What was found
FLIGHT 7820 EN-ROUTE, AIR TURN BACK, DISCREPANCY. WHILE EN-ROUTE THE CREW RECEIVED AN EXCESSIVE CABIN ALTITUDE WARNING. THE CREW PROCEEDED WITH THE QRH, BUT WERE UNABLE TO CONTROL THE CABIN ALTITUDE. THE CREW ELECTED TO RETURN & NO EMER DECLARED. THE ACFT LANDED WITHOUT INCIDENT. PERFORMED OPERATIONS CHECK OF BOTH LT & RT PACKS. BOTH PACKS OPERATING NORMAL. PRESSURIZATION CHECK OPERATED NORMAL, COULD NOT DUPLICATE.