What was found
BOS - AFTER T/O THE ACFT EXPERIENCED RUDDER CONTROL PROBLEMS. AN EMERGENCY WAS DECLARED AND THE ACFT RETURNED, LANDING WITHOUT INCIDENT. MX INSPECTION WAS PERFORMED WITH NO DEFECTS NOTED. ALL FLIGHT CONTROLS WERE CHECKED, NO DEFECTS WERE FOUND. BOTH ENGINE TRUSSES INSPECTED AND BOTH TORQUE AND ITT SYSTEMS WERE CHECKED WITH NO FAULTS FOUND. RUNS PERFORMED ON ENGINES WITH NO DEFECTS, THE GYRO FOR THE YAW DAMP SYSTEM WAS REPLACED. THE TEST FLIGHT CREW REPORTED THAT THE ACFT STILL TENDED TO ROLL. THE YAW DAMP COMPUTER AND SERVO WERE REPLACED, NO DEFECTS NOTED. THE YAW DAMP COMPUTER AND SERVO WERE SENT FOR INVESTIGATION. REF PLGA316951.