Sorry to hear about your troubles. Have you opened it up to see if you can see anything loose? Possible something simple to fix. Might be a simple solder break or connector from the right channel supply. Why not take it to a technician?

I will indeed do that - before I make that move, I was hoping for one or two guidelines if possible.  I'll be looking for ?? the toroidal transformer?  I'll search up a schematic and post it...

First steps:

Is it just the R main output that is mute? Do the remaining three main outputs (L,C and M) work normally?

If all the others work, check that there is not a TRS plug stuck in the R main insert jack. If there is not, use an insert cable (TRS to 2x TS plugs) plugged into the R main insert jack to see if the R signal arrives at the send tip on the cable. This can be checked by touching the tips of the two TS plugs together and looking for output on the R main, and then if not, by plugging the send TS plug into anything that will amplify it, e.g. a guitar amplifier.

If shorting the plug tips produces signal at the R main output, then the likely fault is with the normalling contact on the R main insert jack, and the jack needs to be replaced.

If using an amplifier shows that signal does arrive at the send plug tip but shorting this to the return plug tip produces no R main output, then signal is not passing through the R main VCA, and further investigation is needed.