grindy

I have a Zoom pedal like that:


The silver knobs a the top don't really work anymore. When I try to turn them down the setting still goes up. I have to turn them down very slowly and even then it jumps up at some point.
I seem to remember that I had a similar problm with another zoom pedal I had, so this might be a widespread issue with the brand.
Does anyone know how to fix it?

PS: The warranty expired a long time ago.

PPS: Yeah, I tried googling, but couldn't find anything.

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