One of the wires (control lines) to the motor drive amplifier simply switches the motor on and off.
All of the circuitry that regulates the motor's speed and torque, is on the board under the flywheel, that flywheel is actually a ring shaped flat cylindrical shaped ceramic magnet and the metal part of it, that you see, is one of the magnet's pole pieces. On the pcb underneath this, is a circuit track that acts as a coil, which generates an AC voltage, the frequency of which is proportional to the angular velocity of the rotating ring magnet. This is fed back to the MDA IC and processed by a frequency to voltage converter and applied as negative feedback to the motor drive circuitry, and this regulates the speed. There are Hall sensors on the pcb which detect the magnet poles too so as to coordinate the drive to the coils, and the motor coils underneath are glued to the pcb. The design is all very typical of a direct drive motor.
If you can acquire the schematic, you can find out the connection to the control line that starts and stops the motor. If grounding that & un-grounding it doesn't start and stop the motor, then there is a failure on the MDA pcb, if that is the case, most likely, the electrolytic caps on the side of it, that you cannot see without removing it, have failed. There is always the chance of another type of failure on that board, but it is a lot less likely than the electrolytic cap failure.
