SCT CTLR TAKES INITIATIVE WHEN PERCEIVING POTENTIAL CONFLICT BTWN A VFR C182 AND B737 ON APCH TO SNA.

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THE SKYLANE WAS MANEUVERING IN THE CLASS C OUTER AREA; RECEIVING TA'S ON MY FREQ. HE HAD BEEN MOSTLY AT 5000 FT; BUT W OF THE ARR CORRIDOR WHERE THE SNA JET ARRS TRANSITION AT 5000 FT. WHEN THE B737 SHOWED UP; I NOTICED THE SKYLANE HAD TURNED EBOUND; PUTTING HIM ON A CONVERGING COURSE WITH THE B737. I ASKED THE SKYLANE IF HE WAS CONTINUING THAT DIRECTION. HE REPLIED THAT HE WAS. I INSTRUCTED THE PLT TO EITHER REVERSE COURSE; OR CLB TO 5500 FT FOR TFC. THERE WAS NO RESPONSE. I ISSUED THE TFC TO THE B737 (APPROX 4 MI AWAY). THE PLT REPLIED THAT HE WAS LOOKING. I CALLED TFC TO THE SKYLANE AND INSTRUCTED THE PLT TO CLB -- THERE WAS NO RESPONSE. I ISSUED TFC TO THE B737 AGAIN; AND ADVISED HIM THE OTHER ACFT WAS NOT LISTENING. THE PLT OF THE B737 DID NOT SEE THE ACFT. AS MY AIRSPACE LOWER LIMIT IS 5000 FT; I QUICKLY COORDINATED WITH THE ADJACENT SECTOR; ISSUED THE B737 A TFC ALERT; AND DSNDED HIM TO 4000 FT. THE ACFT PASSED WITHIN 1/2 MI LATERALLY AND APPROX 300 FT VERTLY. WHEN I FINALLY REGAINED RADIO COM WITH THE SKYLANE HE APOLOGIZED FOR NOT HEARING MY CALLS. A CTLR'S ONLY TOOL IS A FREQ. IF ACFT ARE NOT LISTENING TO THEIR RADIOS; IT IS IMPOSSIBLE FOR US TO DO OUR JOBS.