Sometimes scratchy pots are just that, worn out pots.
Another cause for scratchy pots is not so much the fault of the pot but the circuit associated with the pot has DC voltage leaking across the pot.
Hmm, I did find some loose earthing and suspect that related to poor sound within the monitoring system, the main and AUX outs sounded clean except when adjusting 3 or so specific pots.
The metal chassis is not grounded to its own supply. In case you're wondering, the power plug is 3 pin and the lead is a flat 2 core (active & neutral only). There is no evidence of tampering and the smaller mixer I bought for parts has exactly same hardware and setup so this all appears to be original. The service manual clearly shows the US and Canadian models are grounded to the supply but the European model (with a voltage selection switch) is not grounded to the supply.
I reckon most electricians here in Australia would be shocked by this.....
Ken