What was found
IN LEVEL CRUISE AT 41,000 FEET, PRESSURIZATION STARTED TO CLIMB RATHER SWIFTLY. WITH O2 MASKS ON, WE STARTED AN EMERGENCY DECENT. WITH O2 MASK ON, CABIN PRESSURE REACHED APPROX 20FT, PICTURES ATTACHED. SEVERAL CAS MESSAGES WERE ASSOCIATED. PAX MASKS ALSO DROPPED. REMOVED AND REPLACED DEFECTIVE AMS CONTROLLER. OPERATIONS CHECK GOOD.