REPAIR ASSISTANCE

I would prod and poke the channel with an insulated screwdriver this will cause buzz sounds. This should find the cause of the problem. Remember that the left hand channel is identical so you can compare the right with the left. For example if you get a weird buzz on the right when you touch it, but not on the left at the same point you might have found the faulty part.
Don't go near the power supply section as it's nothing to do with that the fault.

Try interchanging the speakers. If the right channel works, you have a problem with your speaker.

Hi,
I think RA 12 is not existing type, but RA 12xx types exist.
Please try this link, and I hope you will find the relevant documentation.
Good luck!
gylacz

check stereo settings after that if it still persists your left channel is goin out

I'd suspect that one of the two output transistors on that channel has gone bad. If you are unsure how to test or troubleshoot this, then you might try replace both the NPN and PNP outputs and catch the bad one that way. If your amp uses an output module then you would of course have to replace the module which would replace all the output transistors on both channels. Good luck. Another thing to look out for, is the emitter bias resistors. They are the large white ceramic ones near the outputs. Sometimes one of those will open up too. They will be of a very small resistance value like .5 ohms (point five ohms) or something in that general ballpark, and have a wattage value of typicaly 5 watts. So they are pretty big resistors. Be sure to check them all even if you do find a bad output transistor. They will often go out with the transistor. Good luck.