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SERVICEMAN'S LOG Lightning strikes again In the May notes, I told of the havoc caused to a TV set by one of a series of thunderstorms which struck in my area some months ago. There were other casualties from those same thunder­storms as well and this is about one of them. This time, it wasn’t a TV set but rather a portable stereo CD player. And it was another National Panasonic product, a model RX-DT610. The complaint was typical: “it doesn’t go. It wouldn’t go after the blackout when we had that thunder- 40  Silicon Chip storm”. I wasn’t sure which thunderstorm “that” one was but it didn’t really matter; the obvious conclusion was that unit had suffered a strike or power surge, with disastrous results. The RX-DT610 is a very nice looking unit. This one was probably around three years old and would have cost between $300 and $400. It features detachable speakers, twin tape decks, a CD player, a radio tuner covering the broadcast band and the FM band, and a full digital LCD readout. It operates from either mains power or internal batteries. In fact, the unit uses two sets of batteries: a main power supply pack consisting of 10 “D” cells (15V) and a memory backup battery consisting of four “AA” cells (6V). There were no “D” cells in it when it came in and the “AA” memory cells had long since died (they were probably the originals). It looked as though the owner had never used the set on batteries. The set performs very well when it is working. But it wasn’t working now and a quick check with the ohmmeter indi­cated at least one reason; the mains input was open circuit. I was not familiar with this model and, because units like this are so physically cramped, I needed a manual before doing any serious work on it. Fortunately, one was available although it wasn’t cheap. Before ordering it, I decided on a brief visual inspection. This revealed that there was no fuse in the transformer primary circuit although there was one in the secondary circuit. Just why this was so I cannot imagine. The transformer primary is in the most vulnerable position and the transformer is an expensive component. Having confirmed that it was the transformer which was open circuit, the replacement cost was the next thing I had to consid­er. This was over $100 and I hadn’t even checked for whatever other faults there might be. Was I justified in pressing on? Ordinarily, the answer would be not. But it transpired that the damage was covered by the owner’s house and contents in­surance policy, so that wasn’t any real worry. In any case, the owner wanted it fixed regardless. So I ordered a new transformer and a YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM SATELLITE ENTHUSIASTS STARTER KIT YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Fig.1: part of the Panasonic RX-DT610 portable stereo unit. IC305 is at top, IC306 at lower right and D310 at bottom left. IC306 is a multiple supply rail device. Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● manual and put the unit aside until they arrived. When they did, I set to and replaced the transformer. And I say “set to” advisedly. This model – and a lot more like it – is a right proper swine to service. In this case, access is only through the front of the cabinet and, need I spell it out, the power supply board is right at the back, behind all the other boards. And that means that all the other boards have to be pulled out of the way, involving the removal of countless screws and clips. Anyway, the transformer was duly fitted and I replaced the other boards, at least to the point where I could safely switch on. This produced some signs of life but not much. The LCD read­out came on and there was a loud hum from both speakers. And that was it. That meant that I had to pull everything out again and start searching for faults. This wasn’t easy because I had to work on the various boards while they were half hanging out of the cabinet, on leads which were not designed to aid servicing. Nevertheless, I managed to make a series of voltage checks and I found several voltages which were quite wrong. These were mainly around two ICs – IC306 and IC305. IC306 is a rather unu­sual device, best described as a multiple supply rail source. IC305 is, basically, the audio amplifier. But it also appears to perform a supply rail function, delivering 9V at pin 4. Well, it should have been but it wasn’t. Cooked ICs At this point, I could only conclude that at least one of these ICs had taken a wallop when the trans­former was destroyed. To cut the losses in time and effort, I decided to order and replace both ICs without further mucking about. As it transpired, this was a wiser move than I imagined. I later learned from a colleague that failure of one of these ICs can destroy the other, so replacing them one at a time can lead to further destruction. ● ● ● ● ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: __________ Phone: (_______) ________________________ ACN 002 174 478 July 1996  41 Serviceman’s Log – continued So the two replacement ICs were duly obtained and fitted. Now, I hoped, the thing should work. And it did. There was sound in the speak­ers, the tape was working and there was some life in the radio. The latter was not fully operational, however, due to the failure of the memory backup batteries. The radio is pushbutton operated, the wanted channels being selected and stored in the memory. Since this had failed, new cells had to be fitted and the radio reprogrammed. But when this was done it worked perfectly. So we had the two tape decks and the radio working. The only thing I hadn’t checked was the CD player. This was awkward, because the physical location of the board and the length of the leads was such that it was not possible to check it until every­thing was back in place. I reassembled the unit (a somewhat lengthy and tedious procedure), cross­ ed my fingers and pushed a CD in. Nothing happened; the drive spindle simply didn’t work. Which meant, of course, that the whole thing had to come apart again. Troubleshooting guide I thought I’d pull a swifty here. The service manual con­tains a troubleshooting guide – one of those flowchart arrange­ments using a “yes/ no” sequence to direct the user from section to section in the hope that it will eventually pinpoint the faulty one. It started with the assumption of no CD playback and asked: “does the disc rotate?” A “no” response instructs you to remove the disc and open and close switch S790. I wasted a lot of time looking in vain for S790 and finally concluded that they probably meant either S822 or S823 on the leaf switch board (board “D”). This achieved nothing. The next questions on the flowchart were directed to the optical pickup; whether it moved and what was its position. These checks achieved nothing either and I seemed to be getting nowhere. I decided it was time to abandon the scientific and resort to the primitive –well, basics anyway. When in doubt, check the supply rails. My first inclination was to go directly into the CD player and look for supply rails but I soon realised that this was not practical. The lid to the CD player operates two interlock switches (the previously mentioned S822 and S823) and so this section is effectively disabled while the lid is open. Any meas­urements could thus be meaningless. The nearest point to the player itself is the leaf switch board (board “D”) which carries the two interlock switches. And this is fed by two plug/ socket assemblies, W302, sections “B” and “F”. It was section “B” which attracted my attention because it connects to the main circuit board and carries two supply rails – 8V and 5V (pins 3 and 4). The 8V on pin 3 was OK but there was no 5V on pin 4. From there, I moved back to the main board to trace out the path. This was easy enough on the circuit, which indicated that the 5V came from pin 6 of the previously replaced IC306 via fuse ICP5. This, however, is not a conventional clipin glass fuse. Instead, it is more like a small transistor encapsulation and is wired directly into the board. Fuse ICP5 was intact but there was no sign of the 5V right back at pin 6 of the IC. So was the new IC306 faulty? That was too horrible an idea but the only logical alternative was a short circuit on this rail. And there was, a check from pin 6 to chassis confirming this. But where? Somewhere in the CD player seemed to be the most likely so I disconnected this by unplugging board “D”. This made no difference, which meant that the short was on the main board. Was it in the IC? I sucked the solder Especially For Model Railway Enthusiasts Available only from Silicon Chip Price: $7.95 (plus $3 for postage). Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 42  Silicon Chip off pin 6 but the short remained. Well, thankfully, that cleared the IC. I moved along the rail and removed fuse ICP5. Ah! A result – of a kind – at last. The short was on the side of the fuse remote from pin 6 of the IC. So why had the fuse not blown in this situation? That puzzle aside, I still had to find the fault. Fortunately by now, there weren’t many likely places left and I eventually traced it to zener diode D310, from the 5V rail to chassis. And that was the final step. A new zener diode restored the 5V rail and the CD player came to life. So, after a day’s soak test, it went back to a happy customer. But it is not a very satisfactory episode in my long love/hate relationship with fuses. There was no fuse where it would probably done the most good and saved the power transform­er. And the one fuse which was where it could have done some good didn’t work. Same thing again My next story is about a video recorder and the awkward situation which can arise when a service job bounces – when the customer returns the device with the complaint that “it’s doing the same thing again”. It seldom is the same thing of course but some customers take a lot of convincing. This set was an Akai model VS-8, an older machine but one which in its day was in the top range, with lots of features. These included stereo sound (with Dolby), long play, slow motion, insert sound dub, and so on. In short, it was a very nice ma­chine. I had first serviced it about five years ago but had not seen it since until it came in a few weeks ago. And the complaint now was that it was chewing tapes when they were ejected. When I opened it, it was clear that, apart from the specif­ic problem, it needed a fair amount of work. A common problem with these machines is failure of the memory backup battery, which then leaks onto the power supply board and attacks the copper tracks. This had happened to this set but, fortunately, only to a minor degree and I had caught it before any major damage had been done. It also needed a new set of belts and tyres and a set of brake pads. And it was these latter items which were the cause of the complaint. When in the eject mode, the system is supposed to pull the tape tight, against the brake pads, before the cassette is actu­ally ejected. This wasn’t happening properly, or at least not every time. As a result, a small amount of tape was left protrud­ing from the cassette and this was fouling on the way out. So it was a major overhaul job: clean up the battery area and fit new batteries; fit new belts, tyres and brake pads; replace the pinch wheel (which had become hard and shiny; and clean the heads and the tape path generally. I also changed the sensing lamp, something which I do as a matter of course in a major overhaul. After that the machine behaved like new and the new brake pads were obviously doing their job. I returned it to the cus­tomer and thought no more about it. The same thing? Until about three months later, that is. Then the customer was back with the machine, complaining that it was doing the same thing. The first thing to do in such cases is to check the ma­chine, in front of the customer, and see whether it really is doing the same thing. In fact, it wasn’t. The fault now was that the cassette wasn’t being accepted properly and it wouldn’t play. And this was intermittent. The situation was a little dicey at this stage. While it clearly was not the same fault – and I made sure that the customer understood this – I couldn’t rule out the possibili­ty that I might have done something wrong during reassembly. So I said leave it with me and I would check it out. The reason the cassette wasn’t always accepted wasn’t hard to find. It was due to a faulty leaf switch which senses the cassette’s position and activates a sensing light and the indica­tor light on the front panel. In fact, the cassette was in posi­tion; it was just that the system didn’t know this and the indi­cator said it wasn’t. And, of course, it could not be played. Well, that was easily fixed, and I assumed that that would solve July 1996  43 Serviceman’s Log – continued Fig.2: the motor drive circuitry in the Akai VS-8 VCR. IC6 drives the “Rell” (reel) motor (M903), while IC5 above it drives the “Lowding” (loading) motor (M902). which is described as the “Lowding Motor”). Anyway, spelling problems aside, I had to find out why the “Rell Motor” wasn’t working in the fast forward mode, even though it was working in the play and record modes. Unfortunately, access to the mecca drive board is very awkward. It is necessary to remove the top cover, take out the loading cage, and remove the front control panel. And the board is behind the front panel but so mounted that it is almost impos­sible to remove it; any work has to be done with it in situ. Each of the aforesaid motors is driven from its own IC – IC5 for the loading motor and IC6 for the reel motor. Naturally, I was interested in IC6, which is shown as a BA6109. But it wasn’t a BA6109 on the board. This IC had obviously been replaced at some time with a new IC designated BA6209. Unfortunately, I had no idea whether this was a legitimate replacement, or how significant the change was in regard to this problem. More to the point, a voltage check on this IC, in various modes, left little doubt that it was faulty. And as a BA6109 was available, it seemed logical to fit it. Unhappy customer the whole problem. But it didn’t; a routine check showed that there was obviously something else wrong. And it was very funny “wrong” as it turned out. No fast forward When I’d fixed the leaf switch, I pushed a test cassette into the machine and put it through its paces. And at first all seemed well; it played, it recorded and it rewound. But then I discovered that it wouldn’t fast forward. And from this emerged another problem. When the tape was fully rewound, it wouldn’t play. However, if it was only partially rewound – as it was when I made my initial test – it would play, record and rewind as normal –until it was fully rewound again, that is. I pulled the cassette out and checked it and could see immediately why it would not play; it had rewound completely so that only the clear section of tape was visible at the take-up reel. Normally the end sensor, when 44  Silicon Chip it sees the clear tape, initiates a reset (forward wind) function, which brings the active tape up to, or close to, the take-up reel. This cancels the end sensor signal, which otherwise disables the system. That was only an intermediate explanation, of course. So why wouldn’t it reset? My first reaction was to suspect the end sensor. This consists of phototransistor TR2 (PN202S), lamp 1N901 (fed from terminals 15 & 16), and some associated circuitry. But no, the end sensor system was working correctly in all respects. Two faults; one problem Then the penny dropped. The two failures had to be related because it is the fast forward mode which is activated by the end sensor to provide the short burst of reset action, as described above. So solve the fast forward problem and all should be well. This meant delving into the “mecca drive” board, which provides the drive for reel motor M903 (described as the “Rell Motor”, as distinct from M902 But first I contacted the customer and explained what I had found, what would need to be done, and what it would cost. This worked out at around $67, involving $12 (cost price) for the IC and the rest for labour. Even then, this did not nearly cover the time taken to track down the fault and fix it. The customer wasn’t particularly happy about this charge, even though I did my best to convince him that this was a com­pletely separate fault in no way connected with the first service call. Anyway, after some show of protest he agreed to meet the cost and for me to go ahead. Which I did, and everything worked out as expected (I’m not sure what I would have done had another fault turned up). But it was a classic example, not only of an unusual technical fault, but of the problem facing any serviceman when a second, complete­ ly unrelated, fault occurs shortly after any service work. And, of course, it’s always, “... doing the same thing again”. But that’s life SC in this game.