FLC OF AN MD80 MADE AN EMER DSCNT BY FL190 WHILE ON DEP CLB DUE TO LOSS OF CABIN PRESSURIZATION.

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ACFT WAS DISPATCHED ON THIS DATE WITH ONE OF THE AUTOPRESSURIZATION CONTROLLERS INOP. THE ACFT LOG WAS APPROPRIATELY COMPLETED AND THE MEL WAS CHKED BY THE FLC AND FOUND TO PERMIT SUCH AN OP. THE FLT SUBSEQUENTLY LEFT THE GATE WITH THE NORMAL DISPATCH FUEL LOAD AND 76 PAX. A CREW OF 5 WAS ALSO ON BOARD THE ACFT. EWR WX WAS 2500 FT BROKEN AND 6 MI VISIBILITY. SURFACE WINDS WERE 260 DEGS AT 6 KTS. RWY 22R INTXN Y WAS THE RWY IN USE. THE FLT BLOCKED OUT AT XA21Z. AFTER ALL CHKLISTS WERE COMPLETED; AND AFTER RECEIVING CLRNC; THE FLT BECAME AIRBORNE AT XA48Z. THE FO WAS FLYING THE ACFT. AT 10000 FT THE CABIN PRESSURE WAS CHKED. ALL CTLS AND INDICATORS WERE COMPLETELY NORMAL. THE AFTER TKOF CHKLIST WAS COMPLETED. AT FL180 THE ALTIMETERS WERE SET TO 29.92 AND THE FUEL WAS CHKED. WITHIN SECONDS AFTER THAT; THE MASTER WARNING LIGHT ILLUMINATED AND 'CABIN ALT' WARNING WAS HEARD. WE IMMEDIATELY DONNED OXYGEN MASKS AND AT APPROX FL190 REQUESTED IMMEDIATE DSCNT TO 10000 FT DUE TO CABIN PRESSURE PROBS. THE DSCNT CLRNC WAS IMMEDIATELY GIVEN BY THE CTLR. THE RAPID DSCNT PROC WAS INITIATED. IF THE CLRNC WAS NOT GIVEN IMMEDIATELY; I WOULD HAVE DECLARED AN EMER AT THAT TIME. BUT IT TURNED OUT TO BE NOT NECESSARY. THE NEXT OBJECTIVE BECAME TO RAPIDLY DSND SO AS TO NOT ALLOW THE OXYGEN MASKS TO FALL IN THE CABIN AS NOTING THE CABIN ALT; IT WAS NEARING 14000 FT. WE ROLLED THE MANUAL OUTFLOW VALVE CONTROLLER TO THE CLOSED POS AND BEGAN THE RAPID DSCNT CHKLIST FOLLOWED BY THE CABIN PRESSURE LOSS CHKLIST. THE MASKS IN THE CABIN DID NOT DEPLOY AS OUR ATTEMPT TO DSND WAS SUCCESSFUL. ALSO DURING THIS TIME OUR REQUEST TO RETURN TO EWR WAS GRANTED AND ROUTING WAS OBTAINED FROM ATC. THE DSCNT APCH CHKLIST WAS COMPLETED. THE CABIN ATTENDANTS WERE BRIEFED ALONG WITH THE PAX THAT A PRESSURIZATION PROB HAD BEEN EXPERIENCED AND THAT WE WOULD BE RETURNING TO EWR. THEY WERE NOTIFIED THAT THE ACFT WAS IN A SAFE AND AIRWORTHY CONDITION AND A NORMAL LNDG COULD BE EXPECTED AT EWR. THE CABIN TEAM WAS THEN TOLD TO PREPARE THE CABIN FOR LNDG. THE ACARS DIVERSION MASK WAS COMPLETED. EWR OPS WAS CALLED AND INFORMED THAT WE WOULD BE RETURNING. ATC THEN ASKED IF WE WANTED TO DECLARE AN EMER AND HAVE EMER EQUIP AVAILABLE ON THE RWY. I TOLD THEM THAT IT WOULD NOT BE NECESSARY. THERE WERE NO DOORS OPEN OR ANY OTHER APPARENT STRUCTURAL DAMAGE OR VIBRATION. IN FACT; WE WERE AWARE THAT IT WAS A PROB CONFINED TO THE OUTFLOW VALVE MECHANISM. UPON REACHING 5000 FT; I NOTICED THAT THE CABIN BEGAN TO REPRESSURIZE AND WAS NOW APPROX 1500 FT BELOW THE FIELD ELEVATION. SEVERAL UNSUCCESSFUL ATTEMPTS WERE MADE TO OPEN THE OUTFLOW VALVE MANUALLY. THE CABIN HAD NOT YET GONE TO A RELIEF STATE; SO I TURNED OFF BOTH PACKS TO DEPRESSURIZE THE ACFT FOR LNDG. EWR WX WAS THE SAME AS OUR DEP AS WE WERE ONLY AIRBORNE 27 MINS. THE ILS RWY 22L APCH WAS BRIEFED AND COMPLETED TO A NORMAL LNDG. THE FO COMPLETED THE NORMAL LNDG AT XB15Z. UPON CLRING THE ACTIVE RWY; THE AFTER LNDG CHKLIST WAS COMPLETED; THE FO'S WINDOW OPENED; AND THE PACKS TURNED BACK ON. THIS WAS IN AN EFFORT TO RE-COOL AN UNCOMFORTABLY WARM CABIN AS THE OUTFLOW VALVE WAS STILL JAMMED IN THE CLOSED POS. FLT BLOCKED BACK IN AT XB25Z. THE SECURE COCKPIT CHKLIST WAS COMPLETED. APPROPRIATE LOGBOOK NOTATIONS WERE COMPLETED. THE MECHS AT EWR REPLACED THE PRESSURE CONTROLLER AND FOUND SOME SORT OF INSULATION MATERIAL TANGLED IN THE OUTFLOW VALVE ASSEMBLY. THE ACFT WAS REPAIRED SEVERAL HRS LATER.