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SERVICEMAN'S LOG It sounds like Donald Duck Poor old Donald! He takes the blame for a multitude of problems, not the least of which is the way people use him to describe a multitude of distortion effects. So much so that I sometimes wonder what they would do without him. Perhaps he should charge for this service. Truth to tell, not many forms of distortion really do sound like Donald Duck. And that's not surprising I suppose, because I imagine that the true Donald Duck sound is a lot more complex than most common forms of distortion. Anyway, to get down to business, the problem involved another National NV-G7 video recorder, at which I recoiled slightly after recounting last month's notes. However, one has to take the good with the bad. The customer - one of my regulars simply dumped it on the counter and came out with the above heading; "It sounds like Donald Duck". "Any problem with the picture?" "No - picture's perfect. But the sound's just like Donald Duck". And that was about it, as far as the customer was concerned. He was obviously convinced that all I had to do was wave my magic screwdriver and all would be well. I find such faith quite touching. When most people use the Donald Duck analogy they mean a recording that is running fast, even though this is not strictly accurate - at least from Donald's point of view. But I couldn't reconcile such an effect with the fact that the picture was supposedly perfect. If the tape was running that fast the picture would also be running fast - assuming that it remained locked. So I set the machine up and pushed in a test tape. And the result was quite surprising. Yes, the sound was funny and it did seem to be some kind of speed abnormality, although I couldn't define it initially. And, yes, the picture was perfect; normal speed, no noise bars, rock steady. So what was happening to the sound? I listened to it long and carefully a number of times before I came to any conclusion. Finally, it seemed to me that the sound would run at normal speed for one period, then hesitate briefly as though the tape had stopped or slowed significantly, then resume normal speed, then hesitate, and so on. I also judged that the normal period was longer than the hesitant one, while the whole process was happening quite rapidly, producing a high speed chopping effect. The end result was certainly strange, though I doubt whether Donald would have been flattered by the comparison. But Donald's sensitivities aside, I had a problem to solve; what was the most likely cause of such an effect which I had certainly never encountered before - and how was I going to track it down? Transport problem This is the stator from the capstan motor in the National NV-G7 video recorder. It consi!!ts of 6 flat windings on a PC board, together with three small surfacemounted solid state devices at the centre of three of these windings. 48 SILICON CHIP Well, it had to be a transport problem of some kind: the capstan drive, the drum drive or, more specifically, the servo systems associated with them. But which or what? And why was it not affecting the picture? I sat down and had a good long THIS TESTER CAN PAY FOR ITSELF IN LESS THAN ONE DAY <at>Dlliu CHECK,. 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A variation of tape speed will affect the sound directly. So it was conceivable that we had a capstan speed variation which was upsetting the sound, but was within the capture range of the drum servo system. In short, a capstan motor fault. It was all pure theory, of course, but I felt it was strong enough to justify checking the capstan motor and replacing it if that was the only way to prove the point. And at that point I had no knowledge of just what kind of motor was involved. Nor was there much about it in the manual. But when I delved into the innards of the machine I found a capstan motor like no other I had ever seen before. Not that I'm knocking it; on the contrary, it is a robust, well designed component, much larger than the usual run , easy to get at and take apart and, in this case anyway, economical to repair. The two main parts are the rotor and stator. The rotor is simply a metal disc, about 75mm in diameter, which runs in close proximity to the stator. Being such a simple device, I felt it was unlikely to be at fault and turned my attention to the stator. The accompanying photograph gives a good idea of the stator construction. As can be seen, it consists of six flat windings mounted on a PC board. A circle enclosing the windings would have a diameter of about 75mm, matching the rotor diameter. The stator board itself is about 90mm across. In the centre of three of the coils is a solid state device, the purpose of which is not clear. The external connections to these devices and to the coils run via a flexible copper pattern which mates with an edge connector. $1499.00 Plus Tax BONUS - SINGLE CHIP ADAPTER FOR TESTING 64K x 1, 256K x 1, IM x 1, and 4M x 1 DRAM chips PACIFIC MICROELETRONICS PTY LTD 'CENTRAL PARK' UNIT A20, 4 CENTRAL AVENUE THORNLEIGH, NSW, 2120 Telephone: (02) 481 0065 Fax: (02) 484 4460 Australian Representative FUJITSU MICROELECTRONICS PACIFIC ASIA LTD. o) FUJITSU DECEMBER1990 49 SERVICEMAN'S LOG - CTD "' ~ '6> .,.· + _y,. Jl . • .., • .· .,. As encountered in the machine, the stator is the other way up to that shown in the photo. It is held by three screws, the holes for which can be seen, and across the back of the board is a bar, held by two screws, which carries one bearing for the rotor shaft. So undoing five screws takes the whole thing apart. But before I took it apart, I gave it a physical check-out. In particular, I checked for free movement of the rotor, adjustment of the bearings, etc. As far as I could tell there was nothing wrong here, which really left only the stator assembly. That led me to another pleasant surprise; a replacement stator was available for around $30 which is quite a modest figure considering that some capstan motors are well over the $100 mark. On that basis I decided on a replacement rotor. Even if I was wrong (and I felt pretty sure I was right), it wasn't a great deal to risk. 50 SILICON CHIP But I was right; the new rotor fixed it and Donald was sent packing. And the final cost was quite reasonable, partly because of the modest replacement cost, plus the relative ease of fitting. So I finally had another satisfied customer. Two Donald's While on the subject of Donald and his mutilation of the English language, I had another video recorder recently against which the same accusation was levelled. In some respects, the cases were similar, in others quite different. It was one of those sameonly-differnt situations. For a start, it was a completely different brand of recorder; a Sharp VC381X. It was also intermittent and, probably as a by-product of this, had something of a history to it. The customer was a stranger, having recently moved into my area from the Orange district. And that was where the fault first appeared and was tackled by a local serviceman. Judging from what I found later, he had tackled the problem logically, had obviously gone to a lot of trouble, and had done everything in a thoroughly professional manner. And, according to the customer, he seemed to have fixed it. It ran for a couple of months before they moved and apparently survived the move, for a few weeks at least! But then suddenly the old fault was back. And the nature of the fault? "It goes Donald Duck. Y'know, everything speeds up. But only sometimes; usually after it's been on for about 20 minutes". The speed-up included the picture which, he explained, was still visible but running fast, with lines across it. "Y'know, like in fast forward, only with Donald Duck sound" . All of which was a pretty good fault description; better than I get from most customers. That, and the history I related above, at least gave me a head start. In fact, I had already mentally .registered that it sounded very much like a capstan fault; possibly in the capstan servo system. At first switch-on, the machine behaved normally and I let it run with a test tape. Then, just as the owner had predicted, after about 20 minutes it suddenly ran amok, with speeded up picture and sound. I switched it off and let it rest for half an hour or so, then tried again. But it was still faulty. I removed the top cover, then lifted the top board, which is hinged at the back, thus gaining access to the mechanical area. The machine performed in the same faulty manner while I looked over this section but there was nothing obvious. I turned the machine off, turned it on its side, removed the bottom cover, and swung out the bottom board, which also hinges at the back. This carries the capstan and drum servo circuits, although it was the capstan system that I suspected. As I explained in the first story, incorrect capstan speed can normally be accommodated by the head drum, which is locked to the reference pulses on the tape. The picture speed will be incorrect and it may display cross-over lines, but it should remain locked. The sound speed will likewise be incorrect. completely aimless exercise. I turned it off in disgust. Later, I switched the machine on again, only to find that all the faults had vanished; it was working perfectly. So what had I done during my aimless probing. I didn't wake up immediately, but at that stage the machine was on its side, with the top and bottom boards folded out at right angles. In order to turn the machine over and get a better look from the top, I closed the bottom board. Whereupon it shut down. I swung the board out again and it was working. I immediately suspected a hairline crack in one of the tracks, and went through the routine of twisting, prodding, and generally abusing the board in an effort to get a lead on it. All of which proved completely fruitless. Suddenly, I woke up to the fact that it was the position of the board that was critical. Swing it in from the open position and all would be well until it was within about 50mm of fully closed, whereupon the machine would suddenly shut down. Open this gap ever so slightly and it would work; close it again and it would stop. But then, when the board was fully Closer inspection of the bottom board around the relevant servo IC, IC706, revealed that the previous serviceman had had the same idea. The IC had obviously been removed and no doubt replaced, along with several other components in this section. And, as I said earlier, it had all been very neatly done. At this point, I turned the machine on again. And lo and behold, it was back to normal. But not for long. Its next trick was to really run amok, displaying not only the high speed fault, but a swag of other faults as well. A revolting development Pressing the PLAY button could send it into the fast forward wind function. Or rewind the tape. Then the dew light came up and the machine shut down completely. As another cartoon character is wont to exclaim, "What a revolting development"! Yes, revolting it was because, right then, I didn't have a clue where to look next. I went through the motions of going over all the components, prodding and poking, but it was a CROSSOVER FOR BETTER PERFORMANCE TO BRIDGE STATIC RAM DYNAMIC RAM 4164-10 41256-8 4464-10 41C1000-8 44C256-8 64k x 1 100NS 256K X 1 SONS 64K x 4 100NS 1M x 1 SONS 256K X 4 SONS EPROMS 2716 2732 27C64 27C256 $9.95 7.95 6.50 7 .50 $2.60 3.00 3.00 9.50 9.50 61161 6264L 622561 2k X 8 BK x 8 32K x 8 100NS 100NS 100NS $2.50 3.80 9.95 FD 55BR 360K FD235F 720K FD55GFR 1.2M 235HF 1.44M TEAC TEAC TEAC TEAC 5.25" 5.25" 5.25" 3.5" $126.00 126.00 130.00 140.00 (AUSTRALIAN MADE) 12 Months Warranty SHORTED TURNS TESTER Built-in Meter to check EHT transformers including split diode type, yokes and drive transformers. 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 VOLT AGE PROBE Ideal for checking microwave ovens and TVs. The ranges are from O to 5kV Negative and from Oto 10kV Positive. Double insulated fo r safety. 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After several tries, I happened to move the AN (12-conductor) cable a certain way and the dew light came on and the machine shut down. The necessary movement was quite critical but, with care, also quite predictable. So, after putting it $180.00 $8 4 INPUT / OUTPUT CARDS AT 1/0 CARD 1 SERIAL, 1 PARALLEL, 1 GRAPHIC AT 1/0 CARD 2 SERIAL, 1 PARALLEL, 1 GRAPHIC $45.00 55 .00 .oo + $5 .oo p&p · ·. TO ORDER PLEASE RING (02)369 2745 WITH YOUR CREDIT CARD DETAILS - WE WILL SHIP THE GOODS THE SAME DAY. If you need any computer product not listed here, please give us a call. . , $75.00 + !_~.DO p&p_ '(~-..... ,"°_. .. ~ Prices include 20% Sales Tax. Delivery $4.00 anywhere in Australia REMOTE CONTROL TESTER " n ¼,, . ) ;) $85.00 + $4.00 p&p l2.,.......,.....,._.....i.....,.,. i;:10 [D)<at>Dll BRIDGE ELECTRONICS ~ PO Box 272, BONDI, .. NSW 2026 ~ j ;(•l: ' (02) 369 2745 FAX (02) 389 2063 PHONE ! :(3i He£: DEGAUSSING WAND 7,700 apere turns. Strong magnetic field , larger than usual coi l with multicore centre. Double insulated for safety with momentary on/off switch. 240V AIC 2.2 amps. This one is just about as important as having a soldering iron in your toolbox! (INFARED OR ULTRASONIC) 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. TUNER REPAIRS trom only $11.00 + p&p Chekque,dMonMey Qtrder,d 8 an car or as erca~ 11JNERS1\216 Canterbury Road, Revesby, • ~ NSW, 2212, Australia Telephone (02) 774 1154 We also repair most tuners on an exchange basis. TV -.,• DECEMBER 1990 51 SERVICEMAN'S LOG - CTD through the go/no-go cycle several times, I knew I was close. After the first failure I imagined that the plug and socket assembly would be the most likely culprit but, after several of the above cycles, I changed my mind. The fault was definitely close to the bottom board end of the cable, where it was soldered directly to the copper tracks. So it was either a break in one of the conductors, or a dry joint at the board. Now I reckon, after all these years, I can pick a suspect joint with the best of them; which is not the same as claiming to be infallible. Nevertheless, if there was a dry joint there I felt sure a close examination would reveal it. So I turned the machine over, reached for the jeweller's loupe, and went over each of the 12 connections from every angle and with every lighting angle. And I would have passed every one of them as a perfect joint; there was absolutely nothing I could see to arouse any suspicion. On the other hand, a broken lead was a long shot. There is no flexing of the cable, apart from servicing activities, so why should it fail. In any case, the easiest thing to try was the soldering. I reached for the iron, unsoldered each joint, examined it, and carefully remade it. And this exercise did nothing to change my mind; I found no evidence of any fault. I was convinced that I would have to replace the cable. But I was wrong; when I put the recorder back into operation it came good immediately, and nothing I could do with the bottom board would create the fault. I was only partly convinced, of course; I've been caught like that before. I closed both boards, refitted the covers, set the machine up on the end of the bench, and put it through a number of cycles for the rest of the .1.m.,.;. r..1;;,:•.;: RCS Radio Pty Ltd is t he only compar,y which manufactures and sel ls every PCB & front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. 52 SILICON CHIP TETIA TV TIP Hitachi CTP-229 (NP8CA Chassis) Symptom: excessive brightness and not enough control to bring it down. A "sort of" picture could be obtained by reducing the screen voltage but this revealed heavy shading on the left hand side of the screen. Cure: C719, a 4. 7µF 250V electro, open circuit. This capacitor is a bypass on the 180V rail feeding the video output transistors. This fault is rather like a screen-voltage problem but the clue is that with this one there are no retrace lines visible. 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. day. It never missed a beat. Dew sensor At the end of the day I disconnected it completely from the mains and let it cool over night. But when I switched it on next morning we were in trouble again. No, it didn't Donald Duck but the dew sensor came up. My first reaction was to rip everything apart and start again. Fortunately, second thoughts prevailed. I simply left it on with the dew sensor flashing. After about an hour, the sensor shut down and the machine came good and stayed that way for the rest of the day. I shut it down again that night, and the dew sensor came on again next morning, clearing itself after an hour. And the same thing happened the next day. By which time I was more than a little suspicious of the dew sensor itself. So much so that I ripped it out and fitted a new one. And that really fixed everything. Many days of testing, under all conditions, failed to reveal any problems. So Donald had been sent packing again, and everyone was happy. But I make it a practice in all cases involving intermittent faults to warn the customer that it might recur, and to come back immediately if it does.