Be sure to clean the function switch real good.
After that its a case of injecting or tracing signals.
After the pre amps there will be a coupling cap to the PA's. From
that point everything is direct coupled so things get a little
more tricky. Run it a few minutes & see if either channel is getting hot.
When outputs short often the "bad" channel sounds better. The "good"
gets low B+ & barely works. Confusing !
A manual would be nice but in the amps the mirror image makes
things quite doable.
BTW with direct coupled PA's its best to change all semi's & lytic
caps in the bad stage. If you leave a leaky transistor behind everything
goes again. Especially on high power amps. Takes a microsecond to
blow everything out again.
73 Zeno
LFOD !