I have a Marshall valve VS30R. it only operates in on the clean channel, i have a reasonable knowledge of elecronics and the circuit diagram to hand i'm struggling to find good test points, at the moment i see no variation on the overdrive volume variable resistor but i'm not sure if i'm testing right. Please help any tips would be greatly appreciated.

STG, December 2009

Sometimes when measuring components in-circuit, other circuit elements interfere with the measurement.

To test a potentiometer temporarily unsolder the wiper wire and make two tests with your meter. Left terminal to wiper and check resistance as your move the control and then right terminal to wiper. Then a very small squirt of servisol contact cleaner.

You will be surprised how cheap a new pot might be at an elctronics shop - just a couple of bucks...

Good luck...

Peccavi, December 2009