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SERVICEMAN'S LOG To tip or not to tip – a few tips Ever wondered what happens to equipment which is written off as not worth repairing? Typically, it would be stripped for any worthwhile spares and what was left would go to the tip. But not always. Yes, sometimes there is the temptation to try to salvage an item, even if obviously uneconomical at a commercial level and there is a risk that the attempt may not be successful. One may spend many hours searching for an elusive – possibly intermittent – fault and not find it. Or, if a fault is found, it may turn out to be a component which, for one reason or another, cannot be replaced. But that’s the risk one has to take. I encountered two such exercises recently which illustrate this situation very clearly. One is from my own bench and one is a colleague’s experience. My own story involves what the makers (Grundig) simply call a receiv- er – Receiver 3000 (GB) – although it would be better described as a tuner/ amplifier combination. It consists of an elaborate stereo amplifier plus an AM/ FM stereo tuner, the latter featuring press-button tuning, as well as continuous tuning and a “Digital Frequency Indication Module”. There are several input sockets to take external signals of various kinds and an appropriate switching system to go with it. It also features both automatic and manual muting systems, and these operate when switching between stations or other signal sources. All-in-all, it is a most attractive unit and this example was in good physical condition. So what was the story behind it? It belonged to a colleague and apparently had had a rather che­quered service history, having previously belonged to someone else. But now, as my colleague summed it up, it didn’t go and he had earmarked it for the tip. When I expressed regret that such a nice unit was to meet such a fate, my colleague responded in­stantly: “take it if you want it – you can send it to the tip as easily as I can” (he is not given to undue optimism). And so I took it. I wasn’t sure what I was going to do with it, assuming I could fix it. For the moment, it was mainly a challenge. A nasty mess At the first opportunity I put it up on the bench but it was dead. I pulled the covers off and this revealed a rather nasty mess. There were three fuses, one of which was obviously the mains fuse while the other two were on the sec­ondary side of the power transformer. All three were blown. In addition, there was a swag components which had been unsoldered. Someone had really gone to town on it. Fig.1: the power supply circuitry in the Grundig 3000. Mains fuse Si.I is at extreme right, while fuses Si.1 and Si.2 are to the left of the transformer. T2 and IC1 are at the extreme left. 40  Silicon Chip I decided the only logical approach was to put everything back and start from taws. I re-soldered all the components, then looked at the fuse situation. The mains fuse, designated as Si.I was marked 2A and the other two were designated as Si.1 and Si.2. Si.1 was marked as 250mA and Si.2 as 1.25A. (No, there weren’t two Si.1s; the mains fuse designation used a capital “I”. Talk about planned confusion!) I replaced the mains fuse and Si.2 and switched on. Splat! Si.2 blew immediately. I realised then that it would be fruitless to go on without a service manual or, at least, a circuit. I rang Southern Cross Electronics, the local agents for Grundig, and asked about a manual. There was some mucking about here. I had to fax a request and they replied a week later quoting $25 for a circuit photostat. I placed an order and it ar­rived after another week. There were 10 A3 sheets in all. Six were circuit diagrams taken from what were originally two large foldout sheets. The rest were parts lists, etc. After dispensing lots of sticky tape and patience, I eventually reconstructed the foldout sheets, each of which turned out to be 1.2 metres long! After all that, I started over again. I tackled the Si.2 fuse circuit first. Si.2 is between a 12V secondary winding and a full-wave bridge rectifier (GL2) which generates a 15V rail. This is then applied to a voltage regulator (IC1) to provide a 5V rail. I suspected IC1 and I was right but in more ways than I expected. First, it was short circuit, which didn’t surprise me. What did surprise me was that it turned out to be a bodgie com­ponent. It was not a 5V regulator at all but, in fact, a 12V unit. Just why this had been changed and by whom remains a mys­tery. It had probably failed because it was the wrong type, par­ticularly as it was working directly into a 5V zener diode (although the zener’s role is something of a mystery in itself). Anyway, I replaced the regulator with the correct type, fitted another fuse and switched on. This time the fuse held and we had a 15V rail and a regulated 5V rail, with no signs of distress. So far, so good. Now to fuse Si.1. This is part of another supply rail which is derived from a 63V transformer winding. This drives bridge rectifier GL1 which in turn drives another voltage regulator based on T2. T2 is not a regulator within itself, however. It is a Darlington pair, housed in a TO-220 flat pack encapsula­tion, and takes its reference from external zener diode D4. This arrangement provides a 55V rail. When I switched on there was an immediate response – R8, a 47Ω 1W resistor in the collector line to T2, began to overheat. T2 was the obvious suspect but an ohmmeter check failed to reveal anything wrong. Nevertheless, I unsoldered it and pulled it out. And there was the fault in full view – the insulating washer between the heatsink (collector) and chassis had punctured. And it was obviously a voltage sensitive breakdown, immune to the low voltage of the ohmmeter. I fitted a new washer and tried again. This time Si.1 held and I had October 1996  41 Serviceman’s Log – continued tran­ sistors form part of the muting circuit. When an appropriate voltage is applied to their bases, they turn on and mute the tuner signals into the amplifier. I confirmed this operation by the simple expedient of shorting the base of each transistor to chassis in turn, whereupon I had normal output from the amplifi­ers. So, the problem was really quite simple – the “muting” voltage (or possibly some other voltage) was being applied to the bases of these transistors and turning them on, even though the muting switch was off. All I had to do was find out what was causing this. Complicated circuits a 55V regulated rail. I had rather hoped that the thing would burst into life now but it didn’t. Granted, some of the LED displays and other lights were now on but the frequency display was dead. A healthy buzz I wasn’t sure of the significance of this but decided to ignore it for the moment and concentrate on getting the sound path working. I pushed a scrap of bare wire into the various amplifier input DIN sockets and, eventually, was rewarded with a healthy buzz from each of the speakers. So, the amplifiers were working – we were making progress. But there was no sign of life from the tuners. Initially, I suspected that the frequency display failure could be a symptom of a major failure in the tuner section, which was rather a nasty thought. But then I noticed something else. If I turned the volume control fully up, I could detect faint sound when I pressed some of the channel selector buttons. So, was the tuner working but unable to pass its signals to the amplifier? After poring over the FM tuner circuit on the other foldout sheet, I pinpointed the stereo outputs as being, initially, at transistors T5 and T6. From there, the signals went to T8 42  Silicon Chip and T9 and from there – on the other sheet – to the switching circuits ahead of the power ampli­fiers. One of the most useful pieces of test gear I have is a small audio amplifier which is equipped with a probe. I use it to trace audio signals and track down losses and distortion. And this quickly confirmed my suspicions; there were strong healthy signals at both T5 and T6 and also at T8 and T9. OK, over to the switching circuits. The tuner stereo sign­als come into the switch bank on terminals 12A1 and 12A3 and emerge on terminals 4A1 and 4A3. From there, they go to the amplifiers via switch position A3/B3 and plug socket SA10. Only they didn’t. The signals were present at the outputs of transistors T8 and T9 but not at the 12A1 and 12A3 switch input terminals. This drew my attention to another part of this circuit. Although the tuner signals are routed through the switches to the amplifiers they also go directly to another pair of transistors, also designated as T8 and T9, just to make it harder. These two transistors are connected between the audio lines and chassis in such a way that, if they are conducting, they pull the audio lines down to chassis. In greater detail, these But what had gone before was merely routine compared with what lay ahead. It was a real round-theworld-for-sixpence job. These circuits are drawn using what I call draughtsmen’s cables; long thick black lines into which individual lines disappear, identified only by a number. One has to follow the line until the number is found, usually on another sheet. And as likely as not, after a small digression into a piece of circuitry, it will go back into the cable on its way back to the first sheet. Believe me, it’s easy to go bonkers trying to trace a circuit like this. Thankfully, I didn’t go bonkers or at least I don’t think I did. I won’t bore readers with all the details of my circuit tracing. In any case, without the circuit, which is much too large to reproduce here, any such description would be meaning­less. I actually lost count of the time I spent on it and as readers will appreciate, there is no way one could ever charge a customer for this work. In summary, I first tracked down the manual mute switch (i1/i2) and backtracked from there to an 8-pole switch which is used to select the FM preset channels. Only seven poles of this switch are used for the actual channel selection – the eighth pole is in the muting circuit I had been tracing. And its func­tion is to momentarily activate the muting function whenever any of the channel selection buttons is pressed, thus masking any clicks, bangs, or crackles, generated in the process. It was here that I struck oil – the switch was faulty. Not only were the muting contacts jammed closed but the whole mechan­ ism was giving trouble. In a sense, I had already been made aware of this. I had noticed that, when a button was pressed, it often took several attempts to get it to lock into position. However, I had previously put this fault to one side, as something to be attended to when the electrical problems were solved. In fact, it was causing one of those problems. As a practical short term solution I removed pin 1 from the plug assembly connecting to this switch, which permanently disa­ b led the muting contacts. I could still mute the system via the aforementioned manual mute switch and the auto-muting, on weak stations and between stations, still functioned correctly. The digital readout Putting the switch problem on hold for the moment, I turned my attention to the only other remaining problem: the Digital Frequency Indication Module. This is in a small metal box and, on removing the covers, I was rewarded with the sight of numerous dry joints – more than I could be bothered to count, in fact. How many more less obvious ones there were I had no idea. To solve the problem, I finished up resoldering every joint but it was worth it. The thing came to life and worked perfectly. And that’s how things now stand. I consider it a pretty good effort, especially as I had done what, apparently, those before me could not. So, what about the switch? Should I repair it? No way; it is made up of numerous tiny pieces, many of them under spring tension. Tackle that lot and there would be bits flying every­ where. What about fitting a new switch? That’s the logical answer but it’s no longer available off the shelf and finding one may be difficult. It was most likely specially designed for this set, which is probably now about 10 years old. The agents are currently checking to see if one can be obtained from the manufacturer. And that’s about the best I can hope for. Scrounged video recorder My second story, from a colleague, is about a device he scored from another colleague – a National NV-180 portable video recorder. Because its fault had proved elusive and so was poten­tially expensive, the customer had written it off and so it had been sitting in a corner of colleague No.2’s shop for about a year. But it left him in a quandary. He wasn’t keen to spend more time on it, yet felt guilty about sending it to the tip. So, when my colleague showed an interest, a deal was struck. My colleague’s interest was understandable. He has a per­sonal interest in video cameras and associated portable recorders. The NV-180 was originally supplied with the models A1, A2 and similar video cameras. Although bulky by modern standards, it was regarded as a major breakthrough in its day, weighing only 2.3kg without the battery. Apart from its portable role, it is an attractive unit in its own right, featuring a large multi-function digital display, slow motion and variable speed stop motion. Its accessories include an AC adaptor, a tuner and a remote control unit. Unfortunately, after a year, the original fault details were rather vague. All that my colleague could find out was that it was something to do with tape speed and a possible faulty capstan motor. As a result, he had to start from scratch. However, before presenting his story, a brief review of the transport control system may help the reader to follow it more readily. In considering the playback mode, it is obvious that the speed of the drum and the capstan – and therefore the tape – must be held constant, at a speed very close to the recording speed. During recording, the speed is controlled by the incoming signal but there is no such reference during replay; the system is on its own. In this mode, it is controlled by an internal reference; eg, a crystal. The capstan motor itself is equipped with a pulse generating device, typically a sensing head (inductive or capaci­tive) mounted close to a rotating wheel or magnet. The resulting pulses are fed to a servo system which com­pares them with the reference (crystal) frequency. This system then generates error correction voltages which hold the speed of the motor constant. A similar system is used to control the drum motor speed. But that is only part of the story. As well as running at the correct speed, the system must also be in correct phase. The drum must be positioned so that a head, when it meets the tape, exactly engages the beginning of a track. And not just any track. If we are talking about head No.1, then it must engage a track recorded by head No.1. It’s a similar story for head No.2. The way in which this is done is quite straightforward. When a tape is being recorded, square-wave reference pulses, derived from the vertical sync pulses, are recorded every 40ms (alternate field) on a control track on the lower edge of the tape. These are used to provide the aforementioned phase control and also the switching between heads. OK, here’s my colleagues story, as he tells it. Donald Duck sound My mate had been right about there being something wrong with the capstan speed; it was fast, much too fast. As a result, the sound had gone “Don­ ald Duckish” and there were noise bars running up the screen. But I didn’t buy the idea of a capstan motor fault; capstan motors normally either work or they don’t. Perhaps they might run slow but I’ve never ever seen one run fast. The first thing I did was to give the machine a good once over mechanically. This involved a routine clean, belt tension and pinch roller checks, and a check of the pause and search functions. I found nothing wrong. I then checked the main supply rails. There was 5V at pin 13 of IC2505 and 9V at pin 14 of plug FJ24 – exactly as marked. My next step was to check the electrolytic capacitors around the capstan motor drive, mainly C2532, C2533, C2534, C2535. These were checked by simply bridging them with another unit of the same value but this had no effect. It was time to put the CRO to work and check pulses. Unfor­tunately, the compact nature of the device means that servicing it can be difficult. For example, I needed to check the Servo/ Power PC board which mounts hard behind the front panel. In order to gain access to both sides, it is necessary to remove the front panel and then mount the board in a special jig – Service Connector Jig (VFK0275) – which sits it at an angle of 45 de­grees, while maintaining all October 1996  43 Serviceman’s Log – continued connections. Fortunately, I have such a jig. I started by checking for the reference (FG) pulses generated by the capstan motor and the CRO confirmed that these were correct. The FG pulse (FG1) appears at terminal 4 of the capstan motor block and, via an allover-the-place path, finishes up on pin 25 of IC2001 (AN3615K), which is also test point TP2015. I traced the pulses right through to this test point. Next, I checked the internal reference (clock) frequency to which the drum and capstan are locked. This is a 4.43MHz crystal oscillator which applies a 1.2Vp-p signal to pin 26 of IC2001. And as a matter of routine, I also checked the control pulses from the Audio Control Erase (ACE) head, although these are basi­ cally phase rather than speed control pulses. These were present and checked through to pin 9 of IC2001. So, we had FG1 pulses from the capstan, clock frequency pulses from the crystal and control pulses from the control head, all being fed to IC2001. But for some reason, the capstan motor was out of control and running free. What followed was a laborious check of various voltages and waveforms on the Servo/Power PC board. This was at times quite difficult but it eventually lead to pin 4 of IC2001 (test point TP2004) where there should have been a 4.43MHz 50mV p-p waveform. However, this waveform was missing; nor was there any voltage on this pin, shown on the circuit as 3.2V. The upshot of all this was that I concluded that the IC was faulty and ordered a new one on spec. And that was a big mistake. When it arrived I found I’d been billed for $93 – yes $93, for one IC. Move over Mr Kelly. There was worse to come. It was a small IC, with closely spaced pins, and mounting it on the double-sided PC board was not easy. The job took a long time – and achieved absolutely nothing. The fault was there exactly as before. Words failed me – well, in print anyway. The real fault I had to find the real fault now. Taking a closer look at the circuit around the IC, I noted that pin 4 44  Silicon Chip was internally connected to two functions: (1) a playback control amplifier (P.B. CTL AMP); and (2) a tracking mono multivibrator (TRACKING MMV), the latter connecting to pin 13. Pin 13 then goes to the tracking control. It was supposed to be at 0.6V – or higher – but was in fact at 0V. I hadn’t checked this voltage before, due to the difficult access. Nor had I previously checked the tracking control. I checked it now; it wasn’t working. I traced the circuit through to the tracking control (R6562, 100kΩ). This pot is panel-mount­ ed and is connected via a short length of 3-conductor ribbon to connector P205. And the one which ultimately connects to pin 13 was broken where it joined the connector. It wasn’t immediately obvious, however, as it is normally obscured and the other two conductors held the ribbon in place. Of course that was it, although how it happened is a puz­ z le. I can’t imagine any kind of user abuse which would cause it. More likely, I suspect, the unit had originally suffered from a quite different fault. The serviceman had fixed this but had broken the lead in the process. And the resulting symptoms had proved too tricky and confusing for the fault to be traced. In fact, it is not immediately obvious just how the track­ing control circuit upset the speed. But, as far as I can see, the loss of a connection to pin 13 was sufficient to upset the whole capstan servo function within the IC. If only I had checked the tracking control first off. And that’s my colleague’s story. My first reaction is to quote another of my colleagues who, in such situations, was wont to remark, “that’s a decent sort of an oops”. Which it was and I’m glad I didn’t make it. But that’s not to say that I might not have in similar circumstances. The bright side On the bright side, my colleague scored a very nice machine for the price of the IC, plus his labour. Not bad, really and he does have a spare IC in his drawer. But I feel the moral of both stories is obvious; think very carefully before you tackle an undertaking like this. And be prepared for a lot of work – and SC the risk of failure.