What was found
AIRCRAFT DEPARTED PHOENIX SKY HARBOR INT'L AIRPORT AND RETURNED WHEN CREW ENCOUNTERED A STABILIZER TRIM MALFUNCTION. THE FLIGHT CREW REPORTED INTERMITTENT OPERATION AND THEN A LOSS OF BOTH THE PRIMARY AND ALTERNATE SYSTEMS. TROUBLESHOOTING DETERMINED THE TRIM SWITCH IN THE FIRST OFFICER'S CONTROL COLUMN FOR THE TRIM MOTOR SIDE HAD FAILED INTERNALLY CAUSING UNCOMMANDED INTERMITTENT OPERATION OF THE PRIMARY TRIM MOTOR CAUSING IT TO OVERHEAT AND TRIP THROUGH THE INTERNAL THERMAL PROTECTIVE DEVICE. THE SWITCH WAS REPLACED AND THE TRIM SYSTEM FUNCTIONED NORMALLY. (X)