Power Amplifier Sony TAN5550 V Fet Problem at low frequencies
 

One of the channels on this amplifier is misbehaving . When there is a
loud low frequency sound, the sound becomes distorted., returning to
normal after a few seconds. Since this uses V Fet semiconductors, I
believe that there is a defective coupling capacitor an open gate
resistor or a defective Fet (hard to get since this amp is more then
20 years old.
Hard to troubleshoot since I don't have a schematic.
Any suggestions ?

Vlad