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SERVICEMAN'S LOG Now look what ya gorn an' done! We don't hear that expression much these days but it was a popular one years ago. It was reserved for those occasions when someone committed a careless act which resulted in serious - and sometime irretrievable - damage. These days we tend to use the more economical expression, "Oops". Either way, the implication is the same; somebody has goofed and done a lot of unnecessary damage in the process. And since we have all done it at times, it doesn't do to be too cocky when someone else docs it. But we can take heed and be warned. I have two such stories this month. One situation was retrievable, even though it looked a bit sticky for a while. The other one ... ? The first story concerns a Hanimex 18cm monochrome TV set, model HTV7 . It was a brand new set, belonging to a colleague who is one of the lo ca l antenna installers and with whom I work quite frequently. The set was bought specifically for this job, to be used when doing survey work in difficult areas, particularly involving SBS on UHF. It could operate from either the 240V AC mains or a 12V battery. In the latter mode, it used a lead fitted with a lighter plug at one end and a so-called DC plug at the other, this mating with a PC board socket in the set. For those who may not recognise the DC plug by that name, a brief description might help. The best al ternative term that comes to mind is a female plug - a contradiction in terms perhaps, but that is what it is; a plug shaped device for lead termination which is, in the conventional sense, a socket. And the male mating device, designed more like a socket for chassis or PC board mounting, is in the conventional sense a plug. 76 SILICON CHIP While these devices are not often found in everyday TV and . video equipment, they are used extensively on plugpacks, small chargers for nicad battery operated devices, and such like. (An article in an English technical magazine some years ago suggested that, in order to avoid confusion, such devices should be referred to as "sugs" and "plackets". Fortunately, the idea did not catch on). Anyway, battery operation was one TETIA TV TIP Sanyo CTP5604 Symptom: no sound or picture. Higher than normal voltage appears on the line output transistor collector and a faint squeal can be heard from the power supply circuit board. Cure: R473 (100Q 0.25W) open circuit or dry jointed. This resistor is a vital part of the line oscillator and without it the set can never start up. The squeal is the power supply running in its self-oscillating mode, at a much lower frequency than normal. TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania 7015. of the main advantages of this device and the owner had purchased a small sealed lead-acid 12V battery to go with it, making the whole assembly compact enough to carry around with an exp loratory antenna, even onto a roof if necessary. To accommodate this battery, he had removed the cigarette lighter plug, shortened the lead somewhat, and bared the ends to fit the screw terminals on the battery. Which was all very clever and logical. Except that this was where he came to grief. The original lead was fitted with a fuse and his first mistake was to remove this from the modified lead. His second mistake was confuse the polarity of the battery leads after he had removed the lighter plug. He assumed - perhaps not unreasonably that the centre connection of the DC plug would be positive whereas, had he taken the trouble to check the original setup, he could have determined that the reverse was the case. In fact, there is a noticeable lack of standardisation in this regard. Some manufacturers favour one arrangement, others the opposite. I'll have more to say about this later. The inevitable disaster So there was a disaster waiting to happen. And, of course, it did happen. The first I knew of all this was when the set was pushed across the counter, together with a brief synopsis of the above events and a plaintive plea from my colleague. He had planned to survey a difficult VHF area that morning; could I have & quick look and perhaps get the set going? He couldn't have picked a worse time. It was a Monday morning and the weekend's pile-up of problems was being unleashed. The phone seemed to be ringing every few minutes and other customers were breasting the counter. And I didn't even have a circuit of the set. Granted, I had a fair idea of what to look for. Most devices designed for external battery connection use a protective circuit of some kind. The usual arrangement is simply a power diode, reverse connected across the supply leads, so that it is open circuit with correct polarity. Then, in the event of reversed polarity, it is effectively a short circuit, and takes out the aforementioned fuse in the battery lead. Thus alerted, the user can take appropriate action. But, without a fuse, what would happen? An ohmmeter check confirmed that there was a short across the battery leads, but was this simply a damaged diode, or was it something more serious? I tried running it in the 240V mode but it was just as dead. This tended to confirm my worst fears. I opened the set with the aim of finding the diode and working from there. But that was easier said than done. Thme was no diode anywhere near the input socket and my efforts to trace the path through the maze of other tracks on a compact board proved fruitless; it seemed that every time I was half way through a trace there was an interruption of one kind or another. Finally, even my colleague realised that we were battling against the wind; there was no way we were going to .a chieve a quick fix in such an atmosphere. The survey would have to be done without it. So I suggested that he leave it with me and go back to the dealer from whom he bought it, and get either a manual or, at least, a circuit. Then, with a more leisurely approach, we might get somewhere. And so I put the set aside. No circuit I heard nothing more from my colleague for about three weeks, then he turned up with the news that, after much chasing and duck shoving, he had been advised that Hanimex could not even supply a circuit. So we were on our own. The owner wasn't particularly happy about the situation. He had spent around $130 for a set which had never been used and now he feared it might be a write-off. I wasn't that pessimistic but I was worried. While I assumed that there was a protective diode, I wasn't sure. And if there wasn't, the likely damage was anybody's guess. So, at the first convenient slack period, I tackled the set again. And strangely, without all the pressures of the previous occasion, I found the elusive diode without too much trouble. Of course, it was nowhere near the input socket or on/off switch; I suspect manufacturers do this deliberately. Anyway, the diode was found, checked , and proved to have a very short circuit! Replacement was simple enough, any power diode with a rating of lA or better being suitable. Then I set to and re-arranged the customer's lead, clarifying the polarity, and fitting a fuse holder and fuse. That done, I connected the lead to a 12V battery, crossed my fingers, and switched on. Splat! The fuse went instantly and I thanked my lucky stars I had taken the precaution of fitting it. But what was wrong? Once again I had horrible thoughts about what damage might have been done with the reversed connection, in spite of the protective diode. Nor was I at all consoled when I put the ohmmeter across the battery leads and found, not the short circuit of a faulty diode, but still a very low resistance, amounting to only a few ohms. Fortunately, further checking revealed that this low resistance was present even with the set switched off, which left only a short track between the input socket and the on/off switch on the volume control to check. There wasn't much to suspect here. There could, in theory, be a leakage between tracks but this seemed most unlikely. More realistically, the only likely component was the input socket, mounted on the PC board. Closer examination suggested that it looked somewhat misshapen, so I unsoldered it for a closer look. And it was misshapen all right; in fact, it was well and truly cooked. More importantly, removing the socket had removed the short from the board. Of course, there could be other, more subtle damage yet to be revealed, but I was feeling more hopeful. This optimism proved to be justiFEBRUARY 1991 77 - .. SERVICEMAN'S LOG ~ CTD · · . · I). fi ed . I patched a pair of leads directly into the board, connected a battery with due regard to polarity - and gingerly switched on. And, for the first tim e. the set came to life; sound immediately and a picture a few seconds lat er. Well, that was a relief. No serious damage had been done and all I had to do was fit a new input socket to put th e set back into new condition. Serviceman's goof Well, this was where I goofed. The simple truth is that I was not as familiar with DC plugs as I thought I was. I had handled them often enough , as part of various pieces of equipment including some ofmy own - but I had never had occasion to actually work on them in the service sense. For a start, I blissfully imagined 78 SILICON CHIP that I could nip down the street to my local electronics store and purchase a new socket for 80c or so. No dice. They had the plugs and they had mating sockets for lead termination or for chassis mounting, but not for PC board mounting. "Sorry, we don't get much call for them." No problem really. A phone call to one of my regular bits and pieces suppliers confirmed that they had good stocks. I ordered one and was assured that it would go in the post immediately. And they were as good as th eir word; it arrived in the mail the next day. It took only a few minutes to fit it to the board, then I reached for the battery lead to plug it in and give the whole setup a final test. I imagine some readers are way ahead of me; yes, my face was red. I had failed to appreciate that these devices come in two sizes; a 2.1mm pin and a 2.5mm pin. And, apparently, the 2.5mm version is the more commonly used one so, in the absence of any specifications from me, my supplier had as sumed that this was what I wanted. Need I add that the set used the 2.11mn version? And so it took another phone call, another hastily arranged postal delivery, and another solder job on the board. And that finished the job. But what a chapter of accidents and confusion. As it was, I made on ly a nominal charge for the job - much less than was really justified. But then, my colleague has done me a few favours and I owed him one. So everyone was happy, particularly my colleague who had mentally written the set off. But he is also a lot wiser and realises it could have been much worse. Not all devices carry protective circuitry and not all protective circuitry works every time. When you're mucking about with any powe.r leads - even low voltage ones check and double check the polarity. Finally, some thoughts on the polarity conventions used with DC plugs. I happened to relate the above story to an amateur friend, including my embarrassment over the DC plug sizes. He grinned, "Oh yes, I was caught that way myself the first time; it's a common trap." (Now he tells me!) But he was also able to demonstrate a typical conflict of standards in regard to plug and socket polarity. He produced a popular 2-metre hand held transceiver; a Yaesu Musen model FT-23R which, by all accounts, is a very nice little unit. As is typical of such devices it operates from a slide-on nicad battery pack, 7.2V in this case, although a 12V pack is available. The pack is fitted with a 2.1mm phono socket (not exactly the same as the DC plug, but similar) which accepts a plug from a charger. And moulded into the case is a schematic portrayal of the socket, with polarity markings clearly indicating that the centre conductor, which becomes the tip of the plug, should be positive. Fair enough. As well as the battery pack proper, there is also a dummy battery pack available as an accessory. It is similar to the real one but empty, its purpose being to provide access to an externa l power source, such as a car's 12V system. This is fitt ed with the same type of socket ·to take the lead from this source - except that (yes, you've guessed it) this connection uses the opposite polarity. Why this distinction? I have no idea. As far as I can sec, there is no advantage one way or the other. Nor does it matter, provided the difference is appreciated. But wou ldn't there be less chance of accidents if both connections were the same? Anyway, that's my "Oops" for the month. But here's another one from my regular contributor, J.L. from overseas in Tasmania. Here's how he tells it. Crossed wires This story is being written on a 6year-old BBC micro-computer. It's now an old-fashioned machine, with only 32K of user memory. But with its operating system and the word processor routine both in ROM , th e machine has proved more than adequate for the jobs I us e it for. Coupled to the machine is a dual single sided disc drive, a Star Gemini printeJ, an English Nightingale modem and a Thomas green screen monitor. All in all, it represents $2500 worth of computer equipment and I'd be totally lost without it. Alongside me as I write is another, far less fortunate BBC computer, the subject of the photos on the following page. It suffered from the careless machinations of a would-be serviceman and is now a part of $2500 worth of junk. The photos tell most of the story in graphic detail. What more need I say? It seems that the owner of the machine wanted to operate it at a distan ce from th e nearest power point so he asked his '·' friend" to make up an extens ion power lead for him. The so-called fri end did so, but crossed over the active and earth leads, with very expensive results. The 240V AC was appli ed to the computer via the earth line and blew the lids off most of the !Cs on the main board. At the time of writing, it seems that only the keyboard escaped total destruction. But that's not all! The 240V found a path out of the main board to the monitor, disc drive and modem lin es, and played merry hell with all of their Is Your Product Or Service Getting The Exposure It Deserves? 25,000 consumers will read this page. They need to see your advertisement if you want them to buy your product. Contact Paul (018) 28 5532 or Ian (03) 696 5411 to reserve this space. semiconductors along th e way. The resultant current finall y went through the printer, leaving a charred and very exp ensive mess behind it. If it had been my computer, I'd be weeping tears of blood. But then, I would have made up my own extension lead and I would have been more carefu l with the active. neutral and earth leads , wouldn't I? (A lat er investigat ion showed that even th e keyboard did not esca pe entirely. Three chips. three resistors and several diodes were totally cooked . The key switches did survive and they have been salvaged for possible use in mv machin e at a later date!) Well. that's J.L.'s story and, as they say in the classics . that was a fair dinkum, no-mu cking-about. kind of "Oops". Belting a VCR Finally, here is a short story which, as well as highlighting th e tendency of some servicemen to assume th e worst, may also provide a coup le of helpful comments fo r other readers. It was inspired by problems with a Hitachi video recorder, mod el VT-33E. TV TEST EQUIPMENT THAT WILL EARN YOU MORE MONEY (AUSTRALIAN MADE) 12 Months Warranty SHORTED TURNS TESTER Built-in Meter to check EHT transformers including split diode type, yokes and drive transformers. $78.00 + $3.00 p&p HI-VOLTAGE PROBE Built-in meter reads positive or negative 050kV. For checking EHT and focus as well as any other Hi-tension voltages. $98.00 + $5.00 p&p LOW VOLTAGE PROBE Ideal for checking microwave ovens and TVs. The ranges are from 0 to 5kV Negative and from 0 to 10kV Positive. Double insulated for safety. $8_4.00 + $5.00 p&p $75.00 10.00 p&p + <:...... _.. .~ DEGAUSSING WAND 7,700 apere turns. Strong magnetic field, larger than usual coil with multicore centre . Double insulated for safety with momentary on/off switch. 240V NC 2.2 amps. This one is just about as important as having a soldering iron in your toolbox! REMOTE CONTROL TESTER (INFARED OR ULTRASONIC) j""'~ "-· -~·.:} $85.00 + $4.00 p&p TUNER REPAIRS TV . Designed to test any lnfared or Ultrasonic control units. With the extension lead you can also test infared units which cannot be placed in front of the testing unit. Requires a 9V battery.· Output is via the LED diode and piezo speaker. trom only $11.00 + p&p . Cheque Money Order Bankca;d or Masterca~d We also repair most tuners on an exchange bas,s. "• • 11JNERS~216 Canterbury Road , Revesby, ~ NSW, 2212, Australia Telephone (02) 774 1154 FEBRUARY1991 79 This is what 240V AC can do to a computer motherboard. These burnt & melted chips were fed with mains voltage after the active & earth leads were swapped on an extension cord. This model is about eight years old and appears to have been very reli able. I have encountered few faults until recently; just the odd minor one or a routine head clean, etc. But lately I have encountered several with what appeared, initially, to be quite puzzling "funnies". In fact, all were due to the same basic cause, albeit with variations. It's the drive belts that case all the problems. There are no less than six belts in this machin e and failure of any one can produce a range of (sometimes) puzzling symptoms. Five belts are on the underside of the machine, while one is on the top side. On the underside, two belts run from the capstan motor which carries two pulleys. One of these pulleys is about 12mm diameter and engages a wide belt which drives the capstan shaft via an 80mm pulley. The other pulley is about 20mm diameter and engages a longer belt which then drives a larger pulley (about 40mm diameter). This latter is on an intermediate shaft which carries a second pully, about 18mm diameter, and from this another belt drives a second pulley, again about 40mm diameter, on the idler shaft. This carries the tyred idler wheel which drives the takeup or supply reel, according to the operating mode. Also on the underside is the loading motor and this drives the two arms which extract the tape from the cassette and wrap it around the drum. The link between the motor and the loading mechanism is via two belts. 80 SILICON CHIP It looks like a bombed-out factory in World War II. This is another section of the main circuit board of the BBC microcomputer after someone connected 240V AC to the PCB earth track. The loading motor pulley is about 10mm diameter and drives an intermediate pulley of about 20mm diameter via the first be lt. A second (12mm) pulley on the same shaft is then coupled via the second belt to a 20mm pulley on the loading mechanism shaft. That accounts for five belts. The sixth one , above the chassis, couples the takeup reel to an impulse generator which feeds the microprocessor. Its function is to confirm that the takeup reel is functioning. Faulty diagnosis The recorder came to me with the story that it sometimes refused to run a tape. It had been taken to another service organisation whose diagnosis was that it was a microprocessor fault and that, as an old machine, it was not worth fixing. I opened it, confirmed that no belts had been changed, then put it through the load function. It appeared to load normally; the tape actually starting to run. Then everything stopped. Acting on experience, I applied gentle pressure to the last pulley in the loading mechanism chain. It moved a fraction and - bingo! - the tape began to play. It was a classic fault. Both of these belts are prone to go spongy and lose their elasticity. Then they will no longer drive the loading arms fully home; nearly, but not quite - and not hard enough to activate a protective sensor. So the machine shuts down. (I admit that this one tricked me for a while, the first time round). Another rather alarming symptom of this condition is that, as the system attempts to load, it will produce a violent screeching sound; something which sounds exactly like a dry bearing about to sieze. That noise is simply due to one of the pulleys spinning against the belt. The cure was simp le; a complete set of new bolts and a now idler tyre, plus a routine clean and lubricate, restored the machine to first class condition. So much for the "it's not worth fixing" diagnosis. A more obvious belt fault involves the top belt coupled to the takeup reel. This sometimes breaks and, while the machine will load, it will not run. The microprocessor knows the takeup reel is not functioning, and shuts the machine down. Fortunately, this is easy to pick. I have also encountered a broken belt in the drive from the capstan motor to the idler wheel - the larger one in this case. Again , while the machine will load, it will not run, because the microprocessor is being told - genuinely - that the takeup reel is not functioning. This is also easy to see, but a slipping belt in this chain could create more puzzling symptoms if it failed only momentarily, in the middle of a run. All of which adds up to a simple piece of advice. Any VCR of this age and the VT-33E in particular - which exhibits similar symptoms, may well have a belt problem. And don't muck about; change all the belts. If one is dead the rest are dying. SC