So I've got my "hacked-up" PET powering up after redoing a bit of wiring
on the power supply. I replaced a couple of obviously bad 74LS157's in
the video generator section (legs were corroded and some had fallen
off/cracked) and reflowed a couple of suspicious looking joints.
The machine now powers up, the display is bright and crisp but I just
get garbage characters on the display. Using my logic probe I've
verified that the CPU is running, voltages appear nominal (4.9V at the
CPU & RAM). The CPU seems to be running something, in that about 1-2
seconds after power up a few characters on the screen will change (not
where you'd expect the COMMODORE BASIC banner to be printed, but always
in the same places). Typing on the keyboard produces no effect.
I've reduced the board to 4K of 2114s and tried swapping around the
2114s used for the display, but the only effect this has is to change
the pattern of garbage characters displayed (interestingly (at least to
me) each RAM chip produces a different pattern, which is reproducible).
If I run without any video RAM installed, I just get a checkerboard
pattern onscreen, which would make sense since character FF is the
checkerboard tile. Unfortunately I have no idea if any of the 2114s I
have are good or not, though I'd wager that they can't ALL be bad. I
don't have any other machines that use them and I don't have any spares.
Any suggestions on logical places to start looking from here?