Quoted from n1teowl:The credit wheel immediately goes to zero,
This is likely due to the small ratchet arm located at about the 2 o'clock position on the cam side of the stepper unit being gummed up and not moving quickly enough to stop the cam from resetting fully. It's supposed to ratchet down the cam one step at a time. When it gets gummed up, the first decrement brings the whole cam back to zero because the ratchet arm isn't there to stop it. Needs to be degreased.