Unfortunately there are no Main in and Pre out jacks , so you can't easily understand if the problem is related to preamp or
power amp .
At this point 3 suggestions :
a) touch the bords ( amp and preamp ) with a plastic pen in order to find the area where
there is the bad contact
b) short with a piece of wire L703 ( output relay )
c) a total resoldering of power amp could be fix the problem
Don't worry , they are normal issues due to age .
Last month I became crazy with my Tandberg TCD 320 .
The solution was the resoldering of the mother board .
Ciao
Marco