Well, I'd check the obvious first. Look the motor over and see if you have any issues there. Unplug the motor connection from the ACDR board. Measure the field (green wire) to ground, it should only be a couple of ohms. Then measure between the brushes, that should come in at 3-4 ohms. Rotate the armature while taking the reading, see if it ever goes to infinity. You can also test the motor with AC power from that plug. Connect the field to one of the brush leads. Connect the frame ground to one side of the transformer and the remaining brush lead to the other side. Apply 6-8 volts from the transformer and the motor should rotate.
If it's not the motor, and you've verified the connections from the motor to the ACDR are good, you likely need a new motor driver (ACDR).