Exceptional work, I will never be grateful enough my friend...
I suspect that in mine the problem resides in the preamp section, so I need the part of the scheme with the inputs, the source switch, the tones, balance and volume controls, essentially what is before the power amp IC. Now I am at work, when I will return home I will give a good view at the various scans.
Thanks again a lot, this is the definitive help I needed to try and solve this problem.