I assume that the power-amp is good.
This noise, of course due to defective parts inside.
But, which section is it? Chances are:
- Amplifier circuit in the Main-Outputs. If the MAIN MIX in the minimum position and still noisy, then the damage in that section.
- At one or several channels. Let's test.
Set to minimum volume for all channels and MAIN MIX. EQ=flat. FX=minimum. TRIM at half (let's try at 12 o'clock).
Raise the MAIN MIX, let's say at 15.
Gently, raise channel 1 volume to maximum, listen, if there is no noise, channel 1 is safe. Lower the volume to the minimum again. Move to another channel in the same way.
If found noise, that is where the problem is.
Actually, many more detailed steps for testing, but the description is too long. This above is a simple way.
Circuit power supply should also be checked, whether all normal voltage.
It is part of the image of the amplifier section circuit.
Parts which may be broken is the IC 64,65,77.