Hunter rotary sprinklers operate on a “gear drive” system. When in operation, water turns a small turbine in the base of the sprinkler, which drives a series of gears that cause the head to rotate. The Hunter gear drive mechanism is sealed from dirt and debris. Age, excessive pressure, or poor water quality may eventually allow particles of contamination to enter the gear drive.
If the sprinkler is no longer rotating and the water flow coming out of the sprinkler has decreased, you can first try cleaning out the filter by removing the pop-up assembly from the main body.
Unscrew the body cap counter clockwise, sometimes after use pliers may be necessary to free the body cap from the rotor's body.
After the body cap is unscrewed the internal assembly can be removed.
The filter on the bottom of the internal assembly can be easily removed using needle nose pliers.
Generally, if cleaning the filter does not solve non-rotation then the sprinkler must be replaced.