{Collins} 75S-1 Tuning Dial Drive Slippage

I've just about completed the refurbishing of the 32S-1, 516F-2, and 75S-1.  One nagging problem is remaining with the 75S-1 PTO tuning drive.  When trying to tune in a station, there is about a 1KHz slippage in the mechanical drive system when changing the tuning direction using the main tuning knob.  The problem seems to be more pronounced at certain locations/positions of the tuning dial.  The set screws on the PTO shaft are so tight I have not been able to loosen them - so that should not be the problem.  The only thing I can come up with is that the split metal drive disc (friction drive) must be slipping.  Has someone been down this road before that could advise me on how to fix the problem?
Thanks & 73,
Bill, K5GCW

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