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The most likely cause is that the ground of the PC board has become disconnected from the outer metal chassis.
Take the cover off….. by undoing the 5 screws on the back and the 2 screws at each end, this exposes the PC board.
There should be a wire link connecting the ground of the PC board to the body of the input gain pot on channel 1… and the fixing nut for this pot should be good and tight, and there should be a shakeproof washer under the nut.
Alternatively, if that is all correct, then there could be a discontinuity in the mic cable… the ground (pin 1) not being connected to the outer case of the XLR connectors.
Check all these and you should find the problem.
One other thought just struck me….. are you using the correct power supply??
I think I have covered all the possibilities! The P4 is a very fine little mic amp…. I use one as a precision amplifier for acoustics measurements because of its extreme frequency response and low noise. Originally it was designed as a recording channel for ‘voice-overs’; the limiter compressor is perfect for good punchy voice sounds.