Apple II+ Video questions

David Williams nospam212-cctalk at
Mon May 10 14:27:29 CDT 2021

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

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