Don Bruder wrote:
Sounds like scragged RAM to me...
What happened? Did it take a lighting hit or something?
Turns out it was really only the 74S175 at B1 that was broken, which I
had already replaced. The other problems came because pin 8 on the
74LS08 at H1 was bent when I inserted a replacement chip in there while
still searching for the original problem. Thus the "set A14 line to
active while fetching graphics data" that's really only applicable in
HGR2 mode was always active, so that the screen showed the $4400 area
in text mode, and $6000 in HGR mode. Some games seem to store a
"working area" copy of the HGR screen at $6000, which was the origin of
the botched graphics I was seeing.
The speckles in HGR2 and the rebooting were because one RAM was
likewise not making good contact. And the odd-sounding disk and speaker
sounds I mentioned were just figments of my imagination combined with
bad memory (my own, not the Apple's; the machine was out of working
order for about a month or so), in reality everything sounded normal.