I have a 110v dc motor on my treadmill that surges speed at times. The motor speed is surging at times to the point where the lights in the room dim. I have attached a schematic of the controller I made up and a picture of the controller itself. It has a PWM motor controller that receives a signal voltage from upper user console on pins #2 and #3 of connector A1. Voltage varies from 0.8v to 5.9v according to requested console speed. Also voltage does not change on pins #2 and #3 when motor surges. Am I correct to say the DC motor speed is controlled on the negative side of the motor through pin #7 of the mother board? I checked the diode rectifier bridge with multimeter in diode setting. Seems to check out ok. Any help diagnosing this problem would be greatly appreciated.

 

Am I correct to say the DC motor speed is controlled on the negative side of the motor through pin #7 of the mother board?

Appears so.
Did notice the on the schematic the polarity of the filter capacitors is incorrect.