I have had this Concept 16.5 for 20yrs. Been great. 10yrs ago replaced power switch. I had lost a channel. This time it was not the power switch. I put a new one in anyway.
I opened up the removed switch, checked the contacts, other than being not true to each other, they were fine.
Recently I had the R channel fail (A&B speakers) here and there, no real pattern.
Both A&B at same time. Went inside the Concept, found dirty contact connection at the Loudness plug to the board. Touched it and it R channel would go off and on. Cleaned it up, all good.
While in there, thought I would see why my "C" speakers will not output. (I know only two sets of speakers at a time)
So "A" and "B" speakers off, Relay for "C" never picks up, when "C" button is pushed.
I have the 38+/- volts at the "C" switch. Goes thru 270 ohm resistor, each speaker switch goes thru a 270 Ohm to drop it to 24+/- to the relay, to pick up the speakers A,B, or C.
So if a relay is picked up you will see 24+/- volts at it, but an open circuit to the relay you will see the 38v+/- (no load) I never see ANY voltage going to the relay/board for "C" speakers.
I see 38v at the last place I find the lead from the switch going into the
Protect (?) board.
At least I believe it goes to the protect board.
It takes a different route than A&B speakers.
The schematic shows the switches for speakers going direct to the relays, Not true, now that I can see in the receiver.
I chased down "A" and "B" and they do go to the relays, but "C" speakers relay circuit does not. (unlike the schematic shows)
I really do not use "C" speakers, but would like to have the old Concept work as it should.
So If anybody has any ideas on this old receiver, maybe have had this issue before, I'd like to hear it.