[Mondial]

Could be a tube or AVC problem, or a sticky vane.

To determine if it is a mechanical or electrical problem, place a voltmeter across the leads of the shadowmeter and monitor the voltage. Should be 10 volts or more when fully open. When the vane shadow is closed there should be little or no voltage across the meter. If there is a low voltage across it but the shadow remains open then it is mechanically sticking.