Anyone have any idea how to fix a servo?

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Hi, i would like to know is it possible to fix a servo? I am playing Radio Control Rock Crawler then the servo is quite important for me cause when we crawl it play the most important role. But recently the servo keep on shaky and it stuck when i switch off the car. I do change a lot servo before but every time met such problems and i feel such a waste on those stuck servo and i am thinking of repairing it myself. So anyone here do know how to fix a servo? Can help me up? Thanks a million!!!