There is a McIntosh forum that is more tech oriented in audiokarma.org that may be more helpful but +1 @mwood for the Audio Classics recommendation. I use them for all my vintage gear. Good luck.
McIntosh MC20 - 1960's - No Left Channel
Total noob here.
I sort of discovered some McIntosh equipment that has been in this house for 40 years in an old handed down stereo cabinet. Finally decided to see if I could get it to work. It's made a great little side table for the past few decades.
In any case, it's an original McIntosh 275 and C20 Preamp (and reel-to-reel/turn table/ and tuner)
I'm trying to get the Pre-amp/Amp working.
Inputting from a computer 3.5mm to RCA on Preamp for testing.
Left channel is not woking.
Tried balance of course and mode selector.
Confirmed that both speakers work by switching inputs in the 275.
I've done a continuity test on all the tubes (they were all the same, so I'm guessing the test was good. The same three pins show continuity with each other on all7 tubes)
I switched the input cables to confirm they were both working.
Changed the input to connect with Aux, Tape, tuner… all no left channel.
Tried High Level and Low level inputs... still no left channel.
Swapped V2 - V1 tubes (12AX7's)
Swapped V4-V3 (12AU7's)
Swapped V5-V6 (Also 12AX7's)
Then swapped V2 and V1 with V5 and V6 (as they're all 12AX7's)
No obvious issues to inputs internally when I open the back.
Next steps?
Thanks!