If it hasn't been mentioned yet, I'd start by unsoldering the DC-DC
converter that supplies -12 and temporarily replace it with a 9v
battery to see if the converter was at fault. Check for a dead short
before you hook the battery in...
FWIW, I've blown 1488/89s by dragging my feet across a carpet to
generate sufficient potential to cause a small arc when I plugged in
the comms cable (the far end wasn't yet connected).
Cheers,
Chuck