What was found
SEA - FLIGHT 2090 - JUST AFTER TAKEOFF, AND GEAR SELECTED UP, THE FIRST OFFICER USED THE RUDDER TRIM SWITCH, AND IN DOING SO, THE SWITCH BROKE TO THE RIGHT POSITION. RUDDER TRIM MOVED 3/4 TO THE RIGHT. CAPTAIN HELD THE TRIM SWITCH TO THE LEFT AND TRIM MOVEMENT STOPPED. CREW HELD SWITCH TO THE LEFT FOR THE REMAINDER OF THE FLIGHT. CALLED CFR AS A PRECAUTION. RETURNED TO THE FIELD AND ACCOMPLISHED A UNEVENTFUL LANDING ON 16L. MAINTENANCE REPLACED THE TRIM PANEL. OPERATIONAL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE.