OK chaps, I just bought a trace AH240 or (whatwever it's called) 300W head. Sounds really great etc... BUT.... The master volume pot is dirty and crackles like the hand of God when turned up above about 6... I'm no elctronics buff, but I know a dirty pot when I hear one and I'm sure that I can clean it myself... What should I clean it with? - and how should I clean it?