What was found
GOING THROUGH 15000 UNCONTROLLED TAIL OSCILLATION BACK AND FORTH - DISCONNECTED AUTOPILOT STILL OSCILLATIONS - CALL FROM F/A - VERY VIOLENT IN TAIL- SLOWED ACFT & STOPPED - NO WARNING LIGHTS. ON INVESTIGATION FOUND RT YAW DAMPER CHANNEL FAILING TEST, WITH INOP LIGHT REMAINING ON. TRIED REPLACING RT YAW DAMPER MODULE, NIL FIX. ORIGINAL PART RE-INSTALLED. BITE DISPLAY ON YDM SHOWED 26VAC FAIL. REPLACED RT CSEU NR 1 POWER SUPPLY MODULE IAW AMM 27-09-00/201 TEST PASSES OK. YAW DAMPER SYS OPS TEST PASSES OK. YAW DAMPER SYS OPS TEST CARRIED OUT PASS OK.