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…)
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