Apple II+ Video questions

[Brent Hilpert]

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.