A COMMUTER FLC OVERSHOT THEIR ASSIGNED ALT DURING DEP. THE OLD MODEL ALT ALERTER HAD ONLY GIVEN A FAINT AURAL SIGNAL WHICH WENT UNNOTICED.

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ON CLBOUT FROM SWF WE WERE CLRED TO 5000 FT AND THAT WAS SET INTO ALT ALERTER. AT APPROX 3000 FT I SWITCHED TO THE SECONDARY RADIO TO GIVE THE COMPANY OUR OFF TIMES. THEY ASKED ME A QUESTION THAT REQUIRED ME TO LOOK UP INFO FROM A PREVIOUS FLT. AFTER FINISHING THAT TASK I RETURNED MY SCAN TO THE INSTS. THE CAPT WAS PASSING 6000 FT AND CLBING AT 1800 FPM; BUT THE ALT ALERTER WAS STILL SET AT 5000 FT. I WAS POINTING AT THE ALERTER SETTING AND ASKING IF WE'D BEEN CLRED HIGHER WHEN DEP ASKED WHAT WE WERE DOING AT 7000 FT. CAPT LEVELED THE ACFT AND STARTED US BACK DOWN WHEN DEP CLRED US TO 7000 FT AND SWITCHED US TO ZBW. OLD MODEL ALERTER WAS INSTALLED WHICH HAS A VERY FAINT AURAL ALERT ALLOWING SIGNAL TO GO UNNOTICED. I STARTED OTHER DUTIES BEFORE REACHING THE ASSIGNED ALT AND THUS FAILED TO PROVIDE ADEQUATE BACKUP. THIS WAS THE 4TH LEG OF A 7 LEG DAY AND AS THE ACFT HAD ARRIVED LATE FROM A PREVIOUS FLT WE WERE PRESSED TO MAKE UP TIME TO GET BACK ON SCHEDULE. I HAD BEEN IN THE SEAT FOR 5 HRS WITH ONLY A 20 MIN BREAK PRIOR TO THE INCIDENT.