What was found
AT 30,000 FT HEARD LOUD POP. CABIN ALT ALERT ON. DESCENDED TO 10,000 FEET. MASK DEPLOY IN CABIN. INSPECTED AND FOUND LARGE CHECK VALVE ON AIR CONDITION SYSTEM LOCATED AT AFT PRESSURE BULKHEAD WITH DAMAGED FLAPPER VALVE AND SPIDER DUCT SEPARATED. REMOVED AND REPLACED CHECK VALVE AND SPIDER DUCT ASSEMBLY PER AMM 21-22. PERFORMED CABIN PRESSURE CHECK. REF AMM 21-30, OPS CHECK GOOD.