CLOSE PROX ACR-MLG GA-SMA NEAR ORD TCA MDW ARSA.

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WHILE ON CHICAGO APCH FREQ ON A HDG OF 270 DEGS; WAS GIVEN A CLRNC TO DSND FROM 5000 TO 3000'. WE WERE GIVEN A FREQ CHANGE AT THAT SAME TIME OR SHORTLY AFTER STARTING DSCNT. AFTER CHANGING FREQS I SAW TFC JUST TO THE LEFT OF 12 O'CLOCK AND LEVEL; ABOUT 1/2 MI AWAY AND COMING TOWARDS US. THE TFC WAS A SINGLE ENG; LOW WING; ACFT APPARENTLY CRUISING AT 3500' MSL. I POINTED OUT THE TFC TO CAPT; WHO WAS FLYING; AND I PUT MY HANDS ON THE YOKE TO TAKE CTL IF REQUIRED. THE CAPT SAW THE TFC IMMEDIATELY AND STARTED A BANK TO THE RIGHT AS TFC PASSED ON OUR LEFT AS WE PASSED ABOUT 3400' MSL. AFTER CHKING IN ON FREQ WE LEARNED THAT THE TFC WAS VFR ON RADAR ADVISORY WITH THIS SAME CTLR. CONTRIBUTING FACTORS: WE WERE GIVEN A DSCNT THROUGH AN ALT WHERE THERE WAS KNOWN CONFLICTING TFC; WHO WAS TALKING TO THE SAME CTLING AGENCY (DIFFERENT FREQ). WE WERE NEVER ISSUED A TFC ADVISORY OR GIVEN AN AVOIDANCE VECTOR. VIS WAS REDUCED HDG W DUE TO SETTING SUN AND HAZE. CHICAGO APCH CTL WAS UNUSUALLY BUSY THIS PM. NOTE: I MADE A NEAR MISS RPT TO CHICAGO APCH CTL. AREA SUPVR TOOK THE RPT.