Yeah, I doubt you would find a replacement. That's why I suggested that you remove it and open it up. I think the problem will be obvious. You might or might not be able to fix it, but you should be able to get it back to its current state.
Do you use all three speaker output sets? Do you want one set always ON? Perhaps you could change the wiring on the PCB so that you don't need to use the A+B+C position?
Before i remove the switch, i was gonna ask you something. It has resistors and capacitors on the board, and two coils. Is it possible one of them went bad and isnt letting the A+B to work when selected? All A,B and C channels work when selected separately.
You can check for the correct continuity patterns in all positions of the switch. There's not enough information on the schematic for me to write up a checklist, but you may be able to figure out which pins are supposed to have continuity in each position by looking at the diagram of the switch in post #7 and matching that up with your memory of how the switch was constructed internally.
It looks like on each position you should have four independent continuity paths, and the pin layout on the diagram should match the actual pin layout on the switch.
Are you testing the A+B switch position with speakers connected to A an B at the same time? In the A+B position, speakers A and B are wired in series, so both must have speakers connected to work. You can't test one at a time.
Thank you. All consumer amplifiers operate in this way, as putting 2 sets of speakers in parallel could easily put too much load on the output section, causing overheating and shutdowns. I have a feeling that nothing was ever wrong with that receiver and the time spent was a lesson learned...