Jim,
First thing to do is swap the connectors between the rotators and see if the problem follows the rotator. The controllers are essentially identical, except for the meter scales. That test will eliminate anything external to the rotators.
If the problem does follow the rotator, check the DC continuity of the 3 motor leads. That should be the bottom 3 screws on the controller. Expect to see about 5 ohms or so with the wires disconnected from the controller.
If that is good, then it is _probably_ the phase shift capacitor. With that open, the motor will hum, and may even move a bit with no load, but will have little or no torque. Unfortunately the capacitor is inside the elevation rotator. Replacing it is straight forward, but tedious, and messy. Been there, done that, last month.
Good luck,
Alan WA4SCA