EMB120 FLC OVERSHOT THEIR ALT DURING DSCNT. CREW WAS BUSY AND FAILED TO PAY ATTN TO THEIR DSCNT PROFILE. ACFT WAS BEING OPERATED WITHOUT THEIR TCASII SYS. OTHER ACFT NOTED THEIR PRESENCE; APPARENTLY WITH THEIR TCASII.

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THE WX AT TLH REGIONAL ARPT WAS IFR WITH A 500 FT OVCST AND THE TOPS WERE ABOUT 2000 FT. THE WX ON TOP WAS VMC WITH GOOD VISIBILITY. AT APPROX AX25 WE WERE LEVEL AT 9000 FT AND CLRED DOWN TO 7000 FT AND WERE ALSO ASSIGNED A HDG. THE CAPT; WHO WAS THE PF; STARTED THE DSCNT AND ASKED FOR THE IN-RANGE CHKLIST AND THE INST APCH CHKLIST. I; THE FO AND PNF; PERFORMED THE IN-RANGE CHKLIST. I THEN BEGAN THE INST APCH CHKLIST BY FIRST BRIEFING THE ILS TO RWY 27 AND THEN I BEGAN PERFORMING THE OTHER CHKS INCLUDED; WHEN I HAD A QUESTION CONCERNING THE CHKING OF THE RADAR ALTIMETERS. THE CAPT EXPLAINED TO ME THE ANSWER TO MY QUESTION AND AT THE SAME TIME WE BOTH LOOKED UP AND REALIZED WE WERE AT 6500 FT AND DSNDING. THE CAPT IMMEDIATELY ARRESTED THE SINK RATE AND BEGAN A CLB AT 6450 FT. AT ABOUT THE SAME TIME THE ACFT OFF OUR R WING AND SLIGHTLY AHEAD QUERIED THE CTLR ABOUT OUR ALT. THE CTLR THEN GAVE US OUR ORIGINAL ALT OF 7000 FT AGAIN AND GAVE US A HDG OF 10 DEGS TO THE L. THE REST OF THE FLT AND APCH WAS UNEVENTFUL. AT THE TIME THE CTLR GAVE US 10 DEG TURN TO THE L WE WERE AT 6700 FT AND CLBING. SOME OF WHAT I BELIEVE TO BE THE CONTRIBUTING FACTORS ARE: 1) FOR SOME REASON THE ALT ALARM DID NOT GO OFF. 2) I AS THE FO HAD A QUESTION ABOUT AN ITEM ON THE CHKLIST THAT I SHOULD HAVE KNOWN. 3) THE CAPT (PF) AND MYSELF (PNF) WERE PREOCCUPIED AND NOT FLYING AND MONITORING THE AIRPLANE PROPERLY. 4) THE TCASII WAS INOP. 5) I BELIEVE THAT ALSO; ON A SMALL SCALE; WE WERE A LITTLE TIRED; WE WERE IN THE LAST 20 MINS OF A 12 HR DUTY DAY; ON THE LAST DAY OF A 4- DAY SCHEDULE.