ALTDEV ALT EXCURSION OCCURS DURING FLC'S ATTEMPT TO CORRECT AN ACFT EQUIP PROB MALFUNCTION.

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DURING MY CLB (FO) OUT OF BDL THROUGH 12000 FT FOR 17000 FT THE CABIN ALT WARNING VOICE CALLED OUT ALONG WITH THE HORN. (THE CAPT TOOK OVER) THE ACFT AND LEVELED OFF AT 12000 FT. I RECOMMENDED A DSCNT DOWN TO 10000 FT AS I INFORMED ATC THAT WE HAD STOPPED THEIR REQUESTED EXPEDITED CLB TO 17000 FT. THE CAPT SAID TO GO AHEAD AND REQUEST 10000 FT. I ASKED FOR 10000 FT AND TOLD ATC THE NATURE OF THE PROB. THEY CLRED US TO ONLY 11000 FT SO I RECOMMENDED TO THE CAPT THAT WE DECLARE AN EMER AND GET TO 10000 FT. HE OK'D IT AND I INFORMED ATC THAT WE WERE NOW AN EMER. ATC SWITCHED US BACK TO BRADLEY APCH AND CLRED US DOWN TO 10000 FT. WHILE REVIEWING THE QRH PROCS I CAUTIONED THE CAPT AT 11000 FT THAT WE WERE 1000 FT TO GO AND AT 500 FT TO GO AGAIN AND TO WATCH HIS HIGH RATE OF DSCNT. HE LEVELED OFF AT 10000 FT BUT LATER DSNDED DOWN TO 9500 FT WHILE WATCHING ME READ THE QRH. HE THEN CLBED BACK UP TO 10000 FT. THE CABIN OUTFLOW VALVE HAD BEEN CLOSED SOMEHOW DURING THE HAND ON THROTTLE EXCHANGE BTWN PF AND PNF. IT WAS CORRECTED BY MYSELF AND THE EMER WAS TERMINATED. THE FLT RESUMED NORMALLY AND UPON CONTACT WITH THE CABIN NO DISCREPANCIES WERE NOTED BY THE FLT ATTENDANTS. ALSO TO NOTE FOR THE ABOVE: WHEN THE DSCNT BELOW 10000 FT WAS ACCIDENTLY MADE I RECOMMENDED THAT THE CAPT ENGAGE THE AUTOPLT TO HELP EASE HIS WORKLOAD. NO COMMENTS WERE MADE FROM ATC ABOUT THE ALTDEV.