Marshall Guv'nor oscillation

[Psabin8951]

I have been given an in houses original Marshall Guv'nor which has/had a few issues.
The gain pot was noisy and switch didn't always work. These issues have been sorted and it kind of works but with gain and level anywhere past 12 o'clock I get a high pitched oscillation which changes pitch with tone controls and increasing level/gain. Below 12 o'clock it works fine. Any ideas?

[GibsonGM]

I think you need to open it up and check out how the pots are connected - be sure there are good solder joints.  If any mods have been doing, if they routed wires anywhere then perhaps they are coupling signal to each other. 

Best thing is to carefully open it, check those, maybe post some pics here of board front and back, and then take voltage measurements per the "Debugging: what to do when it doesn't work" thread.      That will give us more to go on!

Welcome to the forum