JP 13 7 Mono output problem

When I plug a mono guitar cable into the mono output of my 13, only the piezo signal works; the magnetic signal is dead. However, the magnetic signal works fine when plugged into the stereo output.

Sometimes, I encounter problems with the battery box, and the guitar completely loses its signal. However, when I remove the battery from the box and then place it back in, slowly closing the cover, the signal returns.

What could be causing the magnetic signal to not work on the mono output? Is…

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