I think I got it sorted out... I checked for sound coming from the tape, which sounded okay, so that told me that the control head and speed control circuitry were working fine. That led me to checking the voltages on the "cylinder phase speed control" and "cylinder drive" chips, which all measured fine. Next, I started probing the pulse generator circuit from the cylinder, and found a really bad looking waveform on pin 27 on the "phase speed control" IC. This pin goes through a 100k resistor to a 12v rail and a .33uf capacitor going to ground. I measured the capacitor, and it was only slightly off, but when I replaced it with a .22, the waveform looked better, and I had video. I think this capacitor needs to be .33 uf, because when I added some up to make .3, the waveform and video looked better. I never knew a capacitor drifting from .33 to .51 uf could kill a vcr.
04-15-2020, 08:20 PM
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