What was found
FLT NR 4724 - BOS - ON CLIMB-OUT, ELEVATOR TRIM WAS SELECTED, WHICH CONTINUED TO TRAVEL BEYOND THE SELECTED COMMAND. THE CREW SELECTED THE TRIM TO OFF. MANUAL TRIM WAS SELECTED, WHEREUPON THE TRIM SYSTEM WAS FOUND FROZEN. THE AIRCRAFT RETURNED TO BOS LANDING WITHOUT INCIDENT. BOS MAINTENANCE FOUND THE ELEVATOR TRIM SERVO ACTUATOR HAD SHORTED INTERNALLY CAUSING THE SYSTEM TO LOCK. THE SYSTEM WAS DEFERRED PER THE MEL AND THE AIRCRAFT RETURNED TO SERVICE. A REPLACEMENT SERVO IS ON ORDER. (M)