Sometimes,if you haven't used the pedal for some time, a layer of oxide can build up on the Stereo jack plug contact surfaces. Often, you can't see this contamination. Try cleaning the jack plug thoroughly with some solvent, dry it off before refitting in the keyboard socket. If the fault does not occur when the pedal is disconnected, then it would suggest that the problem is either the jack plug, the lead or the pedal itself. As has already been suggested, if possible, try another pedal. Another thing to try would be to set the keyboard playing - say a sequence and then 'waggle' the lead, rotate the jack plug in it's socket and give the pedal a gentle 'kick' to see if any of these treatments make the problem appear.

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Willum