The service manual is available here. The fault may be due to power supply problems, but in my opinion, changing components without diagnosing the problem is a bad idea - if you do fix the problem you will have learned nothing, you may not fix the problem at all and are likely to make mistakes and make things worse. When the fault appears, do all of the PHONES and L and R outputs go silent? If the instrument goes totally silent I'd be looking at the muting circuitry to the bottom right of sheet 15 of the service manual, specifically the relay and its control circuit that isolate the output from the delay. Do you have an oscilloscope? If you do, you could take a look at the output of the delay before the muting relay by 'scoping the relay side of R18 and R19 on sheet 15.