What was found
PDX - ON CLIMB FOR FLIGHT 2527 AS PASSING 14,000 FEET THE RUDDER TRIM RAN THROUGH SEVERAL INCREASING CYCLES RIGHT TO LEFT. DISCONNECTING THE AUTOPILOT HAD NO EFFECT. TRIM ENDED FULL LEFT, WAS ABLE TO CENTER TRIM WITH THE RUDDER TRIM KNOB. TRIM INDICATIONS WERE ACCOMPANIED BY YAW. RETURNED TO THE FIELD AND ACCOMPLISHED A UNEVENTFUL LANDING. MAINTENANCE FOUND THE RUDDER TRIM KNOW WAS STICKY AND HAD EXCESSIVE FREE PLAY. REPLACED THE AILERON/RUDDER TRIM CONTROL PANEL. OPERATIONAL CHECKS GOOD AIRCRAFT RETURNED TO SERVICE.