What was found
A/C YAWED LT ON CLIMBOUT. UNCOMMANDED YAW OR ROLL CHECKLIST WAS RUN. AUTOPILOT WAS TURNED BACK ON AND NO MORE INPUTS NOTED WITH YAW DAMPER SWITCH OFF. A/C RETURNED IAH. YAW DAMP COUPLER REPLACED AND TESTED PER MM 22-12-31, PAGES 501-505. ALL TEST STEPS GOOD. A RUDDER PCU DUAL SERVO SLIDE CHK PERFORMED IAW ECRA 2721-01012, ALL CHKS OK. AUTOPILOT ACCY UNIT WAS R/R'D IAW ECRA 2720-01162. TEARDOWN OF YAW DAMP COUPLER REVEALED RATE GYRO AT FAULT. VENDOR TEARDOWN OF AUTOPILOT ACCY UNIT REVEALED FAILURE OF DIODES CR1-CR4.