VFR C172 IS CONCERNED WITH ENTERING SFO MODE C VEIL WITHOUT FUNCTIONING XPONDER.

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FLEW FROM F70 SANTA YNEZ (IZA); WITH FLT FOLLOWING; COMPLETELY WITHOUT INCIDENT. AFTER REFUELING; TOOK OFF HEADING FOR HAF; WITH SQL AS MY ALTERNATE. SHORTLY AFTER LEAVING IZA; WAS INFORMED THAT MODE C INOP/INTERMITTENT; BUT HAD GOOD PRIMARY TARGET. AT REQUEST OF ATC; RECYCLED IT NUMEROUS TIMES. UNABLE TO GET CONSISTENT MODE C OP; AND WAS TOLD TO RPT ALT CHANGES. WAS INFORMED BY ATC THAT WOULD NOT BE ABLE TO ENTER CLASS B AIRSPACE. I ACKNOWLEDGED THIS; BUT ASKED IF I COULD ENTER THE MODE C VEIL; AND FLY BELOW CLASS B. HE ADMITTED HE DIDN'T KNOW ('A GOOD QUESTION' WAS THE ACTUAL REPLY); BUT THAT HE DIDN'T DEAL WITH THAT AREA OF AIRSPACE; AND THAT NORCAL WOULD MAKE THAT DECISION. SHORTLY AFTER; CONTACTED NORCAL APCH; WHO RESTATED NO ENTRY INTO CLASS B AIRSPACE WITHOUT MODE C XPONDER. ASKED ABOUT ENTERING THE MODE C VEIL BELOW CLASS B; AND WAS TOLD 'SQUAWK 1200; RADAR SVCS TERMINATED.' I CIRCLED DOWN TO 3500 FT MSL; ABOVE EUGEN; AND PROCEEDED TO HAF; WHICH WAS IFR CONDITIONS. SO HEADED TO SQL AT SAME ALT; LNDG WITHOUT INCIDENT; AND WITHOUT FURTHER COM WITH NORCAL. ON AUG/SUN/03; I LEFT SQL; AND FOLLOWED THE HAYWOOD DEP (THINK THIS IS CORRECT NAME) AT 3500 FT MSL; REMAINING OUTSIDE CLASS B AT ALL TIME.; ON REACHING EUGEN; CLBED TO 7500 FT MSL. OBTAINED FLT FOLLOWING -- AGAIN; MODE C WAS INTERMITTENT; WITH GOOD PRIMARY TARGET. REFUELED AT IZA; AND SHORTLY AFTER; OBTAINED FLT FOLLOWING; THIS TIME WITH PERFECT MODE C RESPONSE; WHICH LASTED ALL THROUGH THE LOS ANGELES BASIN; BACK THROUGH MARCH GCA; AND TO F70. FROM THE RESPONSE OF NORCAL; I BELIEVE THAT THIS IS A KNOWN PROB WITH THE MODE C. IT IS TOO MUCH OF A COINCIDENCE THAT THE MODE C STOPPED WORKING AT IZA; AND THEN STARTED WORKING AT EXACTLY THE SAME PLACE ON THE RETURN JOURNEY. I FLEW THE ACFT; 2 DAYS LATER; AND THE MODE C WORKED PERFECTLY. SPOKE WITH OTHER PLTS; WHO INFORMED ME THAT THEY HAVE ALSO HAD PROBS IN THAT AREA. I FEEL THAT THE PROB LIES WITH ATC RADAR SYS BTWN IZA AND THE GENERAL AREA OF SAN FRANCISCO. THIS PROB NEEDS TO BE ADDRESSED.