TASK DISTR CONTRIBUTES TO AN ALT OVERSHOOT IN DSCNT.

Narrative

THE FLT WAS ASSIGNED A DSCNT TO 8000 FT. THE ALT WAS PROPERLY SET IN THE ALT REMINDER WINDOW AND CALLED OUT 1000 FT PRIOR TO LEVELOFF. WE WERE IMC AND THE WX RADAR WAS ON. PASSING APPROX 8200-8300 FT WE RECEIVED A TCASII TA. BOTH OF US LOOKED AT THE SCREEN TO IDENT THE TFC. THE SCREEN WAS ON TOO LARGE A SCALE AND WAS CLUTTERED WITH THE WX RADAR. AS A FAIRLY NEW FO WITH THE COMPANY; I HAD ONLY SEEN THE RADAR ON TWICE BEFORE AND HAD NEVER FOUND IT ON OR OFF MYSELF. I ADJUSTED THE SCALE AND THEN FUMBLED WITH THE SWITCHES TO TURN OFF THE RADAR IN ORDER TO IDENT THE TFC. AFTER WHAT COULD ONLY HAVE BEEN 5 SECONDS; THE CAPT REACHED OVER AND TURNED OFF THE RADAR HIMSELF. AS WE INSPECTED THE SCREEN WE RECEIVED A TCASII RA 'REDUCE DSCNT.' THE CAPT THEN NOTED THAT WE HAD DSNDED THROUGH OUR ASSIGNED ALT. I DON'T RECALL SEEING ANY LOWER THAN 7700 FT. AS WE LEVELED OFF AT 8000 FT MSP APCH ASKED US TO CONFIRM 8000 FT ASSIGNED. WE THEN CONFIRMED THAT WE WERE LEVEL AT 8000 FT AND THE MATTER SEEMED CLOSED. CONTRIBUTING FACTORS: A CLUTTERED SCREEN; FO UNFAMILIAR WITH SWITCHES ON THE OTHER SIDE OF THE COCKPIT (RADAR); CAPT DISTRACTED BY TCASII AND FO. IF I HAD BEEN MORE FAMILIAR WITH THE RADAR SWITCHES THEN THE INCIDENT MAY HAVE BEEN AVOIDED. PERHAPS THIS SHOULD BE ADDED TO SIMULATOR/CLASSROOM INSTRUCTION. IF THE CAPT HAD CONCENTRATED ON FLYING THE ACFT FIRST THEN THE INCIDENT WOULD HAVE BEEN AVOIDED.