Hello again - sorry to be a "post hog"..........I am restoring this K300 PA head and the master Treble pot is very noisy/scratchy. I sprayed it down with electrical contact cleaner and worked it for about 3 mins - this usually does the trick, but not this time. So, is there a special method for these Kustom pots to alleviate the scratchiness? Or is the pot just shot?