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Controls go scratchy because the sliding contact inside the control makes intermittent contact with the resistive element, and you hear a little "gritch" every time it makes or breaks contact. This can be caused by:
You can sometimes restore a control to non-scratchiness by spraying contact cleaner inside the pot and turning the shaft rapidly to clean the dirt and stuff off the resistive element; however, the problem is likely to recur. Getting a new pot is the best fix. If you decide to try the spray cleaner trick, use one that leaves a lubricant on the surface to lessen the wear and make it longer until you have to do this again.
Scratchiness is a thousand times worse if there is DC across the pot, so you might check for that. DC across any pot except the bias pots is usually an indication that a coupling capacitor is leaking, so check for that.