[old_tv_nut]

OK now - if the picture control goes through a range of weak picture to too strong, and there is no change to the sync, then it is likely the sync separator circuit that is the problem.

An oscilloscope would be helpful if you have a reference telling you what amplitude the waveforms should be. If not, you would just be guessing. I can only make this general statement, but maybe someone else here can give you a reference.

It sounds like you need to know the normal signals at the input to the sync separator and in various parts of the sync separator. Before you do that, though, a check of DC voltages in the sync separator circuit may point to something.

Edit: I see Riders has waveforms, so there you go.