1. Reverse the right and left input cables (Phono, or Aux, whatever you are testing with), and see the issue moves to the other speaker. If so, then the issue is not with the receiver.
2. Here's why you get sound from both speakers when you switch it to Mono. The signal in the right channel is getting lost somewhere, so it does not play when set to Stereo. But when switched to Mono then both channels are merged into one channel, and played through both speakers.
Because you get sound from both speakers when on Mono, this means that the main amplifiers both work. A good thing. Now the trick is finding where the signal is getting lost. If it is happening at a particular switch or control, then cycling that switch several times may start to clear up what is blocking the signal. So, systematically work every control 5 or 10 times to see if any of them directly affect getting both channels when in Stereo. If you find one, you'll know what needs cleaning.
The Mono switch is often a cause of dropped channels. In this case it might be the dirty switch, but not necessarily. It may just be doing what it is supposed to do. Cycling the Mono switch a dozen times will reveal if the switch is dirty.
