My garage door has been closing to about 12" above the floor, then stops and opens again. (I have disconnected it and now open and close it manually). I tried adjusting the force down knob on the back o the unit - but that caused the circuit breaker to trip. When i reset the breaker, the opener hums but does not turn the chain drive. Any idea what i did wrong or what may have broken? Model 139.53615SR Craftsman 1/2 HP Garage Door Opener. Thanks - Frank