Thank you! According to this good old logic approach we found out a problem at section "mode selector" (reverse, stereo, chanel A, B) and "Balance". That's it!
But even these two control knobs were same construction thru 3 generations of C-20, McIntosh used two different ways of wiring them and we tried to understand why (I have pictures of that). A black wire instaled in three point and in between Mode and balance is there... or not, depending of the C-20 serie.
I read on the net there was "new born C-20 balance problem" that was set in the sixties (false or true?) in accordance to McIntosh communication.
It's the only section of this C-20 that was not untouched when I bought it. Many traces of soldering...
The thing is, I dont have any close-up, any revealant picture of internal mode + balance control and wiring. Everything seems ok to my eyes.
I really dont know what I should try. But still...