JCM800 style amplifier troubleshooting

[Tallablokki]

Just a guess but maybe try separating that blue wire that is above the board and those other wires zip-tied to it. Move them around and see if it affects the oscillation.

I tried it and no effect. The wires are grounding the boost control, channel control and effect loop control terminals in pcb board. They are grounded at footswitch jack and front panel switch and they control relays.

[Helmholtz]

The frequency of the audible oscillation seems to be changing. And I am also a beginner with oscilloscope. There is so much happening in the screen and I don't know how to isolate things.

To read the oscillation frequency you need a steady trace.
Can you dial in the oscillation without any input signal?
If not, try a 400Hz sine input of maybe 50mV and vary the gain of the amp.