...the voice is omnipresent. Some instruments look (too) loud (as the guitar) and I suspect other frequences are quite missing.
When I turn the "mode selector" to "reverse".. here come the voice! but the guitar sound decrease too much and the snare drum sound just appeared.

As you see it's seems that some frequences are available on one side (reverse switch on) and other frequences are there when "stereo" switch on. I cannot get full sound even reverse or stereo and of course both position can be set on.

My technician and I just dont understand. Somebody from HotGlass Audio made a bad job inside (just replace all bumble bees by japanese commun stuff and made lot of cold soldering) but even after my actuel technician replace all again (by auricap) he cannot find what's going on with this frequencies mixed up problem.

If this tell you something please advise. I can't get original C20 papers (visible) from McIntosh.

Thank you

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...