Ah - that is much more than just a powerfeed. Unfortunately, that means the fix is likely to be much more $$. I think it is highly likely that the problem is in the circuit board, so the easy fix is to replace the circuit board. But if you can even find a replacement circuit board, it is likely that it will be very, very expensive. (My guess only - maybe you will get lucky and someone has 100's of these on a shelf somewhere ...)
If it were me, I'd take some time to look over the circuit board very closely, looking for any components that look burned. Could be as simple as a resistor or capacitor ... but if I had to guess -- and again, it is just a guess, with just enough knowledge behind it to be dangerous and not enough to be confident -- if I had to guess, I'd guess that this board uses a power MOSFET to control the PWM current supplied to the motor. If I recall correctly, at least some MOSFETs can, if they burn out, wind up in a shorted condition. Thus, rather than turning off and on to send controlled pulses to the motor, it is just on - leading to full-speed travel regardless of settings.
All of the above is assuming that this is using a PWM (pulse-width-modulated) driven DC motor; presumably the motor (or at least the circuit) has some sort of feedback, since the controls suggest that it can seek to a specific point - sort of an almost-but-not-quite CNC, perhaps. But it could be some other sort of motor, e.g. a stepper - in which case my MOSFET theory goes out the window.
Hopefully, wiser and more experienced users will step in and confirm or correct my guesses above!
