As peresumably another bloody PbF fault both channels are fine with me and
failing channel not known. From my reading of the circuit as the owner could
carry on with
one channnel (half mono rather than half stereo, as a mono amp) the
protection circuit must have stayed inactive. So ps and DC of both pa must
be ok so leaves output wiring from an amp which is pretty robust or mixer
stage between the dividing path of IC203 , IC206 , speech/music switch and
the "stereo" master pot or preamp area of an output amp, or is there another
area that could lead to this situation. When it was consistent failed, the
owner swapped leads around and confirmed not a speaker or speaker-lead
problem

That daughter board with the reverb that slots down into the small fan
outlet duct is also in line. If absent then it kills the output audio , both
ch, and protection stays inactive. The 4 opamp section rather than the
reverb section
is a line in or out there is broken then loss of a channel, reverb filter is
monofied I think.
Nothing hot on that board in use, just
somewhere to stow it presumably.
Incidently its headers need a needle inserted to separate as nibs hold the
header in more firmly than usual headers.
As this is an early one without the extra vent holes then perhaps overheats
a PbF joint on this board or headers although no obvious cracks/movement
seen under x30.
Looks like one of those suck-it-and-see / cross-fingers jobs, redo main
solder joints there, sets to header pins and return to owner if engraving
tool vibration/hot air ellicits nothing. If it recurs then at least get them
to note which channel