my W.A.G. is that you have a 'leaky' (maybe all the way bad) fly-back diode. one of the 2 large stud/pigtial diodes on their own heat-sink in the chopper panel. you [most likely] would have to test it with 1 of 2 possible methods, either a "handyman' tester, or 'check by replacing with a known good one'.
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