Barry777
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Jul 24, 2010, 3:45 PM
Post #39 of 41
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Re: [jts1957] Fisher VCR Model FVH-720 (very old) No Power
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I guess at this point, we would have to measure the current draw of both motors after replacing the IC again, and look on the IC's data sheet to see if it's capable of supplying that much current. If you're not familiar with current measurement, it's done in SERIES - meaning you have to disconnect one of the motor wires and insert the meter into the path. Most meters have a separate jack for current measurement, and it's important to IMMEDIATELY plug your test lead back into the "voltage" jack when finished. Or, you'll forget and end up trying to measure voltage with the meter set up for current and blow something - usually the meter's internal fuse, but possibly something in the equipment you're working on. This is because a current meter presents a short circuit at the test probes, due to an internal shunt that routes most of the current around the actual meter.
jts suggesting motor replacement along with the IC bears out my previous suspicion, that the motors are drawing too much current. When I talked to the guy about my ADAT machine and he warned me to check current draw when replacing the drive IC, I thought "The motor is fine" because it turned freely. But sure enough, it was drawing more than twice its normal current and would have blown the new IC had I not kept a finger on the chip to monitor its temperature. If your tape drive motor is one of those large flat "pancake" types, I would definitely suspect excessive current draw from that puppy. Half of my drive IC runs the flourescent display, so it sounds like a fairly similar IC.
In my case, my motor had about a quarter-inch-thick circular magnet (looks like a large, very thin doughnut) around the inside of the case. Since magnets are quite brittle, it developed a crack from either temperature variations or something. When a magnet cracks, it radically changes its magnetic properties as the edges of the crack suddenly become north and south poles. That poor motor was working its butt off trying to overcome it, and appeared to be working fine in operation. But the driver chip got super-hot in FF and REW modes.
Since this would be a very hard motor to find (and cost $50 to boot), I'd scrap the machine if new PS caps don't do anything for it. Since I stock thousands of all standard values, recapping is a very common practice in my shop, and often fixes mysterious problems.
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Barry Fone - VCR Repair hobbyist and professional FAA Repair Station Avionics Bench Technician (top level). TEST EQUIPMENT: (4) Sencore VC93 VCR Analyzers, (11) Tentel gauges, Sencore VA48 and (2) VA62 Video Analyzers, Sigma Electronics TSG-375 NTSC/SMPTE Video Generator, several VCR Alignment Tapes, plus countless Oscilloscopes, Frequency Counters, Wow and Flutter Meters, Distortion Analyzers, Vectorscope, 136-channel Logic Analyzer, Signature Analyzer . . . . VIDEO GEAR: (6) JVC BR-S822U's, (3) JVC BR-7000 series, (3) JVC BR-S500U, (2) JVC BR-S800U, JVC GR-800U and (2) GR-860U Editing Controllers, Sony FXE-100 Video Switcher/Effects Generator, (5) Time Code Generator/Readers, (1) Sony SLO-1800 (Beta), (2) Alesis ADAT-XT, (1) Sony DXC-1200 TV Camera, Pioneer VP-1000 Laser Disc Player plus many consumer grade Beta and VHS VCR's. Passionate electronics enthusiast since 1973...most anything except computers. Check out my collection at www.barrys8trackrepair.com/MyVideoGear.html
(This post was edited by Barry777 on Jul 24, 2010, 3:48 PM)
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. I know full well the VCR isn't worth 2 cents of anyone's time. It's just that I feel that the darn thing as gotten the best of me, as insane as that may sound. I think I'll hang on to the unit just to see what jts has to say. I do recall him mentioning in a prior post about a RESISTOR that may have something to do with that specific IC. 
