ok...
not a contactor issue
but i doubt its a card issue if forward is working perfect
if your getting just a hum like the system is pulsing in reverse but just no increase in oscilation when the pot voltage changes then it must be either the range on the pot or something else related to the reverse circuit. You said new contactor? Are you sure the power circuit was put back exactly the way it was when it was reassembled? Could it be possible one lead was put on incorrectly on the reverse side?

I'm just guessing here but it sounds like something that was mistakenly done during a repair job maybe? Go back and retrace the steps made and insure EVERYTHING is wired up correctly.