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go thru the circuit a component at a time, check voltages to ground on each side.. often the resulting disturbance is enough to show the problem component if you’re monitoring the circuit while checking voltages. anywhere that looks suspect, check it for a cold joint. it isn’t as hard as it seems, and can often quickly find the issue.

the gain pot isn’t supposed to go all the way to off, btw. look at the schematic and how the components connect. in order for it to go fully off, the ground pin of the pot has to go directly to ground, in this case its more of a variable resistance in the feedback loop of the opamp. different resistances will get different levels of gain. its common to see up to 1 meg pots in this application, but only if the circuit can support it.

that said, i found the gain pot came to life by replacing r6 , the 3.3k resistor, with a 1m one. more gain, nice sweep and seems to open up the tone some more. do this at your own risk, of course.

hope that helps man