Non-CRTC PET video circuitry...

[tony duell]

> t's only drawing a single line of characters (garbage with some noise
> due to a very terrible socket holding the character ROM), centered
> vertically.  I initially thought this was a VRAM addressing issue (a
> stuck counter or whatnot) but it turns out the VSync signal is much too

Since the vertical timing signal is triggered by the counters getting to end of frame, are
you sure it's not a counter problem? Are D6 and D7 counting correctly?

> fast -- it's only about 1.2ms long (it's supposed to be about 16ms).
> I'm actually surprised the monitor can deal with that without damage, so
> I've been leaving the monitor disconnected while debugging.  The HSync
> period is correct (63.8us or so), and most of the addressing / character
> display logic (which is driven by earlier portions of the video timing
> chain) seems to be running correctly, it's just only getting a chance to
> draw one line of characters before the next frame starts.

> I'm not having much luck tracing down the fault; I have a schematic
> (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320008-3.gif)
> but it's not making it easy to work out exactly how everything ties
> together.  I can't find anything resembling a service manual for this
> model, but perhaps I haven't looked hard enough yet.

I agree, that's  not a nice schematic. It's not obvious if an input (e.g. a reset input) is actually a logic
signal or just pulled high. You have to trace it all over the diagram. 

Anyway, I think I would start at C6 pin 13. Is it stuck high, or totally the wrong period?

Then check the gates ay the bottom next to the copyright notice, E8, etc.

-tony