What was found
AT FL290, APPROXIMATELY 75 MILES SOUTH OF PHL, CREW REPORTED THE CABIN PRESSURE WARNING HORN SOUNDED AND CABIN PRESSURE INDICATOR READ 10,000 FEET. FLIGHT CREW SWITCHED FROM AUTO PRESSURIZATION TO MANUAL PRESSURIZATION AND CLOSED THE OUTFLOW VALVE. CAPTAIN DECLARED AN EMERGENCY AND DESCENDED TO 10,000 FEET. THE CABIN OXYGEN MASKS DID NOT DEPLOY. FLIGHT LANDED IN PHL WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED THE PRESSURE CONTROL PANEL PER MM 21-31-00. OPERATIONAL CHECK GOOD. COMPLETED TWO TESTS FOR OUTFLOW VALVES PER MM 21-31-00. OPERATIONAL TEST GOOD. AIRCRAFT OK FOR SERVICE.