SCI Drumtrak Volume Knob

Hi, there are for rotary knobs (pots??) on the SCI Drumtraks that need to come off in order to access the PCB. Does anyone know how to get these off without breaking them. I took mine to a highly recommended repair depot and the guy ended up breaking two of the four!! Now I want to clean the contacts but can’t get the PCB board exposed without taking the knobs off.