51S1 Tuning slippage

[N9AXl]

I’ve picked up a Collins 51s1 which seems to have slippage with the tuning knob.
It is smooth in some places and slips in other. Just hard to keep on frequency.

Otherwise it is fully functional with just minor cosmetic issues.

I’ve read an earlier posting on a similar problem here. Any suggestions as to where to start would be welcome. I’ve done many repairs on other brands but this is the first Collins.