ZSU CTLR EXPERIENCED OPERROR AT 6000 FT BTWN ARR AND DEP ACFT WHEN DISTR BY OTHER EVENTS IN ANOTHER SECTION OF ASSIGNED AIRSPACE.

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I WAS WORKING R-1; THE APCH CTL TO SIG AND SJU ARPT. WX WAS VFR; BUT SOME CLOUDS AND RAIN PRESENT; MOSTLY TO THE S. I WORK ALL INBOUNDS TO SJU AND SIG; PLUS OUTBOUNDS GOING N; W; AND S. I ACCEPTED A HDOF ON THE ACR X COMING IN FROM THE NE AT 6000 FT MSL. JUST BEFORE THE ACR Y DEPARTED; I BECAME PREOCCUPIED WITH AN LR55; IFR INBOUND TO SIG; WHO WAS NOT RPTING SIG IN SIGHT FOR VISUAL APCH. THE ACR Y DEPARTED AND I JUST CLBED HIM TO 10000 FT ROUTINELY (DID NOT SEE THE CONFLICT WITH ACR X. MY INITIAL PERCEPTION WAS THAT ACR X WAS FAR AND NOT A FACTOR; BUT I DIDN'T ANALYZE HIS SPD; CLOSE TO 300 KTS). I WENT BACK TO THE LEARJET LNDG SIG; AND STILL NOT VISUAL; SO I COORD ON LANDLINE TO ASK SIG TO ENTER THEIR AIRSPACE. ALL THIS AFFECTED MY AWARENESS OF THE CONFLICT DEVELOPING; I GOT TOO FOCUSED ON ONE THING ALONE. I FINALLY GOT THE LEAR ON HIS WAY; WHEN I THEN REALIZED (TOO LATE) OF THE LOSS OF SEPARATION WITH ACR X AND ACR Y. I SHOULD HAVE SWITCHED TO INST APCHS SOONER WHEN REALIZING WX WAS DETERIORATING AND PRECLUDING VISUALS. ALSO; THE FACT I WAS 'FOCUSING' TOO MUCH ON ONE THING; SHOULD'VE ALERTED ME TO 'LOOK AROUND' AND SCAN MY ENTIRE OP MORE.