What was found
CABIN PRESSURE CREW REPORTED DURING CRUISE, AT FL 330, CABIN ALT. HORN SOUNDED. CABIN ALT WAS AT 10,500 FT & DUCT PRESSURE AT 38 PSI. BOTH ENGINE BLEEDS WERE ON AND BOTH PACKS ON. UNABLE TO CONTROL CABIN IN STANDBY MODE. DESCENDED TO FL 240, & ABLE TO CONTROL CABIN IN MANUAL MODE, CABIN DESCENDED TO ALT OF 7500 AT FL 240. LANDED WITHOUT FURTHER EVENT. MX PLACED AUTOMATIC & STANDBY CABIN PRESSURE CONTROL SYSTEM ON MEL. REPLACED CABIN PRESSURE CONTROL IAW BOEING 737-300 MM 21-31-21. OPS CHK GOOD. COULD NOT CONFIRM FAILURE. UNIT WAS INSPECTED AND TESTED IAW MFG SPECIFICATIONS PER ATA 21-31-11 REV 14.