[Pamphonica]

I suggest that if you are worried about damaging the pcb, make up a small pcb for the -12V supply and test that outside the amplifier first so you can check the results.

Or at least disconnect the -12V line from the rest of the amplifier while you are testing.

Pin 1 (input) of the 7812 goes just to the +ve of the big 1000uF capacitor (rectified dc from the diodes), nowhere else.
Pin 2 ("ground" or common) goes to the -12V line (the other side of the 1000uF capacitor)
Pin 3 (output) goes to digital ground and diode cathode (white ring)

Just try removing the 7912 (or the transistor) and putting the 7812 exactly in its place. But make sure pin 2 (centre/tab) does somehow end up connected to the -12V line, isolated from chassis.