I have an Activator PJ set up in a project bass that has the Vol - Vol - Filter set up with added 3 way Q switch. The Filter sweep knob seems to have a loose shaft, and when I turn the knob, it makes a thumping sound in the speakers, and even cuts out. I replaced the 9V battery with a fresh one, and flipped the Q switch, and changed the position of the volume knobs, but those function perfectly. Is there a way to tighten the shaft inside this pot? Or is it in need of a replacement?