FAI CTLR EXPERIENCED OPERROR WHEN AN ALT; BELOW MVA; WAS ISSUED AND A CLRNC READBACK WAS MISSED.

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WHILE WORKING THE W RADAR POS; I TOOK A HDOF ON A C208 INBOUND IFR TO FAI. AT THE TIME I WAS ALSO VECTORING AN IFR B190 TO AVOID AN IFR PA31 THAT I HAD LOST COM WITH AND WAS TREATING AS A NORDO ACFT. WHEN I TOOK THE HDOF ON THE C208; I WROTE ON ITS FLT PROGRESS STRIP THE APCH I INTENDED TO GIVE IT AND THE ALT I WANTED IT TO DSND TO (5000 FT MSL). I WAS PREOCCUPIED WITH THE NORDO ACFT DUE TO UNCTLED TFC IN FRONT OF IT AS WELL AS THE OTHER IFR ACFT I WAS VECTORING BEHIND IT; WHEN THE C208 CHKED ON FREQ. WITHOUT REALIZING IT; I ISSUED A DSCNT FROM 7000 FT TO 4000 FT. I INTENDED TO ISSUE 5000 FT AND MISSED THE PLT'S READBACK OF 4000 FT. I SAW THE ACFT DSNDING BELOW 5000 FT. I ASKED THE PLT TO CONFIRM LEVEL AT 5000 FT; AND WAS SURPRISED WHEN IT ADVISED IT HAD BEEN ISSUED 4000 FT. I THEN ISSUED 5000 FT AND THE ACFT CLBED BACK UP. THIS IS AN ERROR BECAUSE THE MVA IN THE ACFT'S AREA IS 5000 FT. THIS RESULTED IN A LOSS OF SEPARATION WITH TERRAIN. NO ALT ALERT OCCURRED. THIS PROB OCCURRED BECAUSE I BECAME PREOCCUPIED WITH THE ABNORMAL SIT. THE B190 WAS IN AN AREA TO DSND TO 4000 FT. I THINK THAT WHEN THE C208 CHKED ON; I WAS IN A HURRY TO GIVE HIM AN INSTRUCTION AND GET BACK TO SEPARATING OTHER ACFT. AS A RESULT; I INADVERTENTLY GAVE THE C208 THE ALT I WAS PLANNING ON ISSUING TO THE OTHER ACFT. I ALSO DID NOT FULLY PAY ATTN TO THE PLT'S READBACK. THE CTLR WORKING WITH ME ADVISED ME AFTERWARD THAT HE DID NOT HEAR ANY OF MY CONVERSATION WITH THE C208 BECAUSE HE WAS OCCUPIED ORGANIZING THE INBOUND FLT STRIPS. IF WE BOTH HAD BEEN PAYING A LITTLE MORE ATTN; THIS ERROR COULD HAVE BEEN AVOIDED. FACTORS AFFECTING THE QUALITY OF HUMAN PERFORMANCE: IT WAS MY SECOND DAY BACK FROM A 1 1/2 WK VACATION; AND I FELT A LITTLE 'RUSTY' ON THAT POS. THE OTHER CTLR HAS ONLY BEEN AN FPL FOR 1 MONTH AND I BELIEVE IT WAS HIS FIRST TIME WORKING THE HDOF POS BY HIMSELF. THIS POS USUALLY IS COMBINED WITH THE ONE I WAS WORKING DURING TRAINING RESULTING IN A LACK OF REAL PROFICIENCY ON IT (IN MY OPINION).