What was found
ON CLIMB-OUT, GEAR RETRACTED NORMALLY AND GEAR HANDLE MOVED TO THE OFF POSITION. LEFT GEAR RED LIGHT ILLUMINATED FOLLOWED BY THE LEFT GEAR GREEN LIGHT. HANDLE RAISED AND HEARD A THUMP FOLLOWED BY A YAW. ALL LIGHTS WENT OUT. HANDLE RETURNED TO OFF AND ALL LIGHTS REMAINED OUT. ACCOMPLISHED INSPECTION OF MAIN LANDING GEAR (MLG) RETRACTION SYSTEM. FOUND LEFT MLG UPLOCK ACTUATOR BAD. ADDITIONAL VISUAL INSPECTION FOUND NO ADDITIONAL DEFECTS. REPLACED UPLOCK ACTUATOR. LEAK CHECKED GOOD. ACCOMPLISHED GEAR SWING AFTER ACTUATOR REPLACEMENT.