I doubt that the fault is on the input selector part of PCB2 where you appear to have been changing components. Whilst it has the circuit for the input select and line condition change circuit are on this module, your "Problem Statement" seems to suggest that for the few seconds it apparently works i.e. the green light is on and the sound lasts for a much shorter length of time, so the amplifier is more likely to be drawing more current than it should.
I suggest that in its failing state you check the + & - 68 volt supply. You can check the +68 volts by measuring across C46 on PCB1 and the -68 volts check across C49 on PCB1
If those are fine, then check the -15 volts at pin 4 of IC1 on PCB2, we know that +15 volts is fine because the Red/Green Led is working. The input selector area only uses +15 volts.