PLT ON VECTOR FOR GPS APCH EXPERIENCES NMAC WITH 1 OF 2 ACFT RPTED IN THE VICINITY BY SOCAL.

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I WAS FLYING A C210 FOR AN AVIONICS BUSINESS IN ORDER TO CHK OUT AND CERTIFY AN IFR/GPS. AN FAA INSPECTOR WAS IN THE BACK SEAT AND THE MAN WHO OWNED THE AVIONICS BUSINESS WAS IN THE R SEAT. WE HAD BEEN CONDUCTING GPS/VOR MULTIPLE APCHS AT CAM/OXR/VNY AND WERE ON AN APCH TO BURBANK. WE WERE AT 3500 FT AND I BELIEVE GIVEN A HDG OF 330 DEGS IN ORDER TO INTERCEPT THE 075 DEG RADIAL TO VAN NUYS. WE WERE OUTSIDE CANOG. UPON TURNING TO THAT HDG; SOCAL CALLED TFC; 2 TARGETS; I SAW ONE AND ACKNOWLEDGED THAT I HAD 1 TARGET. A FEW SECONDS -- MAYBE 30 -- MY R SEAT PAX (ALSO A PLT; COMMERCIAL RATED; NO INST) SAID 'OH MY GOD' AND THE NEXT THING I SAW WAS A CHEROKEE UNDERNEATH ME BIG ENOUGH TO READ THE WHITE AND BLUE TAIL NUMBER. (I DID NOT 'READ' THE NUMBERS BECAUSE IT WAS THERE AND GONE; BUT WE WERE ABOUT 100 FT APART.) I NEVER SAW THE ACFT UNTIL IT WAS UNDERNEATH ME. MY R SEAT PAX SAID HE SAW THE PLANE AT 2-3 O'CLOCK BUT WAS UNABLE TO RESPOND IN TIME BECAUSE IT WAS SO CLOSE. SOCAL LATER ACKNOWLEDGED THAT THEY SAW THE OTHER ACFT CLOSE TO US; BUT SINCE I HAD 'CONTACTED' 1 TARGET -- THEY DIDN'T KNOW WHICH ONE I WAS REFERRING TO. MY R SEAT PLT/PAX FEELS SOCAL TURNED US INTO THE TFC. I AM NOT SURE. I FLY IN THE AREA AND TEACH. OVER THE PAST 5 YRS TFC HAS BECOME INCREASINGLY DENSE AND EVEN THOUGH I USE RADAR ADVISORIES WHETHER VFR OR ON IFR; I NEVER ASSUME I AM PROTECTED. I THINK WE MUST ALL BE VERY DILIGENT IN THE AIR AND ON THE GND AND NEVER ASSUME ANYTHING. I WILL NEVER SAY 'I HAVE TFC' AGAIN BECAUSE I REALIZE IT MAY BE OTHER TFC.