A PA28 PLT RPTS AN NMAC EVENT WITH A BONANZA IN A BUSY CCR TFC PATTERN.

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AT ABOUT XA15 ON APR/SAT/05; I WAS ON L DOWNWIND FOR RWY 19L AT CCR. I HAD CALLED THE TWR FROM JUST W OF MT DIABLO; AND WAS INSTRUCTED TO MAKE L TFC FOR RWY 19L. THE PATTERN WAS FAIRLY BUSY; AND I WAS HAVING SOME TROUBLE HEARING ALL TWR COMS. IN FACT; PRIOR TO CALLING TWR; I HAD JUST COMPLETED A 360 DEG TURN NEAR DIABLO SINCE THE FREQ WAS SO CROWDED. THIS WAS MY FIRST LNDG AT CCR; SO I WANTED TO BE EXTRA CAUTIOUS ABOUT MAKING THE CORRECT APCH. AS I ENTERED L DOWNWIND FOR RWY 19L FROM THE 45 DEG ENTRY; I WAS AWARE FROM THE RADIO THAT THERE WAS OTHER LNDG TFC IN THE AREA; BUT I DID NOT HAVE IT IN SIGHT. I BELIEVE IT WAS A BONANZA; BUT I'M NOT CERTAIN. AFTER PASSING ABEAM THE NUMBERS FOR RWY 19L; I STILL DID NOT HAVE TFC IN SIGHT. ABOUT THIS TIME; TWR CALLED TO 'ACFT N OF THE ARPT' (OR SIMILAR PHRASE). AS I WAS THEN PASSING TO THE N; I ASSUMED HE WAS TALKING TO ME AND IDENTED MYSELF. WITHIN THE NEXT FEW SECONDS; I ASKED THE TWR TO CALL MY BASE; AND THE NMAC WITH THE OTHER ACFT OCCURRED. I DON'T REMEMBER WHICH OF THESE 2 EVENTS WAS FIRST. THE OTHER ACFT (PROBABLY BONANZA) PASSED NEARLY HEAD-ON FROM SLIGHTLY ABOVE AT MY 1 O'CLOCK POS; AND THEN ACROSS TO MY L. I WAS STILL ON (NOW EXTENDED) DOWNWIND AS IT PASSED. AT ABOUT THIS POINT; TWR REASSIGNED ME TO RWY 19R. I TURNED BASE AND FINAL AND MADE A NORMAL LNDG ON RWY 19R. IF NASA HAS ACCESS TO TWR VOICE RECORDS AND/OR RADAR TRACES; PLEASE CALL ME WITH ANY INFO. I WOULD LIKE TO BETTER UNDERSTAND THE CIRCUMSTANCES LEADING TO THIS INCIDENT; PLUS ANY HINTS FOR AVOIDING SAME/SIMILAR.