LESS THAN STANDARD SEPARATION AND CLOSE PROX BETWEEN ATX AND SMA. BOTH IFR. OPERATIONAL ERROR.

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WHILE WORKING 2 COMBINED RADAR POSITIONS; I ACCEPTED HDOF ON ATX X ON V214 EBND; NO MODE C READOUT; BUT ON FLT PROGRESS STRIP NOTED AT 5000'--HIS NORMAL ALT. I WAS ALSO WORKING SMA Y ON ROUTING OVER MRB V501 NBOUND AT 7000'. AFTER COMPLETING AUTOMATED HDOF ON SMA Y TO ZDC; I TRANSFERRED COMS TO THE CENTER FREQ. AT THIS POINT; SMA Y WAS ABOUT OVER MRB AND ATX X WAS ABOUT 5 MI W OF MRB. ATX X'S TRACK READ AN INTERMITTENT '70' INDICATED MODE C READOUT OF 7000'. I XMITTED TO ATX X TO VERIFY ALT AND TURN RIGHT TO 180 DEGS FOR TFC WITH NO RESPONSE. (SINCE I ROUTINELY CHK THE ALT ON CONTACT AND X'S ALT WAS NOT CHKED; IT IS PROBABLE THAT HE NEVER MADE RADIO CONTACT.) I CALLED ZDC TO ISSUE AN EMER TURN FOR TFC AND ALT VERIFICATION. THE CENTER SECTOR SAID HE WASN'T ON FREQ. I THEN CALLED THE CENTER SECTOR X WHOM I HAD TRANSFERRED ACFT Y AND ASKED FOR AN EMER DSCNT AS TFC AT 7000' HAD JUST IDENTED. THE CENTER CTLR COMPLIED IMMEDIATELY. PROBS: 1) COMBINED SECTORS REQUIRE TOO BROAD A SCAN SO THAT IF ACFT Y HAD READ OUT 7000' EARLIER; I MISSED IT. 2) NO UPDATE OF FLT STRIP INFO BY CENTER OR TOO BUSY AT DATA AND MISSED IT. 3) NO COM TRANSFER TO VERIFY ALTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR GAVE APPROX YRS IN ATC. POSITIONS COMBINED WERE N ARR AND W ARR; WHICH IS AN UNUSUAL COMBINATION AND COVERS A LOT OF SCOPE AREA. ACFT AT THE LOWER ALT COMING FROM CUMBERLAND AREA ARE NOT IN RADIO OR RADAR CONTACT WITH CENTER AND OFTEN ARE HANDED OFF DIRECTLY FROM ZOB TO IAD APCH. THE RADAR AND RADIO COVERAGE FOR IAD APCH IS INTERMITTENT UNTIL ACFT GET CLOSE TO MRB. ALSO; THIS ATX COMPANY HAS SEVERAL ACFT WHICH DO NOT GIVE GOOD XPONDER RETURN. ACFT Y HAD STARTED DSCNT AFTER CALL FROM CENTER; SO ACFT HAD ABOUT 200' VERT SEP WITH TARGETS MERGED. RPTR HAS NOT HEARD DATA REF LAT SEP. SYS ERROR GIVEN TO ZDC FOR REVIEW. ATX X SAW ACFT Y AS THEY GOT CLOSE. RPTR HAD NEITHER ACFT ON FREQ. FLT DATA POS HAD RECEIVED A REVISED STRIP ON ATX X WHICH SHOWED CORRECT ALT OF 7000'; BUT THAT NEW STRIP HAD NOT BEEN POSTED AT POS.