B737-400 EXPERIENCES SLOW DECOMPRESSION REQUIRING AN EMER DSCNT FROM FL280 AND A RETURN TO DEP ARPT.

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A NORMAL PREFLT; TAXI; TKOF; AND CLB WERE CONDUCTED AND ALL CHKLISTS COMPLETED THROUGH FL180. AT ABOUT FL280 AND AROUND XA05Z; THE ALT WARNING HORN SOUNDED. OXYGEN MASKS WERE DONNED; COMMUNICATIONS ESTABLISHED; AND THE CABIN ALT WAS NOTED AT JUST ABOVE 10000 FT AND SLOWLY CLBING. NO WARNING OR ABNORMAL LIGHTS WERE NOTED ON THE PRESSURIZATION PANEL (OR ANY OTHER PANEL FOR THAT MATTER). THE QRH FOLLOWED WITH NO RESULTS AND THE OUTFLOW VALVE WAS NOTED STUCK IN THE NEAR CLOSED POSITION. AN EMER WAS DECLARED; A DSCNT TO 12000 FT (ASSIGNED BY ATC FOR TERRAIN CLRNC) WAS COMMENCED AND L TURN TOWARD ZZZ WAS STARTED AS ASSIGNED ATC. AT ABOUT 5000 FT THE CABIN ALT WAS NOTED TO DSND BELOW 0 FT. CYCLING THE PRESSURIZATION CTLR BETWEEN STANDBY AND MANUAL RELIEVED THE PRESSURE; THE PACKS WERE SECURED AS THE CABIN REACHED THE ACFT ALT AT ABOUT 3000 FT AND AN UNPRESSURIZED UNEVENTFUL LNDG WAS MADE AT ZZZ. ON LONG FINAL; THE 'A' FLT ATTENDANT CALLED TO INFORM US ONE PREGNANT PAX WAS EXPERIENCING SOME PAIN SO PARAMEDICS WERE REQUESTED THROUGH THE TWR. THE CABIN ALT NEVER EXCEEDED 12000 FT THROUGHOUT THE FLT. IT WAS NOT REALIZED UNTIL BLOCK IN THAT MANUAL DC HAD NOT BEEN TRIED INFLT. AFTER BLOCK IN; THE FLT ATTENDANT CREW WAS DEBRIEFED AND ALTHOUGH ALL 3 RPTED THEY FELT OK; THEY WERE RELEASED FROM DUTY.SUPPLEMENTAL INFO FROM ACN 687193: THE FWD LAV SMOKE ALARM WENT OFF AT SOME TIME DURING THIS EVENT; CAUSING A VERY HIGH PITCH SQUEAL THAT LASTED UNTIL I SHUT IT OFF AFTER BLOCK IN. DURING THE FLT; THIS WAS A HUGE DISTRACTION AND I THOUGHT IT MAY HAVE BEEN A CABIN/DOOR SEAL LEAK. OVERSPD DURING DSCNT. RAPID DEPRESSURIZATION/CABIN ALT WARNING HORN QRC/QRH SAYS TO PUT PRESSURIZATION CTLR IN MANUAL MODE. ONCE ON THE GND; WE REALIZED WE HAD NOT TRIED THE MANUAL DC MODE. QRH DOES NOT DELINEATE EITHER. ONCE THE EVENT OCCURRED I SHOULD HAVE TRIED TO USE NOT ONLY MANUAL AC MODE; BUT ALSO MANUAL DC MODE.