What was found
SLIGHT YAW OSCILLATIONS AT 250 KTS IN SMOOTH AIR, AUTOPILOT ON OR OFF. OK TO CONTINUE PER MM, THIS IS A KNOWN CONDITION AND IS ACCEPTABLE FOR DISPATCH WITHOUT CORRECTIVE ACTION. THE OUTBOUND FLIGHT CREW DETERMINED THAT NOT ENOUGH WORK WAS ACCOMPLISHED TO CORRECT THE DITHER IN THE RUDDER SYSTEM AND REFUSED TO FLY THE AIRCRAFT. ACCOMPLISHED SERVICE TIP MM, NO OSCILLATION FOUND AND NO UNWANTED MOVEMENT OF THE RUDDER CHANGER MECHANISM, ALSO ACP THE RUDDER RATIO ACTUATOR DITHER TEST AND THE YAW DAMP, ALL NORMAL.