A quick drop by:
The pictures are great, detail i can work with - except for the shadowed area.
I can see signs of previous attempts on this signal loss.
We need to take that board out of the front panel, insulate the daylights out of it between the components and the bottom of the unit, and operate the unit.
then get a almost straight on down picture showing it resting out of the chassis.
just unscrew the panel mechanical mounts
pull it in
about an a few inch(es) or so, and then up.
you have done this if you replaced caps and transistors on it.
This way we will have full view of and access to the circuit patterns on it.
because - wouldn't you know it - the potential problem areas are all hiding in that shadow.
The missing right channel is right next to the edge in spots.
With power off, before insulating and securing it, there are factory jumper wires on it - be sure they are all intact and connected.
(the empty spots are the volume and balance controls.)