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I installed an arm from the salvage yard last year. Earlier this year it stopped working. The gears and mechanism were all perfect. It would retract but not extend. I spent an afternoon reverse engineering the mechansim and found the problem. The arm has two actuators on the bar which engage with two limit switches next to the motor. The limit switch which engages the closing actuator on the bar had become loose. The loctite material cracked and allowed the limit switch to move upward. I adjusted the switch downward until I could hear the click of the switch when the arm was in the retracted position. Once this was done the arm worked perfectly again. I cemented the screws with thick super glue after adjustment.

I hope this helps some of you fix the presenters.