The shaft is binding or the bearing is failing. Your masthead unit either needs rebuilding or replacing. Sometimes you can extend its life by using WD40 or a similar product to release and lubricate the shaft, best done on off the mast
so the unit can be held inverted to allow the lube to get gravity assistance penetrating the shaft.
On some masthead units the bearing is sometimes a part of a circuit board, given the space for soldering the wiring
, not an easy task.
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