On 2021-May-09, at 2:16 PM, David Williams via cctalk wrote: > Working on restoring my very first computer, an Apple II+. Got it mostly > working with just a couple of issues identified at the moment. Some of the > keys on the keyboard don't register but I can work on that. The real question > I have is about the video, it constantly rolls and no amount of playing with > the vertical hold on any monitor I try will completely stop it. I can get it > close but it will slowly roll one way or the other at best. Tried several > different monitors and it is the same on each. Also tried my old Apple IIe > and it seems fine on all the monitors so trying to decide what might be the > issue with the II+. Any ideas or areas to look?
It sounds like the vertical sync pulse may be missing, or 'weak', in the composite video output signal. You 'get it close' with the V-Hold as your adjusting effort brings the monitor frame rate (V frequency) to match the frame rate the Apple II is generating. Pull out the schematic and look at how V sync is generated. Probably easiest to start at the video output and work backwards, to where the combined V&H sync are added to the pixel video to form the final composite signal, then back to where V sync & H sync are combined, and so on.
