VCR for Dad, the odd bit of cash for Johnny the 5-year old. And oh what a disaster this combination can make! 54  Silicon Chip Mr Grey brought in his Akai VSG220EA, with a tale of woe that his youngest son had “posted” some toy or other into it. Of course, it no longer worked and when I shook it I could clearly hear something rattling inside. The coffee had definitely improved my mood and I chose Mr Grey’s still shiny VCR – just barely out of the egg (I think) – to look at next. Removing the cover gave good access to the mid-decked VCR and I quickly found two coins – a 10-cent piece and a 5-cent piece – sitting on the PC circuit board just under the deck. I retrieved the two coins by carefully jiggling the deck upside down in the air, then carefully checked for more and for any signs of damage before powering it up. When I switched it on, the drum motor came to life briefly but no other signs of life were present – not even from the display. I pushed various buttons and nothing happened but when I pushed a prerecorded tape in and pressed play, the tape loaded normally and a picture appeared on the TV with full sound. I pushed all the buttons in turn and it paraded its full box of tricks. In fact everything was working except the display. Unfortunately, I don’t have the service manual for this particu­lar model, which meant that I would have to tackle it blind. Fortunately, the deck isn’t too difficult to remove. It’s simply a matter of removing five screws and three plugs, removing the front escutcheon/control panel, and then lifting the deck out vertically. This gives access to the PC board which is held in via two screws and half a dozen clips. Unfortunately, the clips make it rather awkward to remove the PC board assembly but eventually I was able to free it and lift it out from the rear. This done, I gave it a careful visual inspection but nothing obvious was shouting back at me so I applied power to the board and began checking voltages around the circuit. Because there was no display, I reasoned that the supply rail to it had probably gone missing. Either that or the display itself, or possibly the microprocessor that drives it, had been damaged. My initial checks revealed that a voltage was present bet­ween the two ends of the display where the filaments are connect­ed. This is typically either 5V DC or 5V AC. Having found this, I checked various other points around the display, looking for a -28V (approx.) rail, but there was none. I didn’t have a circuit diagram which was a bit of a hindrance but it all screamed of a failed -30V rail from the switchmode power supply. All I had to do was identify it. There are about 16 diodes in the secondary of the power supply, most of which are protected by low-value resistors. Unfortunately, no voltages were marked on this part circuit but it didn’t take a mental giant to figure out which diodes were in the negative rail, as their anodes connect to the negative side of an electrolytic capacitor. Anyway, I checked each of these in turn and eventually found that D209 was open circuit. I replaced it, plugged the machine into the wall socket and was immediately rewarded by a flashing “AKAI” sign in the middle of the display. Switching on the power at the machine then brought up the word “ERROR”, which is normal at this stage. Getting it all back together again was surprisingly simple, with the PC board literally falling into its supports. The deck accurately followed suit and I powered it up with a tape in place. This time, the display worked correctly and I gave it a thorough soak test before calling the customer. The house call After lunch, I was asked by a little old lady to do a house call on her aging Sony KV2764EC which had sound but no picture. This set is now about 10 years old and not getting any younger. In fact, assuming average use, this is about the “use by” date of a TV receiver. I don’t like doing service calls on these sets as access is to the main circuit board is quite poor. However, she couldn’t possibly bring the set to me, so I had to go to the set! When I got there, I found that the set was on a low table near a window and so the lighting was good. I switched the set on and the symptoms were as described – sound but no picture. This set used a PE3 chassis rather than the Rx chassis. The tube filament was alight and you could hear the familiar rustle from the EHT at switch on. As a first step, I measured the screen voltages on pins 3 and 4 (G1 & G2) of the tube. These were both around 500V which is what I would expect them to be. However, the cathodes were too high at nearly 200V. I then switched the set off and it momentar­ily flashed a white line. Ah, ah, I thought – a vertical deflec­tion failure. When I finally managed to remove the motherboard, I could see what looked like a number of dry joints and even though I worked them over, I knew that the problem just had to be IC552 (TDA3652). However, this IC is no longer available and is now replaced with a TDA3654. At the same time, you also have to replace R518 (6.8kΩ) with a 1.5kΩ resistor. As this is a well-known problem, I had the parts on boards that I carried with me and installing these quickly restored the picture. However, on departure, I advised her to start saving for a new TV. Portable players Back at the ranch, a couple of Pye ND-20 portable CD cas­ sette stereo radios had come looking very much the worse for wear. My instructions from the owner’s financier, namely the father of two teenagers, was to make one good unit out of the two. Much as I hate working on these cheap units, I reluctantly agreed to have a go. Gaining access to the circuitry of such units if often a problem but in this case, the two halves of the cabinet shell can be split after removing only eight screws. One of the sets was completely dead and I decided to work on this first, as I hoped that it would be a simple power supply prob­lem. In the event, the power was OK and I could trace the problem to the cassette deck and function switch SW2. Unfortunately, I didn’t have a circuit and so I couldn’t quite work out the se­quence of events from there on in. The function switch was fairly complex and as power was going in but not coming out, this had to be the logical suspect. To confirm this, I wiggled and twisted the switch and sprayed contact cleaner inside it until finally I managed to get some sound out of the speakers. That was enough to confirm my theory and I placed an order for a new switch right away. What about the other machine? Well, it had a smashed con­trol panel and for a while I contemplated removing its function switch for use on the other machine. However, it wasn’t worth the time that would be spent removing and refitting it, particularly without knowing its condition. Of course, the other option was to repair the smashed unit with parts from the other machine but the hole was too big and the damage too severe. Tarzan’s TV My next customer was a young man who arrived in a small 3-door hatchback. To my astonishment, he removed a 63cm stereo TV from the back seat and effortlessly carried it into the shop as though it was an empty cabinet. He plonked it down on our small counter and cheerfully informed me that there was no picture. The set was a Philips 2B-S chassis KR5987R 25CT8883/75, circa 1988. This was a fairly popular model and is one that I am quite familiar with. March 1997  55 Fig.1: the video control chip in the Philips 2B-S chassis. As the set ages, it is sometimes necessary to add a 33kΩ resistor between the +13a (12V) line (pin 6 of IC7300) and pin 26 (RGB output stage current sensor). Not bothering to even catch his breath, “Tarzan” continued to elaborate on the set’s problems. Apparent­ ly, the picture had been a little “unclear” and intermittently had taken longer and longer to come on. Initially, I was rather reluctant to take the set on, as I didn’t fancy the prospect of it taking up so much bench space for a week or two while I chased an intermittent fault. I mumbled that it could take quite some time and suggested that, in view of the set’s age, he might prefer to spend his money on a new set. His response was that he wanted the set fixed and that I could take as long as I liked. It was the wrong response but still, you couldn’t help liking him for his cheerful manner. In then end, I relented and promised to start on it straight away. All he had to do was move it to my workbench and promise to pick it up as soon as it was ready. No problem – Tarzan made the set’s removal look as though a genie had instantly answered my wish. If only he could have fixed it too! When I switched it on, there was no picture but both sound and EHT were apparent and the CRT filaments lit up. Now, this chassis will not give a picture until the beam current has 56  Silicon Chip reached a certain level. This feature is achieved using a video control chip (TDA4580/V2 – IC 7300), which also deals with brightness, contrast, saturation, beam cutoff stabilisation and beam limiting. However, a problem arises as the tube ages, in that it takes longer and longer for the picture to come on. Often, there is only a white line at the top of the screen but this can usual­ly be fixed by adding a 33kΩ resistor between the +13a (12V) line (pin 6 of IC7300) and pin 26 (RGB output stage current sensor). Unfortunately, this same symptom (ie, the white line at the top of the screen) can also be produced by a variety of other faults. And, in fact, it showed up within a minute or two from switch on. Access to the PC board in this set is not good and I find the best approach is to turn the cabinet on its side, or even upside down, to get to it. The first thing to tackle when you do get access is to remake any suspicious-looking joints, particu­larly around the transformers and ICs. In this case, it wasn’t too bad and my efforts made no difference to the problem. Next, I replaced C2571, a 100µF electrolytic capacitor in the vertical output stage. This is something that I always do as a matter of course with these sets, as past experience has shown that this capacitor can give problems. Again, it made no dif­ ference. Finally, I stopped working as an automaton, put in my re­ m aining braincell, and started measuring voltages and checking waveforms. First, I checked the 1.2V nicad battery which was OK (this battery backs up the memory for the microprocessor). This done, I checked gating pulse waveforms 43 and 44 to the chroma decoder (pin 8M10), to pin 9 of IC7550 (TDA3870/V2), to pin 10 of IC7300 (TDA4580/V2) and to pin 7 of IC7570 (TDA3654Q). I was drawing blanks everywhere, so I decided to go back to first principles and examine the CRT voltages. And as luck would have it, I found the cause almost immediately. As soon as my 100kΩ/V analog multimeter touched pin 7 of the CRT socket, the picture came on and stayed on. There was just one problem – it was out of focus. The voltage on pin 7 of the CRT is marked as 650V and is derived from the flyback transformer 7kV connection via the focus and screen control pots. And, fairly obviously, the very small current through the meter was necessary to make the focus control function. Replacing the focus control pot (33MΩ) fixed the problem. Unfortunately, the picture still wasn’t the best, even after adjusting the focus, and I suspect that the emission was down. However, it wasn’t down far enough to justify the modification mentioned earlier. When I stripped down the faulty pot, I found that the printed circuit on the ceramic base had corroded. Apparently, the extra leakage of the meter was enough for the voltage to arc across the corroded section. I might add that, on some sets, I have also had problems with the screen control (4.7MΩ), especial­ ly on the KT3 chassis, and with C2471, a 68nF capacitor which connects from the wiper of the screen control to the +200a vol­tage rail. Tarzan, true to his word, turned up not long after I made the call, tucked the set under one arm (well, not quite), and plonked it in the back of his tiny car. Fortunately, he was as happy as SC Larry.