A B737-300 IN CRUISE AT FL350 HAD THE CABIN ALT WARNING ACTIVATED. CABIN WAS AT 9700 FT AND CLBING. ACTIVATED PAX MASKS. DIVERTED. CAUSED BY A FAILED FORWARD CARGO DOOR SEAL.

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WHILE AT CRUISE OF FL350 NEAR ZZZ1; ALT WARNING HORN ACTIVATED. CABIN ALT WAS APPROX 9700 FT AND CLBING AT 500 FPM. FO AND I PUT ON OXYGEN MASKS AND PERFORMED OUR QRH ITEMS TO CTL CABIN. AS CAPT; I ASKED FOR LOWER ALT AND TOLD ATC WE HAD A PRESSURIZATION PROB; TOLD THE FLT ATTENDANT TO BE PREPARED FOR ME TO ACTIVATE THE PAX OXYGEN MASKS IF WE COULD NOT GET CABIN CTLED. I ELECTED TO HAVE FO FLY AS IT WAS NOT A RAPID DEPRESSURIZATION PROB AND I COULD COORD FLT ATTENDANT AND PAX. I ASKED FOR 14000 FT WHEN CABIN CONTINUED TO CLB THROUGH 12000 FT AND I ACTIVATED THE PAX OXYGEN MASKS AFTER MAKING A PA TO PAX. ATC CLRED US DIRECT TO ZZZ. OUR MAIN COMPANY FREQ WAS BLOCKED FOR SOME UNKNOWN REASON AND HAD TO CALL MAINT FREQ. ASKED THEM TO HAVE EMT'S STANDING BY. MAINT UNFORTUNATELY CALLED OUT ALL CFR; EMC AND MADE IT A BIGGER PROB THAN IT SHOULD HAVE BEEN. LNDG WAS UNEVENTFUL; BUT EMC/CFR BARGED ONTO MY AIRPLANE AND DEMANDED EVERYONE REMAIN SEATED! THIS UPSET THE PAX. ONLY A LADY WITH HER SICK BABY ASKED FOR MEDICAL ATTN. MAINT HAD NOT FOUND WHAT CAUSED THE PRESSURIZATION PROB. COMPANY RPT FILED. CABIN ALT WAS FINALLY CTLED BY DSNDING TO 14000 FT AND SOMEHOW WENT DOWN TO FIELD ELEVATION WE HAD SET IN STANDBY PRESSURIZATION CONTROLLER. ONCE CABIN WAS BELOW 10000 FT; I HAD FLT ATTENDANT AND PAX TAKE THEIR OXYGEN MASKS OFF AND REASSURED PAX. PLTS TOOK OFF OUR MASKS AS WELL. FLEW FOR APPROX 20 MINS AT 14000 FT MSL; BUT CABIN WAS DOWN NEAR FIELD ELEVATION BY THEN. STILL DO NOT KNOW WHY CABIN PRESSURIZATION COULD NOT BE CTLED IN STANDBY OR MANUAL AC/DC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED AN OLD PLT INSTRUCTOR TAUGHT A VALUABLE LESSON TO FLT CREWS THAT AT FL350 IF THE CABIN PRESSURE ALT HORN SOUNDS; 'IT AIN'T THE TKOF WARNING HORN!' RPTR STATED IT SURE WAS EASY SILENCING THE HORN AND STARTING DESCENT. THE PRESSURIZATION WAS UNCTLABLE IN AUTO AC/DC AND IN STANDBY MODES. ON THE GND; MAINT FOUND THE FORWARD CARGO DOOR SEAL FAILED. SEAL WAS REPLACED INCLUDING THE OUTFLOW VALVE AND PRESSURE CONTROLLER AS PRECAUTIONARY.