What was found
DURING A START-UP FOR DEPARTURE, THE PRESSURIZATION OUTFLOW VALVE FAILED AND LOCKED IN THE CLOSED POSITION. THE CABIN DOOR WAS CLOSED AND THE APU AIR WAS SELECTED ON TO PREPARE FOR ENGINES STARTING. AT THAT TIME, THE CABIN STARTED PRESSURIZING UNCONTROLLABLY!! THE PRESSURIZATION OUTFLOW VALVE COULD NOT BE OPERATED VIA THE (EMERGENCY) DC MOTOR. SUSPECT THE OUTFLOW VALVE PLANET GEAR ASSEMBLY HAD A FAILURE (THE BUTTERFLY WAS FREE).