What was found
SHORTLY AFTER TAKEOFF, PRIMARY TRIM STOPPED WORKING. IT WORKED OK FOR THE FIRST 1.5 TO 2 MINUTES. SELECTED STBY TRIM AND IT ALSO DID NOT WORK. STBY TRIM WORKED AFTER 5 MIN OF NOT WORKING. FIRST INDICATION OF PROBLEM WAS NO FLIGHT DIRECTOR MODES COULD BE SELECTED AND AUTO PILOT WOULD NOT ENGAGE. AFTER 5 MINUTES THE STBY TRIM STARTED WORKING. SET TRIM FOR ABOUT 4 DEGREES, CONFIGURED AC FOR 15 DEGREES FLAP LANDING. WHILE TAXIING TO RAMP PRIMARY TRIM INITIALLY WOULD NOT WORK, THEN STARTED WORKING AGAIN. ABOUT A MINUTE PRIOR TO TAKEOFF, PFD SELECTOR WAS PUSHED ONCE TO MOVE CONTROL FROM LT FLIGHT DIRECTOR TO RT FLIGHT DIRECTOR. PFD SELECTOR WAS STUCK. REMOVED AND REPLACED FGC (GC-810). OPS CHECKED GOOD IAW MM.