What was found
DURING CLIMB THROUGH 26,000 FEET WITH AUTO-PILOT ENGAGED, THE AIRCRAFT SUDDENTLY YAWED TO ONE SIDE THEN THE OTHER. DISCONNECTED AUTO-PILOT AND PROBLEM CONTINUED. SWITCHED RUDDER CONTROL MANUAL; YAWING CEASED. CAPTAIN DECLARED AN EMERGENCY AND FLIGHT DIVERTED TO TYS. NOTE: NUMEROUS RUDDER TRIM CHANGES WERE REQUIRED DURING CONTINUATION OF FLIGHT TO MAINTAIN CENTERED POSITION. AIRCRAFT LANDED WITHOUT INCIDENT. REPLACED THE YAW DAMPER ACTUATOR. OPERATIONAL CHECK OK. AIRCRAFT RETURNED TO SERVICE.