I am getting some incorrect readings on resistors in my Wah pedal when measuring them in situ. I think a couple may be low due to age which could explain a few things

It depends how they are connected - series/parallel.

You could have told me that before Imposted this

You'll be talking to yourself next

Fancy a cuppa?

You're not likely to get accurate measurements on a resistor in the circuit. Even a resistor wired in series will be in parallel with the rest of the circuit. You'll measure a "Thevenin equivalent" resistance in parallel with the resistor. If the Thevenin equivalent has a very high resistance you might get a close measurement.

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In my opinion Leo Fender had essentially perfected the guitar amplifier by 1964.