A B737-300 FLT CREW EXPERIENCED LOSS OF CABIN PRESSURE IN CLIMB ABOVE 30000 FT. THEY REGAINED CONTROL USING THE MANUAL CONTROLLER.

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CLBING FROM FL300 TO FL340; THE CABIN ALT WARNING HORN SOUNDED. FO WAS FLYING AND SILENCED THE HORN. THE CABIN WAS CLBING AT 1000 FPM. I HIT THE ALT HOLD AND TURNED THE PRESSURE CONTROLLER TO MANUAL. THE CABIN ALT CLBING STOPPED. I TOLD CTR WE WERE HAVING PRESSURIZATION PROBS AND WERE DSNDING TO FL300. FO STARTED DOWN WHILE WE ASSESSED THE PROB. BEFORE WE GOT TO FL300; I ASKED TO CONTINUE TO FL280. AFTER THE CONTROLLER WAS IN MANUAL; THE PRESSURIZATION WAS UNDER CTL. BOTH PACKS AND BLEEDS WERE IN THEIR NORMAL POS WITH NORMAL PRESSURE. THE OUTFLOW VALVE GAUGE SHOWED CLOSED. WE WENT THROUGH THE QRH ITEMS. NO MASKS DROPPED. FO CALLED DISPATCH AND TALKED TO MAINT AND THEY TOLD US TO CONTACT MAINT. WE ACARS'ED MAINT CTL AND GOT AN ACARS FROM DISPATCH ASKING IF WE DID AN EMER DSCNT AND WHAT ALT WE WERE AT. WE DID NOT NEED TO DO AN EMER DSCNT SINCE THE PRESSURIZATION WAS UNDER CTL. NEXT; DISPATCH CHANGED OUR DEST TO ZZZ AND THEN ASKED IF WE WERE OK AT FL280 AND IF WE WERE A NON EMER. THERE WAS NO EMER AT THAT TIME. CTR TURNED US BACK TO THE ARR TO ZZZ AND DSNDED US TO FL270 FOR DIRECTION. I TOLD THE PAX WE HAD A PRESSURIZATION PROB AND WERE RETURNING TO ZZZ. WE CTLED THE PRESSURIZATION IN THE MANUAL MODE BACK TO ZZZ. FIX THE PROB FROM THE DAY BEFORE WHEN THE SAME PROB OCCURRED ON THIS ACFT.