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So it turned out to be a bad main filter capacitor. It was only noticeable in the sync amp, because the other amplifiers are push-pull, and cancel hum, while the sync take off point is single ended and does not cancel hum.
All it took was probing the sync input to the oscillator with my EICO 425, and then observing the massive power supply ripple on top of the sync signal.. The ripple was causing the scope to not be able to synchronize properly with any signal. I temporarily connected a capacitor in parallel with the bad filter capacitor, and suddenly everything works properly.
This should have been more obvious to me, but since the scope makes a straight green line, I just assumed the filters must be OK - well, live and learn! I will proceed with a full replacement of all the electrolytic capacitors, and then follow the calibration instructions.
I'm going to restore the EICO 425 while I am at it, it did a great job helping me diagnose the OSK-4.
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