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I think I've finally isolated this problem - would appreciate thoughts from the group before proceeding.
Cooling didn't produce any results, so I started doing voltage and waveform measurements. I have proper waveforms at the AFC diode (W11 and W12) on the Sams) - but virtually no bias or signal on the grid of the 6CG7 horizontal multivibrator. I've checked the values of the resistors that I can access from the pins of the K6 Couplate, and they're within tolerance, and subbing for the 220 mmf doesn't help. So my best guess is that the sync signal is being shunted to ground by a failed 50000 mmf capacitor inside the Couplate.
There's a little bit of really noisy signal (<100 mv) visible on the grid, but nowhere near the 3.5 volts the Sams specifies. Maybe it's enough to sync the horizontal for a few minutes, then the horizontal just goes open-loop which would explain the symptoms I have.
Unless I'm going wrong somewhere I reckon the next step is to make a replacement network using discrete components and see if I'm right.
Thanks again,
Bob
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