PIT APCH CTLR DESCRIBED NEAR SEPARATION LOSS AT 3000 FT WHEN TRYING TO CONDUCT VISUAL SIDE-BY APCHS DURING MARGINAL WX CONDITIONS.

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THE PIT PRIMARY RADAR SITE WAS OTS FOR MAINT. WE WERE OPERATING ON THE OAKDALE BACKUP RADAR SYS. THE PIT SOP RESTRICTS THE USE OF SIMULTANEOUS APCHS WHILE USING THE BACKUP RADAR SYS. WX WAS MARGINAL FOR VISUAL APCHS; BUT IMPROVING. TFC VOLUME WAS MODERATE AND THE RWY 28L FINAL WAS EXTENDED 15-20 MI IN A SINGLE ARR RWY CONFIGN. A B737 RPTED THE ARPT IN SIGHT ON A 10 MI FINAL AT 3300 FT. THIS LED ME TO BELIEVE THAT VISUAL APCHS WERE FEASIBLE FOR RWY 28R WHICH WOULD REDUCE THE DELAYING VECTORS AND SPD CTL FOR 2 ACFT IN MY SECTOR. I CHANGED THEIR ARR RWY ASSIGNMENTS TO VISUAL APCHS TO RWY 28R AND VECTORED THEM ACCORDINGLY. THE #1 ACFT (A BA146) WAS ON A R DOWNWIND FOR RWY 28R. I COORD AN EARLY DSCNT FOR THIS ACFT TO GET DOWN BELOW THE BASES AND ALLOW THE PLTS TIME TO SEE THE ARPT. I TURNED THE BA146 R TO 200 DEGS FOR A R BASE ENTRY TO RWY 28R. THERE WAS A STRONG NW UPPER LEVEL FLOW WHICH CAUSED THE ACFT'S GND SPD TO INCREASE AS IT TURNED TO THE S. THE WINDS ALOFT AND THE USE OF THE BACK-UP RADAR SYS CAUSED THE TARGET TO APPEAR TO JUMP AND THE GND SPD INCREASED TO 320 KTS AT 3000 FT. THIS ALARMED ME AND I BEGAN AN EARLY TURN TO THE ARPT ANTICIPATING A POSSIBLE LOSS OF SEPARATION FROM A B190 ESTABLISHED ON THE RWY 28L FINAL. I COMBINED AN ARPT POS RPT TO THE PLT IN CONJUNCTION WITH THE TURN SO THAT THE PLT COULD BE CLRED FOR A VISUAL APCH PRIOR TO LOSING RADAR SEPARATION FROM THE B190 ON THE RWY 28L FINAL. THE PLT STATED THEY WERE IN AND OUT OF THE BASES. THE ACFT ENTERED AN AREA OF LOWER MVA AND I IMMEDIATELY DSNDED THE ACFT TO 2500 FT AND ALERTED THE CTLR RESPONSIBLE FOR THE B190. THE BA146 PLT RPTED THE ARPT IN SIGHT AND WAS CLRED FOR A VISUAL APCH TO RWY 28R. THE CONFLICT ALERT WENT OFF; BUT I WAS UNSURE IF THIS WAS DUE TO THE TARGET JUMP AND SUBSEQUENT QUICKLY CHANGING GND SPD OF THE BA146 OR THAT SEPARATION WAS LOST.