First where things are :
In front of the jack pack, on the main PCB is the
audio out IC. Its on the heat sink.
The jack pack has all the ins & outs. The SPKRS are
the two wires that plug into the top. Try swapping them.
Also you can do the swap right at the SPKRS. If the dead
channel "moves" the problem is on the chassis. If not
its the SPKR. ( SPKR plugs may be keyed).
If on the chassis look for cold solder at the out IC & the
jack pack. Especially look around any jacks that have been
used a lot putting physical stress on things.
One other thing to try is unplug the set over night
then try again. That will cause the micro to do a complete
reset & can solve a lot of problems.
73 Zeno
LFOD !