Zenith jumpy picture ...

[Electronic M]

If back feeding the video output with input is giving different brightness and sync behavior, then on top of the sync sep being suspect, there could also be an issue with the gain levels in the video chain feeding it.

[rwagoner]

Obviously I am not a tech, though I am good at following directions for building/repairing and soldering/desoldering... Should I be looking (for things like leaky caps, I assume) on the input board (attached to and part of the main board) or just anywhere on the main board? Or -- and I know this is hard to say without actual diagnosis -- is this probably more than a bad cap?

I do appreciate the help, even if the ultimate advice ends up being to either have someone qualified to look at it or give it away if someone wants it. I certainly don't mind trying things.