I have a reconditioned 12 Qt. Hobart A120. I bought it about 6 months ago. Just recently it stopped turning in first speed. The motor hums but I cannot get the shaft to turn. I have tried manually turning the dough hook back and forth while the motor is running (with proper caution to where my hands are so I can get them out of the way) to see if I can get the gears to engage but no luck. I have also tried tweeking the position of the speed lever with no success. . The mixer engages just fine on second and third speed.