A BE90 PLT BEGAN AN UNAUTH DSCNT AND HAD AN NMAC WITH A C208 AT 6000 FT.

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AT THE TIME OF THIS EVENT; I WAS WORKING ERIE APCH/DEP WITH E AND W SECTOR COMBINED. THE WX AT THE TIME WAS IFR; TOPS AROUND 5000 FT MSL; CLR ABOVE. THE SIT INVOLVES A BE90 DSNDING FROM ASSIGNED ALT WITHOUT CLRNC AND IN VERY CLOSE PROX TO A C208 IN LEVEL FLT. EVASIVE ACTION WAS TAKEN. THE BE90 WAS INBOUND TO ERI FROM THE WSW AT 9000 FT MSL. ON INITIAL CONTACT; THE BE90 WAS ISSUED A HDG OF 060 DEGS FOR ILS RWY 24 APCH. PLT CAME BACK WITH A REQUEST FOR GPS RWY 24 APCH VIA DIRECT ERI VOR AND TRANSITION TO 19 NM ARC SE OF ERI; WHICH I APPROVED BUT NEVER CLRED THE ACFT FOR APCH OR LOWER ALT. I HAD APPROX 8-10 OTHER ACFT I WAS WORKING AT THE TIME; ONE BEING A C208 SWBOUND AT 6000 FT MSL ABOUT 10 NM W OF ERI VOR. SCANNING MY SCOPE; I NOTICED THE BE90'S MODE C READOUT AT 7000 FT MSL AND DSNDING. I ASKED THE PLT IF HE WAS STILL AT 9000 FT MSL. THE PLT OF BE90 ADVISED HE WAS IN FACT DSNDING TO 6000 FT. I IMMEDIATELY TURNED THE BE90 R TO 180 DEGS AND CLBED TO 7000 FT. I TURNED C208 R TO 330 DEG HDG. TAGS WERE OVERLAPPING BUT IT APPEARED THE BE90 STOPPED DSCNT AT 6500 FT. THE C208 PLT CALLED FACILITY AFTER LNDG IN CLE AND ADVISED HE WAS HEADS DOWN WORKING A COMPUTER WHEN I ISSUED TURN FOR TFC. PLT ADVISED HE LOOKED OUT WINDOW ONLY TO SEE THE BE90 PASSING IN A BLUR APPROX 500 FT VERT AND 600 FT HORIZ FROM HIS ACFT AND 2 SECONDS LATER FELT THE WAKE TURB AS HE PASSED BENEATH THE BE90. THE BE90 PLT ADVISED THERE WAS TRAINING IN PROGRESS IN HIS ACFT AND THOUGHT HE WAS CLRED DOWN TO 6000 FT. BE90 PLT ADMITTED HE WAS IN ERROR TO ERI AIR TFC MGR; BELIEVING HE MISTOOK THE 060 DEG HDG FOR 6000 FT ALT.