I use a standard, sustain pedal that came with one of my Yamaha PSR keyboards many years ago--it works just fine. It's just a momentary switch in the pedal--nothing fancy. Make sure the plug on the foot-switch is a mono plug--not a stereo connection.

Are you able to turn the harmony on and off using the built-in switch?

Cheers,

Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!

K+E=W (Knowledge Plus Experience = Wisdom.)