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SERVICEMAN'S LOG Ho\V do you get to blast a ghetto? For a change this month, I'm starting off down south. J. L., from across Bass Strait, has another of his unusual stories. And although interesting technically, it is also nearly as much about the customer as it is about the equipment. These pages usually tell stories of difficult repairs to complex colour television sets or video recorders. This story is about something quite different - a Sharp stereo radio cassette, one of the devices commonly known as a ghetto blaster. The model in question was a GF7600Z, fitted with a single cassette deck, a 5-band graphic equaliser and a 4-band radio tuner. It came to me in perfect working order, except that the cassette deck would not rewind a tape. It would play, record and fast forward, but would not rewind. Years ago, when I did a lot of audio cassette servicing, I bought a reel tension gauge, something I should also have for video decks but have not yet got around to buying. It took only a second to pop the FRACARR gauge into the machine and determine that there was less than 5gm/cm ofrewind torque. Play torque was tolerable at 30gm/cm and fast forward was marginal at 15gm/cm. But neither of these last two figures was optimum and I decided that the deck would perform much better if they could all be brought up to specs. Fortunately, in audio cassette decks, this usually involves no more than fitting a new drive belt or belts. Only once or twice have I ever found the takeup or rewind clutches to be the cause of this kind of fault. In this deck, there were two belts involved: the main drive belt linking the motor to the capstan flywheel and a secondary belt from the flywheel to the takeup drive. I had suitable belts in stock so the exercise became one of removing the mechanism, fitting two new belts and reassembling the whole outfit. I can only assume that the people at Sharp had set about making this unit DEL ... best television antenna performance for less cost. as difficult as possible to service. No less than nine screws secured the cabinet halves and, when they were removed, the back of the cabinet separated from the front by no more than 50mm. It was restrained from moving further by the speaker leads, which were so short that I couldn't get my hands inside to release them. I had to use a pair of long-nosed pliers to grab the plugs and ease them from their sockets. After that, it was a simple job to release the cassette mechanism by undoing two screws and separating two plug/socket assemblies. 0103 The troubles begin It was about here that my troubles really began, because there was no way of replacing the drive belts without completely dismantling the mechanism. The motor was mounted upside down to the usual way. It was outside the mechanism, with the drive pulley inside, behind a large mounting plate. This plate carried the main operation solenoid, as well as four microswitches operated by a cam on another plate lower down in the works. The only way I could replace the belts was to dismantle the whole mess, then reassemble it slowly and carefully. There was only one serious prob- Fig.1: the power supply circuit in the Sharp GF-7600Z ghetto blaster. Ripple filter Q103 functions as an emitter follower type voltage regulator. 2SCl684 TAR Ic.+-+-----~~~~----<>-+-__,;111,--, I SWIOt-A I -+-,l--+-+="'F--4---' ... I l~_...,:IWU-V)I 1 Fracarro 10BL4, 10BLS, 10BL45, 1Oelement yagrs that ~ work Peter C. Lacey Services Pty. Ltd. P.O. Box 678 (74Fulton Rd.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 40 SILICON CHIP ' ') I I I I l-o-SWS04 _ _. ' '~I :10001 I I _) 11:.IOC HL!CTOR wow! NIN£ S~L~S AN't> \M\?Oss,~R'-< .... 1"SN 3 S'fLO(Kf;'f - THeN 'Fl~SE~'!AA'\MSt & 6oooovot--r ~~OR:.:1" Sf'E~~R "S~t(E.MN-l'S 1'6~t>-S ! r,, ~- suR~L \~E." SR\,CH E.XPR.ot>l"1G wOOOP~ C.O~t\tOIIJ ~ 'PRUG, c Nv\ ~'fW, ~if'~INJ). 2 ,~ F~~e:: Df'X:>6'1 9 000 / • r o,., CA~ ONL.Y ASSUM~ -rHAi 11-t~ 'PlWPL..e: A-r S HA~P H~'t> SIS."\ ASoOi' MAKI ~ c;. 1'H \ S UN \1'" AS 7::>JFF'ICl/t..T A.'S 'POSSI St..~ -ro S~RVl Ce:..o..lem during the reassembly, as I was trying to refit the cabinet front. One of a row of four function switches along the front of the chassis failed to enter its proper recess in the front panel and the shaft was bent rather badly. It took several minutes to find out what had happened and to determine that no obvious damage had been done to the switch. In all, it was the sort of job that I always wish I had never started when I'm half way through, but then find quite satisfying when it's all back together and working properly. Except that this time, all was not working properly. Sure the mechanism now had a steady 40gm/cm on each function and could rewind a C90 in just a minute or so. But there was no output from the speakers, on either cassette or radio. I immediately thought of the switch that had been abused earlier. This had been the Tape/Radio switch which is effectively the main on/off switch, and a problem there could cause the symptom of no sound. I once again undid the nine fixing screws and exposed the works. I had to arrange extension leads for the speakers, since the short leads provided made it quite impossible to service the chassis when the case was opened. With the chassis laid bare, I soon found that there seemed to be a distinct likelihood that the switch had been damaged in some way. The area around the switch seemed to be very sensitive to pressure and occasionally produced a loud burst of sound for a fraction of a second when the switch was operated. The sound died quickly but when a cassette was playing, there remained a very faint trace of the music if the volume control was advanced to full on. I plugged in a set of headphones and was able to hear quite plainly that the music was seriously distorted. I tried to tune the radio but there was no sign of any program, on any of the four bands. Only the cassette could produce any kind of signal. The distortion made me wonder if the output chip might have suffered some kind of breakdown due to the presumably damaged power switch. But I had no circuit diagram and the boards were a complex m ess of conventional and surface mounted components. I wasn't game to interfere with that lot without a service manual. By this time, I had invested as much effort as I was prepared to put into a job that wasn't going to earn me a fortune. It would have to be put aside until I had had a chance to talk to the owner and find out how much he valued the unit. If he was prepared to pay a reasonable amount, I could afford to buy a manual and spend more time on the job. At that time, I gave no thought to what I would do if h e still wanted it fixed at the old price. As it turned out, the owner must have forgotten all about the radio, since it was still on the shelf four months later. I was reminded of the set when talking with a colleague about another Sharp product. So I asked him if he had a manual for the GF-7600X and, after a quick search, he. turned up the very thing. So I was back in business, whether or not the owner still wanted his set fixed! With the circuit diagram in front of me, and using an au dio probe, I was able to trace the signals through the audio preamplifier from the cassette input to the volume control. These all sounded more or less normal and I was at a loss to know wh ere the distortion I had heard in the phones was coming from. No radio signals However, the audio probe could find no trace of any signals from the radio board. It sounded "alive", but there was nothing that could be called a signal anywhere on any of the bands. One common cause of no response in a radio is failure of the local oscillator. This happens more often than you'd expect and is usually the result of an open circuit oscillator coil. An easy way to test for this fault is to tune the faulty set to a station around mid-scale on the broadcast band and then bring a small pocket radio close to the antenna rod. If the pocket radio is then tuned to the top end of the band, a spot should be found where its oscillator is 455kHz above the wanted station and leakage from the little set will push a signal JUL Y 1992 41 SERVICEMAN'S LOG - CTD through the faulty front end. In my case, however, the exercise seemed be pointless since the fault existed on the FM band, as well as on the medium and shortwave bands. It was too much to expect that both the AM and FM oscillator coils had failed at the same time. So, I had to look for a different cause of my troubles. And, in fact, it was not all that difficult to find. All the inputs and outputs for the RF board came together at a single plug assembly on one corner of this board. And each point was clearly labelled with both its function and the normal voltage on the circuit diagram. The first one I tried was the main supply rail (designated B+) which should have been around 10.SV. In fact, it was only 2.5V - easily low enough to explain the missing radio programs. Without the circuit diagram, I could never have found the cause of this loss of voltage. I traced the 10.SV line back from the RF board to the function switch mentioned earlier. From there , the rail found its way right across to the other end of the main board to transistor Q103. This transistor was wired in emitter follower.configuration and labelled "Ripple Filter". There was about 15V on the collector of this transistor but only 2.5V on the emitter. Sure enough, the transistor was open circuit between base and collector. However, there must also have been some leakage between ~ - emitter and collector, because there was some output from the filter. It · also explained the distortion I had heard from the cassette, because the same rail also supplied the drive stages of the audio amplifiers. The radio was mute because the low voltage killed the local oscillators, although the liveliness I had noticed was probably the result of a still active IF strip. A new transistor soon had the set back in working order but left me wondering about the reason for the sudden transistor failure. It had been perfect right up until the switch was abused but I can't see any connection with that event. The switch wasn't damaged and still works perfectly. It's just one of those puzzling things one sometimes finds in electronic servicing. Funny attitude When I had replaced those nine screws securing the cabinet, I put it back on the shelf and began to wonder if I would ever be rid of it. It had been four of five months since the owner had last put in an appearance and it looked as though I'd have to make a trip to a distant suburb to chase him up. Fortunately, I found his phone number and called to give him the good news. He didn't seem to be the least bit interested but still turned up a few days later, paid the bill without question, and wandered off with the monster under his arm. I got the feeling that he would not - ~ • - P~\ t) '0-\E:- 'gl L,.t.- VJ 1~0\M" /"41, ~£s•no1'l & w~t>&•t.o ~Ff W\1"M 42 SILICON CHIP ~e. N\ON'SrG UN't>Pt ~\S MMu, have worried if I'd never fixed it. So why did I fuss over it? I dunno. I guess I just like fixing things! Thanks J. L., but do you ever worry about the rest of the community who will have to live with the ghetto blaster? Perhaps fate had really intended that it should never be fixed and you have defied the will of the gods. Curses upon you! The pickle foundry And now, here's another one in lighter vein. As C. J. Dennis would have put it, "I know a bloke 'oo knows a bloke 'oo toils in that same pickle foundry." Only it's not a "pickle foundry" and he doesn't boil the "cabbitch storks". No, it's an electronics organisation, and the "bloke" would be better described as an electronics technician. All of which is a round about way of saying that this story came to me very much secondhand, so it's a little short on detail in places. But it goes something like this. The bloke who my bloke knows - hereinafter known simply as the bloke (leaving my bloke out of it) - keeps a plastic ruler handy alongside his video recorder. Why? Well, it's a rather complicated story. While this bloke works in the electronics field, he is not in the service game. Nevertheless, he's clued up enough to tackle service on most of his own equipment. And quite recently he acquired an Akai VS-112EA video recorder - a reasonably modern model with several interesting features. No, it didn't fall off the back of a truck; fell out a garbage can would be more like it. While the history of the machine is rather vague , it appears that it was discarded by its original owner because of its unreliable behaviour. And, after passing through several hands, it finished up on this bloke's workbench - it was his if he wanted it. Of course, by this time, the history of its behaviour had long since been lost. No one even knew what fault it was exhibiting. So he simply connected it to his TV set and gave it a burl. And it worked, in all modes. Too good to be true, surely? And it was. After about 30 minutes it shut down, and wouldn't start until it had been turned off and allowed to stand for some time. Well, it didn't take a genius to con- heated, it could no longer drive the elude that it was a temperature remotor. And when this stopped, senlated fault. We were spared the details of the tracking down but it turned sors in the machine shut everything down. out to be an IC that was overheating. So this bloke simply fashioned a And it was overheating because it had apparently been fitted with a heat- heatsink from a strip of brass. It was roughly "U" shaped, to push over the sink which was now missing. IC, and with a couple of wings to aid There is some suggestion that the dissipation. And it worked; the maheatsink was glued to the IC, since the latter was coated in some kind of chine would now run indefinitely glue-like mixture which was very with no sign of distress. Problem solved? Well, that one, yes. tough but not glass hard. Another suggestion was that this was a heatsink But now another one appeared; percompound which had been baked haps the one that caused the machine to be chucked out in the first place. At hard by the overheating IC. Anyway, the discovery seemed to times, the drum motor would not start, and it appeared to depend on the exmake sense. The IC, a type TA7245F, act position in which it stopped. All fed the drum drive motor and was mounted on a double sided printed it would do was shudder. So the bloke simply put his finger board which carried six flat coils, arranged in a circle, in close proximity on top of the drum and gave Ha spin. to the motor armature. This descrip- And away it went, running perfectly. tion, as it came to me, reminded me of Now this bloke appears to be a pretty a similar type of motor which I de- easy going type. If it was as easy as all scribed in these notes for December that, why worry further? Granted, the timer function could 1990, complete with photograph- the main difference being that that was a . not be relied upon, but the machine capstan motor whereas the bloke's was hadn't cost him anything anyway. a drum motor. And where does the plastic ruler And apparently, when the IC over- come in? Well, he got a bit sick of AUSTRALIAN MADE TV TEST EQUIPMENT 10% discount on all test equipment. 15% discount on two or more items. STES Built-in meter to check EHT transformers including split diode type, yokes and drive transformers. $78.00 + $4.00 p&p TETIA TV TIP JVC HR-7200 video recorder Symptom: no picture, just a number of horizontal lines with snow in between them. The sound seems to be normal. The machine was making a shrill whining sound when in play or record. Cure: the problem is the video head drum running very fast. This is caused by the loss of frequency generator pulses from the drum motor. There are a number of possible reasons for this problem but one unlikely cause was corrosion on connection pins 2/11 and 2/12 on the MDA (motor drive amplifier) board. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute ofAustralia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. pulling the cover off - even with the screws left out - every time the motor baulked. Then he realised that he ANTRIM TOROIDAL TR ORMERS C Built-in meter reads positive or negative voltages from 0-S0kV. For checking EHT and other HT voltages. $98.00 + $5.00 p&p ■ Strong magnetic field. Double insulated for your CAswitch operation. 240V safety with momentary AC 2.2 amps. As important as a soldering iron! $75.00 + $10.00 p&p r TER Designed to test infrared or ultrasonic control units. Supplied with extension infrared detector lead. Output is via a LED and piezo speaker. $84.00 + $4.00 p&p ~~~ - TUNER REPAIRS From $19 exchange plus post & pack Cheque, Money Order, Visa, Bankcard or Mastercard 216 Canterbury Rd, Revesby, NSW 2212, Australia. l".V. TUNER9 • ••~ ~w •·•W, • ~ Phone (02) 774 1154 Fax (02) 774 1154 • • • • Now made in Australia Standard range 1SVA to 2kVA Large ex-stock quantities available Competitive pricing available for OEMs & resellers • Simply the best transformers all round HARBUCH ELECT ON CS T D 90 George St, Hornsby, NSW 2077. Phone (02) ~76 5854 Fax (02) 476 3231 JULY 1992 43 ""<=OU\,t:) ~ 1"1"" WM'"~ ~ ~U~~ 6-IV6 l"'f" 11-let. ~A~ could see the top of the drum through the front loading aperture; and that he could reach it with the ruler and give it the necessary kick start. The theory advanced to explain all this was that a drive circuit in the IC for one of the coils had failed. This may have been because of the overheating problem or, as suggested earlier, as a natural failure which caused the machine to be discarded in the first place. And that is the story as it came to me. Unfortunately, I'm not very familiar with this model. In fact, I don't even have a manual for it and, while I have serviced a few, this was some time ago and did not involve this section. But I did manage to get access to some data and, significantly, it offered the advice that the motor should be replaced if it fails to start. My stock of scrubbers And there the matter rested for a while, as a curiosity story with a number of questions unanswered. But I did make a mental note to keep a lookout for this model should it come into the workshop. Then, more recently, I happened to be scrabbling through my stock of scrubbers - items written off as too costly to repair but kept for possible spares - when I suddenly spied an Akai VCR in one corner. And suddenly its history clicked. It had been a perfectly good machine, relatively new, until Jupiter hurled a thunderbolt into the area and wrecked it, along with several other appliances. Its owner hoped that I could repair it but it was not a proposition. Subsequently, it became the subject of an insurance claim and the 44 SILICON CHIP & Kl Ctc: St"'A~-T;,,,. owner then suggested that I might as well k1:1ep it; was of no use to him. More importantly, it turned out to be a VS-112EA; the very model in question. At the first opportunity I pulled the cover off and took a good look at the drum drive motor. In general terms, it fitted the previous description very closely. The rotor was a mild steel disc, about 18g and 45mm in diameter, and having a flange around its perimeter, about 5mm deep. And sitting snugly inside this flange was a ring-shaped permanent magnet. This rotor assembly was mechanically coupled directly to the drum, immediately above it. The stator consisted of the previously mentioned assembly of six coils, the driver IC, a smaller IC, and some minor components, all on a double sided PC board, and most of them surface mounted. The board was similar in width to the disc but about 70mm long. It was semi-circular at one end and rectangular at the other, this latter area accommodating the IC and various other parts. But there was one very significant difference between this board and the description as it had come to me; there was no heatsink on the IC. And, clearly, there had never been one, nor was there any need for one. I had "known" the machine since it was new and there had never been any suggestion of overheating problems of any kind. So what did it all mean? At this distance one can only speculate. The best suggestion I can come up with is that the whole thing started with a fault in the IC, long before the bloke came into the picture. This caused Top view of the drum motor stator board in the Akai VS-112EA video recorder, with the rotor (magnet) in approximately its normal position. In practice, it is suspended slightly above the board, below the video drum. This underside view of the stator board shows the surface mounted components & IC TA7245F in the top left corner. The motor drive coils are under the circular steel plate. the overheating and subsequent shutdown as already described and someone else decided to fit a heatsink, which they glued on. Only the heatsink wasn't big enough and/or the glue failed, and the experiment was written off as a failure. Then, later, it finished up on this bloke's bench and the rest we know. Anyway, I'm planning to send the motor board down the line via my bloke, so that this bloke can make a proper job of it. I reckon it is a fitting reward for a story like that. SC