9 May
2021
9 May
'21
5:53 p.m.
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.