CL65 CREW HAD AN IMPROPER MEL ITEM THAT MAINT DEFERRED.

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THE FOLLOWING IS TO DESCRIBE AN IMPROPER DEV FROM MAINT PROCS DURING A SCHEDULED REVENUE FLT ON OCT/SUN/00. WHEN 10 MINS FROM OUR DEST; ON A FLT FROM FT WAYNE; IN; TO CINCINNATI; OH; THE CAPT (PNF) RECEIVED A RADIO CALL FROM COMPANY MAINT AND WAS INFORMED THAT THE ACFT WAS IMPROPERLY DEFERRED FOR A LIGHTING PROB BEFORE FLT. AFTER THE RADIO DISCUSSION; WHICH I DID NOT MONITOR; THE CAPT THEN TOLD ME OF THE NEW PROB. BEFORE FLT; HE HAD DISCOVERED THAT THE TEST #1 FUNCTION OF THE LAMP TEST WARNING AND CAUTION LIGHTS HAD FAILED. THROUGH A PHONE CALL TO MAINT AND DISPATCH; HE RECEIVED INSTRUCTIONS TO DEFER THE ITEM WITH AN MEL FOR LAMP TEST SWITCH. THE CAPT QUESTIONED IF THIS WAS THE CORRECT DEFERRAL AND WAS ASSURED THAT IT WAS (HE HAD MENTIONED THE DEFERRAL TO ME BEFORE THE FLT AND WE PROCEEDED). AFTER RECEIVING THE NEW INFO OF THE IMPROPER DISCREPANCY (SHOULD HAVE BEEN A LAMP DRIVER UNIT) HE THEN SHOWED ME WHAT THE PROB WAS BY DEMONSTRATING HOW THE LAMP TEST WAS FAILING AND THAT IT DID NOT APPEAR TO BE A SWITCH PROB. THEREFORE; THE ACFT WAS DISPATCHED WITH AN IMPROPERLY DEFERRED ACFT AND COULD NOT BE CORRECTED INFLT (REQUIRED MAINT PERSONNEL TO PERFORM ACTION BEFORE FLT). ALTHOUGH THE HAZARDS IN THIS CASE WERE MINIMAL; THE SAME ERRORS COULD HAVE OCCURRED UNDER DIFFERENT CIRCUMSTANCES THAT MIGHT CONTRIBUTE TO AN ACCIDENT. ALSO; IF AN INCIDENT OCCURRED; ANY INVESTIGATION COULD HAVE BEEN IMPEDED DUE TO INACCURATE MAINT LOGS AND COULD HAVE RESULTED IN INCORRECT CONCLUSIONS.