If it won't go, check the fuse, then for obvious looking smoke marks on the board, or the main chip may have popped & you may see the side bulging or such.
Look for any leaking capactors, the cans with a plastic coating, they may have a brown gunk comming from out of the top or underneith.
Sometimes Amps are thrown out because the volume goes all crackley, same with switches & bass etc controls. WD40 may fix this, it may not too.
Its best to use a proper contact cleaner.
To clean the volume controls properly they (techs) use a small round tube that screws over the control shaft (once you remove the knob) onto the threaded boss underneith (the nut on the boss normally holds the control onto the backing plate.)
Then they fit the extension tube on the contact cleaner into a hole in the end of this tool & plug the can of spray cleaner onto the extension tube.
Then they spray the cleaner down past the actual control knob into the resistor part of the volume etc control.
That will push the oxides right out. I think they can turn the tool around so it turns the volume shaft around too, just too loosen the oxides. I have only seen a picture of them.
I think I can make something similar by just threading out the end of a bullet shell so it threads onto the control (1/8 NP?. Its 27 threads per inch) and then spraying the cleaner solvent into where the primer hole is, by drilling it out to the right size to fit the extension tube.
I could also drill & tap a small screw in from the side so it locates onto the flat on the control shaft, making it turn when I turn the tool around, while spraying contact cleaner into it.
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