What was found
ON CLIMBOUT AT FL 190 HEARD A LOUD BANG AND FELT AIRCRAFT SHUDDER. CHECKED PRESSURIZATION AND FOUND OUTFLOW VALVE WAS IN FULL CLOSED POSITION, CABIN PRESSURE WAS CLIMBING. MANUALLY DEPLOYED OXYGEN MASKS & MADE EMERGENCY DESCENT. DECLARED EMERGENCY & TURNED BACK TO PHX. MX FOUND A BLOWN DUCT IN RT A/C PACK BAY, REMOVED & REPLACED DUCT ASSEMBLY IN RT PACK BAY, LEAK CHECK WAS GOOD. THE CHECK VALVE THAT PREVENTS REVERSE AIRFLOW FROM THE MIX MANIFOLD ALSO FAILED, FLAPPER DISK WAS TORN OFF HINGES & FOUND WEDGED IN DUCTING. REMOVED & REPLACED CHECK VALVE ASSEMBLY, OPERATIONAL CHECK GOOD. REPLACED ALL OXYGEN MASKS & GENERATORS.