The vertical film reel sensor break (tab) missed sensor, the sensor needs replacing, or the motor needs replacing. 

1) Make sure that the rubber sensor break attached to the "Number 7" frame is properly attached and in good condition.

 2) Check the alignment of the sensor break and the film reel sensor. Make sure that the sensor didn’t vibrate loose which causes the rubber break (tab) to miss the optic beam with each revolution. Also check to make sure the wires are firmly connected.

 3) As film reels rotate, check to make sure that the tab goes through the middle of the sensor and that the red light on the sensor illuminates. This will indicate that the sensor is working properly.

 4) If the sensor is working properly, check the connections on the motor. If the wires are firmly connected. 

5) Replace the motor.