Bought this from a guy on another site a couple of months ago and until today only used two effects so don't know if he sold it knowing it was faulty. However, went to use the synth bass and just got a crackle (similar to what a dodgy guitar lead sounds like) and the effect was on/off intermittently. Nothing to do with the leads as I checked that. All other effects work fine so can't understand why this particular one is playing up. Any ideas on what it could be or how to rectify?