Replaced the Mosfet on the A109 today and everything was working fine for a few minutes then the protection relay clicked on, sound switched off and the input LEDs all flashed on/off which I think is a fault signal. Checked on the heatsinks and they were really warm and the Mosfet that failed and a new one installed was way to hot to touch for long, the other 3 were also very hot. Removed all fets, tested and they all tested OK so re-installed and tested the speaker terminals for the presence of DC and the left channel has a slow creep upto 3.5V before the protection relay cuts in. Back to diagnostics again
Desolder and lift the gate legs of all the FETs, power up with 100Ω resistors in place of the speakers. It may be easier to cut a slot in the pad to isolate the gate; if you do this, make the slot thin enough to easily bridge with solder when you're done diag-ing. If voltage creeps up on the outputs, then you have a FET leaking at operating voltage. Design of the amplifier determines whether that will be the source or drain of the FET; but you'll be able to identify the leaky FET by looking for the same voltage as at the speaker terminals.
If you're lucky, the new parts are okay and its some semi in the next stage up or even the one before that; these are often ICs but may be discrete. You'll be able to find the culprit by tracing back from the similarly creeping voltage on the gate pads of the finals; this is usually because the original failure cascaded excessive voltage up the amplifier stages.
Yeah, it's a total PITA. There's a reason most repair shops stopped fixing those damned things; hard to justify charging for hours of diag time on a stereo that can be replaced for $200-300 with new speakers and remote.
mnem
*tzzt*