What was found
AT 33,000 FT ALTITUDE WARNING HORN SOUNDED (CABIN PRESSURE SHOWED 9,500 +/- FT) HAD TO DESCEND TO 10,000 FT. REGAINED CABIN IN STANDBY MODE WHICH WORKED PROPERLY FOR THE REMAINER OF FLIGHT (STANDBY PRESSURE OK AT 23,000). REPLACED PRESSURE CONTROLLER PER AMM 21-31-21. PERFORMED OPERATIONAL TEST OF PRESSURIZATION CONTROL SYSTEM PER AMM 21-31-00. CHECKS NORMAL.