What was found
PHL FLT 798 EN ROUTE PHL TO AMS. UNABLE TO CONTROL CABIN PRESSURE IN ANY MODE. AN EMERGENCY DESCENT WAS PERFORMED AND THE OXYGEN MASKS WERE MANUALLY DEPLOYED. THE FLIGHT RETURNED TO PHL AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE NUMBER ONE PRESSURE CONTROLLER AND RECONNECTED THE RUBBER BOOT ON THE RIGHT PACK OUTFLOW DUCT FLANGE. ALSO REPLACED THE CABIN PRESSURE SELECTOR PANEL AND NUMBER TWO CABIN PRESSURE CONTROLLER AS A PRECAUTION.