SCT CTLR INCORRECTLY JUDGED TURN MANEUVER BTWN E120 AND B737; BUT MAINTAINS PROPER SEPARATION. CTLR CONCERNED WITH ASR9 RATE.

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B737 WAS ESTABLISHED ON A STRAIGHT-IN ILS APCH. E120 WAS GIVEN A DOWNWIND TURN TO FOLLOW THE B737. THE E120 WAS SUBSEQUENTLY OBSERVED ON A CONVERGING COURSE WITH THE B737 AT THE SAME ALT. AT 4 MI SEPARATION; A CORRECTIVE TURN WAS ISSUED AND TFC EXCHANGED. VISUAL SEPARATION WAS APPLIED. THE PROB I HAD WAS THAT WITH THE SCAN RATE OF THE ASR-9 RADAR; IT APPEARED TO ME THAT A WBOUND TURN FOR THE E120 WAS THE PROPER CORRECTIVE ACTION; BUT SINCE THE E120 WAS MORE FULLY ESTABLISHED IN HIS TURN THAN HE APPEARED; THE TURN WOUND UP POINTING HIM MORE DIRECTLY AT THE B737; RESULTING IN SEPARATION DECREASING TO 2 MI. CONTRIBUTING TO THE INCIDENT WAS A HVY AMOUNT OF TFC; PLUS A BE36 THAT WAS ON A NONSTANDARD ROUTING WHICH DISTR ME FOR A FEW MOMENTS. AS FAR AS PREVENTING FUTURE OCCURRENCES; I AM SURE I HEARD THE PROPER READBACK. I SUSPECT THE PLT DIALED IN THE WRONG HDG ON THE FLT DIRECTOR. I HAVE SEEN NUMEROUS OCCASIONS OVER THE YRS WHERE AN ACFT TOOK A WRONG HDG; BUT DUE TO THE SCAN RATE; IT'S NOT PICKED UP IMMEDIATELY AND THE ISSUED CORRECTIVE HDG DID NOT TAKE INTO ACCOUNT THAT THE AIRPLANE WAS ALREADY DEEP IN THE TURN. THAT APPEARS TO HAVE HAPPENED TO ME TODAY.