I tackled the problem by taking the amp apart, removing the capacitor and the ball drive, cleaning and lubricating both. The front panel of the amp will have to be removed to get the ball drive out. Both Tune and Load capacitors have to come out to get to the screws that hold the capacitors and the ball drives in. Not for the faint of heart. You can try a shot or 2 of WD-40 with an extended spray attachment to get some lube into the ball drive at a minimum. New ball drives are available on the Internet for ~ 45.00.
Jackson Bros. in England make them. You may want to get someone to help you with this, or seek out a repair shop.
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73' Dan W2NO
