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SERVICEMAN'S LOG Working within the customer’s budget With smaller sets, the cost of any repairs must be kept under control. My main story this month is a good demonstation of that & is also an excellent example of how a minor component can cause inconvenience. The story concerns a Hanimex colour TV set, model CTV-10. It is a semi-portable type, with a 25cm screen, and is designed to operate from either 240V AC or 12V DC (eg, from a car battery). And it is around this dual operation feature that the story revolves. The owner had bought it when he retired, primarily for use in his caravan on a round-Australia trip which lasted some 12 months. And, as can be imagined, the 12V system came in for a fair amount of use during that time. But shortly after returning home, he realised that the 12V system could no longer be used, although the set continued to perform perfectly well on 240V. Well, that wasn’t much of a bind; they were not planning any long 34  Silicon Chip caravan trips in the future and, as long it functioned on the mains, it would suit their needs quite well. So that was how things went for the next 12 months or so. In fact, it would probably have continued along these lines indefinitely, if the set had not also failed on the mains. And that’s where I came in. The owner was a new customer and he set out the above his­tory before asking the vital question, “Can you fix it?” I could­n’t be sure, of course. As I explained to him, it was a set I hadn’t seen before and I had no circuit or service manual. Hope­fully, I could get data from Hanimex if necessary but, initially, I would simply look for what was obvious. Filthy lucre That was OK but then came the vital question of filthy lucre; how much was he prepared to spend on it? It was an import­ant question because I didn’t know what I would find or what data or spare parts were available. After some discussion, we settled on a figure of $100. If it looked like exceeding that figure, I would consult him first. And so he left it with me. On closer examination, I learned that the set was made in China. Later, when I delved into the innards, I wasn’t all that impressed with the construction in general. I’d seen worse but I’d seen better too. At a more basic level, I had to try to work out the power supply system and decide where to start looking. The only thing I felt reasonably sure about TC-48P10 Has Live Chassis In the Serviceman story in the October 1993 issue, I men­tioned that the Panasonic TC-48P10, and by implication the TC-1480A, had an M15D “dead” chassis. This is wrong – both sets have the M15L “live” chassis. The mistake has come about partly be­cause I test and repair all sets using an isolation transformer. was that there were two separate faults, since the two failures had occurred at different times. As it turned out, the set was basically a 12V device, with the addition of a 12V mains-driven power supply. The mains cord was fitted permanently, while the 12V cord connected to the set via a plug and socket arrangement. There was no switching involved. The 12V positive battery lead went straight to the 12V rail derived from the mains power supply. The voltage regulator IC isolated the 12V from the rest of the supply circuitry – quite an elegant system. It was the plug and socket arrangement connecting the 12V lead to the set that provided the first clue. It is a commonly used fitting, typically referred to as a DC power plug or DC jack – with matching socket. The plug, which fits on the cable, is a female fitting, and the socket, normally chassis mounted, is the male version. When I tried to push the plug into the socket it wouldn’t fit properly. Closer examination revealed that the end of the plug had been damaged. The insulation between the inner and outer conductors was deformed, as was the end of the inner conductor. Thus alerted, I pulled the back off the set for a closer examination of the socket. And this was the real culprit; it had obviously been seriously over- heated and was grossly deformed. I subsequently measured the current involved and it varied from 2.25A to 2.5A over the range 12-13.8V. That’s pretty solid for a miniature connector like this, particularly as the set would typically run for quite long periods. As a quick test, I patched a couple of leads into the cir­cuit and connected them to the 12V bench power supply. And the set sprang to life immediately, with good sound and a first-class picture. I gave it a quick check across all the channels and satisfied myself that there was nothing wrong with its operation. This clearly suggested that its failure to operate on 240V must involve the power supply. In the meantime, what was to be done about the damaged connectors? The easy way out would be to simply replace them with new units of the same design. However, knowing that they appeared to be inadequate, at least on a long-term basis, should I try to fit something better? I mulled over this latter idea at some length and was even­tually forced to the conclusion that, desirable though it might be in theory, it was not physically practical. It would have meant hacking into the chassis in an awkward spot, could not have been done neatly, and would have only added to the cost. I even discussed the problem with the owner and he agreed that it was unlikely that the 12V system would get much use from now on. So I took the easy way out. But why wasn’t the set designed to take an adequate plug and socket – even if it cost a fraction more? The supply fault Putting such questions aside, I now had to find the fault in the mains supply. It was a simple circuit, consisting of a power transformer delivering around 16V, a bridge rectifier, a 3-terminal voltage regulator and a filter capacitor. It didn’t take long to pinpoint the culprit – the trans­former primary was open circuit. It was a simple job to remove the transformer and, when I did, it revealed a small sticker saying, “Internal Thermal Fuse”. So my guess is that it was this that had failed but for what reason we will never know. In any case, the transformer was a write-off. So where to from here? There were two points to be clari­ fied: (1) the availability of a replacement and (2) the cost. This latter point was most important. Transformers are not cheap and I could see that, with labour costs, the $100 limit might be exceeded. So it was back to the customer. I had to be honest and point out that, depending on the best transformer deal I could swing, the job might cost up to $150. Was he prepared to go that far? He wasn’t too keen at first but I pointed out that we knew the set itself was OK, performed well, and that to replace it, would probably set him back $300 to $350. So, after some hesitation, he agreed. So began the search for the best transformer deal. My first choice was a Sam­ sung unit I had on hand. It was electrically suitable, was fitted with a magnetic shield similar to the origi­ nal, and looked as though it would fit physically. And, most importantly, it only cost me $36, including tax. At that price, I could do the job well under the limit. But alas for my clever thought; it was just marginally too large and there was no way I could fit it. So the next step was a call to the Hanimex spare parts department. It was a good news/bad news situation. Yes, they could supply a transformer ex-stock but the price was $73 plus tax, or about $95. Pack and post would add another $6, giving a total of about $100. I said “Ouch” under my breath and thanked them for their help. That simply was not a proposition. Even without any mark-up on the transformer, labour costs – including the work done on the 12V system – would put the total above the agreed figure. And I couldn’t justify pushing the limit any higher. I had to find a better way. Substitute transformer A colleague dropped in for a chat around this time and I filled him in on the problem, displaying the faulty transformer. His reaction was immediate: “Have a look at an old Akai video recorder, a VS-3 or VS-4. They had a transformer with a number of windings on it and it might just fit.” Being thus reminded, I realised that he could be right and that I might just have one. So it was out to the junk room where we scrabbled through the discards and, sure enough, there was a junked VS-3. So it was back to the bench and out came the trans­former. And it looked quite promising, being of a shape and size which would clearly fit. All that remained was to November 1993  35 SERVICEMAN'S LOG – CTD sort out the secondary windings and, hopefully, find one that would suit. The primary winding was easily identified, so the quickest way was to fit a power cord and plug it into the mains. Unfortunately this proved to be another setback. At switch-on there was a protesting splat and a puff of smoke; storage had apparently not been kind to this device and it was write off. But the exercise had not been completely wasted. My col­ league had started a useful train of thought and I remembered another Akai video recorder in the junk room, this time a VS-112 which is a much later model. And again, the trans­former looked very promising. It was smaller that the VS-3 unit and should fit easily. In fact, when I pulled it out and tried it in position, I realised that I 36  Silicon Chip could bolt it directly into place; a real win. So another mains test was set up. And it didn’t go splat this time. More importantly, it had three secondary windings, one at 30V and two at 16V, the latter being the more useful value. Initially, I sat the transformer on the bench and patched one of the 16V windings into the circuit for a trial run. And it worked but with one reservation; the DC voltage out was barely holding its value. I plugged it into the Variac and reduced the input voltage slightly. Sure enough the picture started to bend and pull. In short, there was not quite enough voltage at the regulator input to cope with line voltage variations. The 16V AC should have been enough but I sensed that the winding was a rather light one and that the load of the set was pulling it down. The logical reaction was to connect both 16V windings in parallel – suitably phased of course –and that solved that problem. And an hour’s bench test confirmed that the transformer was running quite cool. But we weren’t out of the woods yet. The transformer had no magnetic shield and, while it worked fine when sitting on the bench, I was worried that it could cause trouble when mounted in the set. So the next step was to mount it in the set, wire it neatly in the final form, and hope. Well, we won. There wasn’t the slightest hint of interference from the transformer, even when check­ e d on blank red, green and blue rasters. I gave the set a soak test on a daily basis for about a week before finally deliv­ering it to the customer. So what did it all cost? I charged him $125. Yes, I know, the transformer didn’t cost me anything – well not in hard cash – but it did cost me a lot in time, taking into account the problem with the first unit. So I reckoned it was a fair price. The customer thought so too; he was delighted. A day in the life ... And now, for a change of scene – and a change of pace – here are some short stories from my colleague J. L., across Bass Strait. He has sent me a number of such items and I am including as many as space will permit this time around. More next time. Over to you, J. L. The stories selected for this column are usually about unusual faults, or about common faults that require some kind of mental gymnastics to resolve. The simple faults, like replacing a shorted chopper tran­sistor and the resulting blown fuse, never seem to make these pages. Yet they constitute 99% of all jobs passing through the average serviceman’s workshop. There are times when I have a long run of routine tasks that provide no inspiration for Serviceman stories and I’m left bereft of any material to write about. So, at the suggestion of the Editor, I have decided to put together a list of common but interesting faults. One that I have just finished was an AWA video recorder, an AV47. It came in with the complaint that it “would not play through the TV set”. This was a rather ambiguous statement, since it could mean that it would not feed playback signals to the TV set, or it could mean that it would not pass off-air signals as well. The only thing to do in a case like this is to set the machine up and operate it in the way an average owner might drive it. This is not as easy as it seems, since a technician usually knows what the “TV/VCR” switch and other controls mean and would be expected to put them in the correct position. To operate a VCR in “average citizen” mode means that the serviceman has to try to forget everything he has ever learned about these devices. Then, with any luck, he will hit on AUSTRALIAN MADE TV TEST EQUIPMENT 12 Months Warranty on Parts & Labour HIGH VOLTAGE PROBE Built-in meter reads positive or negative 0-50kV. For checking EHT & focus as well as many other high tension voltages. $120.00 + $5.00 p&p This close-up view shows the heat-damaged DC plug & socket from the Hanimex CTV-10 12V supply system. the same problem as the customer did and be able to diagnose the fault straight away. In this case, I was lucky. I connected the machine to the bench monitor and tuned one to the other. The off-air picture was decidedly snowy and the off-tape picture was also extremely poor. It was snowy, had no colour and often rolled. I recorded a few minutes of program material and then played it back on another machine. The results seemed to be reasonable but I did detect some snowy scenes. It was not what I would have called a prime quality picture. Booster/modulator unit A glance at the circuit diagram in the service manual showed that both the antenna booster and the output modulator were contained in a single unit. I had a horrible suspicion that this was the source of the trouble. By attaching a test tuner I was able to determine that the output of the booster was far from up to scratch. In fact, there was far less signal coming out of the booster than there was going into it. Similarly, I was able to feed a good video signal into the modulator part of the unit and found the output to be far less than one would have expected. There is no circuit diagram pub­lished for the booster/modulator unit but I would hazard a guess that some part of the supply rail to the chips inside the unit had failed. Unfortunately, there is no way to repair these devices so I had to place an order for a new one from Mitsubishi AWA. It wasn’t cheap but the new unit cured all the problems and so the customer is satisfied with the result, if not really happy about the cost. The next job was a Sharp VC-8300X VCR, one of the first of Sharp’s front loading models. The problem here was that the machine would load and begin to play but then shut down before it showed any sign of a picture. I took the cover off the machine and tried again to play a tape. The cause of the trouble was immediately apparent. The drum was not rotating. The drum motor in this machine is not one of those modern direct drive types. It consists of a very conventional brush type motor, driving the drum by means of a wide flat belt. It didn’t take long to discover that the motor was being provided with a healthy drive voltage yet showed no signs of life. I disconnected the leads from the motor and tested it for con­ tinuity. There was none. The VC-8300X is a very old machine and probably not worth all that much. I checked with the owner to see if he was prepared to pay something like $80 for a new motor. He wasn’t, so we decided that the machine would probably have to be junked. But before taking that step, I enquired among some of my colleagues who have a stock of junk sets in their store rooms. One of them did have an 8300 and it had a seemingly good motor on the deck. We struck a quick deal and I took the motor off the wreck. I had the motor fitted and the machine up and running just 15 minutes after my return to the workshop. The owner was quite happy to have a secondhand motor fitted to his machine. The alternative would have been very much more expensive. (I later found that one of the brush springs in the origi­nal motor had been bent slightly, thus holding the brush away from the commutator. I might have been able to repair it but as I had already fitted the replacement there was no point). DEGAUSSING WAND Great for comput er mon­­­i t­o rs. Strong magnetic field. Double insulated, momentary switch operation. Demagnetises colour picture tubes, colour computer monitors, poker machines video and audio tapes. 240V AC 2.2 amps, 7700AT. $85.00 + $10.00 p&p TV, VCR TUNER REPAIRS From $22. Repair or exchange plus p&p. Cheque, Money Order, Visa, Bankcard or Mastercard TUNERS Phone for free product list 216 Canterbury Rd, Revesby, NSW 2212, Australia. Phone (02) 774 1154 Fax (02) 774 1154 The next job involved another mechanical problem, this time in a Sharp cassette deck. I plugged in a set of head­­phones, dropped my test cassette into the deck and pressed play. The sound in the phones was hardly recognisable. It was suffering from the most hideous warble I have ever heard. When I looked inside the cassette well, my eye was imme­diately caught by a large dollop of black gloop on the capstan shaft. In fact, it turned out to be a part of the pinch roller. What had happened was that the roller rubber had perished and become very soft and sticky. Part of it had torn off and stuck to the capstan. The job was relatively easy to fix. I first had to work out how to remove the pinch roller arm before I could remove the roller spindle. After that, I had to find out if I could get a proper replacement. In the event, I found an alternative in a junked deck so once again the customer scored a quick fix at a reasonable price. Thank you J. L. I like the idea of dealing with the more mundane, dayto-day, bread-and-butter jobs; it helps strike a balance. More next time. SC November 1993  37