10 May
2021
10 May
'21
7:27 p.m.
On 2021-05-09 16:53, Brent Hilpert via cctalk wrote:
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.
Someone suggested bad ram and as I tried to boot DOS 3.3 it would drop
into the monitor and display some memory. Since it loads into high mem I
pulled the language card and the top 16K of ram leaving me 32K in the
system. Now it turns on and the vertical sync is fine but have other
issues. Namely it wouldn't give me a beep and prompt, just the standard
screen of @ and ? which is normal for boot up but wouldn't respond
otherwise. So pulled the next bank of ram and it would boot and give me
a prompt but was still randomly failing in various ways.
So I am guessing some bad ram is possible cause of the sync issue at
least, though there appears to still be other issues with the board
somewhere else. As the sync made it hard to follow stuff on the screen I
wanted that fixed first. I need to work on the keyboard next as having a
bunch of keys not responding makes it difficult to find issues. For
example the "3" doesn't work so can't get into BASIC from the monitor
even. At least I feel like I'm making progress.
David Williams
www.trailingedge.com