Sometimes when a knob goes bad, it's the knob's potentiometer. What's a potentiometer? It's basically a resistor that can change resistance depending on how far you turn it. Usually there's a metal contact that slides against another metal contact, and often these contacts can wear out or get dirty. If it looks like the potentiometer contacts are reachable, I would try cleaning it (obviously with the power disconnected). If not, you may see if you can find a potentiometer with the same resistance ratings and try replacing it.