B737-300 CREW INITIALLY MISINTERPRETED THE ALT WARNING HORN AS HAVING AN ACFT CONFIGURATION PROB AND DELAYED RECOGNITION OF AN ACFT DEPRESSURIZATION PROB.

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WE HAD BEEN AT CRUISE FL330 FOR ABOUT 10-15 MINS AND THE CABIN ALT WARNING HORN SOUNDED WITH NO OTHER CAUTIONS OR ABNORMALITIES. WE CHKED ALT AND IT SHOWED 10000 FT AND CLBING ABOUT 500 FPM. WE DONNED OUR OXYGEN MASKS AND I TOLD ATC WE WERE HAVING A CABIN PRESSURIZATION PROB AND DSNDING. WE WERE CLRED TO FL240 AND WE STARTED DOWN USING RAPID DSCNT PROCS. WE ESTABLISHED 2-WAY COM; ALTHOUGH I WAS HAVING A HARD TIME HEARING THE FO. I THEN PUT THE PACKS TO HIGH; BUT CABIN CONTINUED TO CLB. I VERIFIED BLEED SWITCHES WERE ON AND DUCT PRESSURE WAS NORMAL. I THEN LOOKED AT THE OUTFLOW VALVE POS INDICATOR AND IT SHOWED FULL CLOSED. AT THIS POINT; I ELECTED NOT TO SELECT MANUAL AND MANIPULATE THE OUTFLOW VALVE SWITCH. WE RECEIVED A CLRNC TO 10000 FT AND CONTINUED TO DSND MAX RATE WHILE CABIN CONTINUED TO CLB 500 FPM. I CALLED THE FLT ATTENDANTS AND INFORMED THEM OF THE SIT AND TO STANDBY FOR FURTHER INSTRUCTIONS. I FINISHED THE REMAINDER OF THE QRH CHKLIST FOR CABIN ALT WARNING HORN AND COORDINATED A DIVERT TO ZZZ. I CONTINUED TO MONITOR THE CABIN INDICATIONS AND NOTICED AROUND FL180 THE CABIN STARTED DSNDING BUT THE OUTFLOW WAS OPENING. AT THIS POINT; I DIRECTED THE FO TO SELECT MANUAL AND CLOSE THE OUTFLOW VALVE. WHEN WE DID THIS; THE CABIN STARTED CLBING. I THEN DIRECTED THE FO TO RESELECT AUTO MODE; AND THE CABIN STARTED DSNDING. THE CABIN NEVER EXCEEDED 13000 FT AND PAX OXYGEN SYS WAS NOT ACTIVATED. WITH SOME SEMBLANCE OF CTL; WE CONTINUED TO 10000 FT AND DIVERTED. AT THIS POINT THE CABIN INDICATOR SHOWED 10000 FT AND DIFFERENTIAL PRESSURE WAS ZERO. I RESET THE CONTROLLER TO THE ELEVATION OF ZZZ AND CABIN STARTED DSNDING 500 FPM. WE CONTACTED OPS AND HAD THEM CALL DISPATCH AND LANDED WITHOUT FURTHER INCIDENT. I SUBMIT THE FOLLOWING OBSERVATIONS ABOUT OUR PERFORMANCE AND HUMAN FACTORS ISSUES IN HOPES OF SOME DEGREE OF LEARNING: 1) I ELECTED NOT TO MANIPULATE THE OUTFLOW VALVE BECAUSE THE FEW TIMES IN THE LAST FEW YRS I HAVE HAD CAUSE TO DO SO; IT ALWAYS MADE THE CONDITION WORSE; IE; HUGE CHANGES IN CABIN RATES OF CLB OR DSCNT. THE OUTFLOW VALVE INDICATED CLOSED TO USE AND IT SEEMED REDUNDANT TO 'CLOSE THE OUTFLOW VALVE.' THIS IS A COMPLETE LACK OF CHKLIST DISCIPLINE; BUT BASED ON PREVIOUS EXPERIENCES; I DIDN'T WANT TO MAKE IT WORSE. AT THIS POINT; I WAS PROBABLY THINKING TOO MUCH; WHICH IS HARD TO DO WITH THE MASKS ON; COORDINATING A DIVERT; TRYING TO ESTABLISH 2-WAY COM; AND WONDERING 'WHAT AM I SCREWING UP HERE?' LESS THINKING AND MORE CHKLIST DISCIPLINE MIGHT HAVE BEEN A BETTER COURSE OF ACTION. 2) I WAS AMAZED HOW LONG (10-15 SECONDS) IT TOOK US TO FULLY REALIZE WHAT WAS HAPPENING. I ALWAYS THOUGHT I WOULD INSTANTLY RECOGNIZE THE 'TKOF WARNING HORN' AS A 'CABIN ALT WARNING HORN' ONCE I WAS AIRBORNE. NOT SO. WE ARE SO 'PAVLOVIAN' IN OUR RESPONSE TO THAT SOUND; ESPECIALLY NOW WITH OUR HEIGHTENED AWARENESS OF THE HORN AND RTO'S. 3) CREW COM WASN'T FULLY ESTABLISHED UNTIL I ASKED FO IF HE WAS USING BOTTOM HALF OF YOKE PUSH-TO-TALK. THESE 3 ITEMS ARE TRAINING ISSUES. MORE PRESSURIZATION PROBS IN PT'S WITH ACTUALLY DONNING OF MASKS; ETC. 4) AFTER I HAD BEEN ON THE GND FOR A LITTLE WHILE; MY LEGS WERE A LITTLE SHAKY. I WAS VERY AWARE OF THIS AND ASKED FO AND FLT ATTENDANTS IF THEY WERE OK. THEY SAID THEY WERE. BUT BY THE TIME WE GOT ANOTHER AIRPLANE; I WAS FINE AND READY TO GO AGAIN. HOWEVER; I WAS VERY DISTR ON THE NEXT LEG RELIVING THE EVENT; SELF-CRITICIZING; REREADING CHKLISTS; READING FLT MANUAL; ETC. I THOUGHT I WAS READY -- MAYBE I WASN'T. I WAS VERY CONCERNED ABOUT THE AMOUNT OF CRITICISM I WOULD RECEIVE FOR LACK OF CHKLIST DISCIPLINE. 5) SCHEDULING REROUTED ME THAT NIGHT; LNDG AT XA20. I HAD AN XI00 THE NEXT MORNING RPT IN ORDER DEADHEADING 2 LEGS BACK ON MY TRIP AND A NEARLY 13 HR DUTY DAY. WHEN I GOT TO THE ARPT THE NEXT MORNING; I WAS NOT PHYSICALLY READY TO FLY AND CALLED IN FATIGUED.