As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE FLIGHT CREW REPORTED THAT THE AUTO-PILOT FAIL WARNING ILLUMINATED AND SYSTEM WOULD NOT ENGAGE. THE MAINTENANCE VENDOR TROUBLESHOT AND DETERMINED THE RUDDER SERVO DRIVE DEFECTIVE. THE SERVO WAS REMOVED AND REPLACED WITH OVERHAULED UNIT. OPS CHECKS GOOD. AIRCRAFT WAS RETURNED TO SERVICE.