A C-172 PLT OPERATED IN THE VICINITY OF THE CVG CLASS B AND FAILED TO REALIZE THAT HE WAS IN VIOLATION OF FAR 91 PT 215 WHEN HIS XPONDER WAS NOT ENABLING A MODE C INTERROGATION 30 NM FROM CVG.

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TOOK OFF OUT OF I69 TO LOU. AFTER TKOF DURING CLB; NOTICED XPONDER WAS NOT RECEIVING ANY HITS. RECYCLED XPONDER. STILL DID NOT INDICATE NORMAL REPLIES. #1 RADIO VOR NUMBER DIGITS BECAME SCATTERED PREVENTING ITS USE FOR NAV. #2 RADIO WAS NORMAL. FLEW APPROX 10 NM S BEFORE TURNING SW TO LOU. I WAS MONITORING 121 PT 50 THE WHOLE TIME. I STAYED BELOW THE 3000 FT CLASS B AIRSPACE. VISIBILITY WAS EXCELLENT. REMAINED TO THE S OF INTERSTATE 71. I KEPT TRYING TO GET THE RADIO AND XPONDER TO WORK BECAUSE IT WAS DISTRACTING. APPROX 30 MI S OF CVG I HEARD CTR CALL OUT TFC TO A DEP ACFT. I THEN REALIZED I HAD GAINED SOME ADDITIONAL ALT BUT THOUGHT I WAS STILL OUT OF CLASS B AIRSPACE. NORMALLY I WOULD USE BOTH VOR'S TO UPDATE MY EXACT POS; BUT DUE TO #1 BEING UNUSABLE; I COULD ONLY SHOOT ONE RADIAL. I CONTINUED TO LOU. WHEN I WAS ON L DOWNWIND THE AIRCRAFT ON THE R DOWN-WIND CALLED THE TWR AND STATED HE WAS TCASII EQUIPPED BUT I DID NOT SHOW MY POS. REQUESTED TWR TO PROVIDE HIM MY POS. THIS IS WHEN IT OCCURRED TO ME THAT MY XPONDER WAS NOT INDICATING CORRECTLY. I THOUGHT THE REPLY LIGHT WAS THE ONLY PROB. UPON LNDG LOU TWR REQUESTED I CONTACT CVG AND TALK WITH THEM. I ELECTED NOT TO TALK WITH THEM AT THAT TIME. I SUSPECT MY XPONDER WAS NOT WORKING OR I MAY HAVE INADVERTENTLY ENTERED CLASS B AIRSPACE. I SHOULD HAVE HAD CVG VERIFY MY ALT READOUT OR FLOWN FURTHER S TO INSURE I WAS CLR OF CLASS B AIRSPACE OR RETURNED TO CVG TO HAVE MY EQUIP CHKED OUT; HAVING RADIO REPAIRED AND AVIONICS SHOP CHK OUT XPONDER. ALSO; IF ATL HAD CALLED OUT THAT 'TFC S OF ARPT (US) IS ABOUT TO ENTER CLASS B AIRSPACE' OR 'XPONDER NOT WORKING' I COULD HAVE CHANGED ALT OR DIRECTION TO INSURE I WAS CLR OF THE CLASS B AIRSPACE. I THINK ATC HAS A RESPONSIBILITY TO AT LEAST TRY TO LET AIRCRAFT IN THE AREA KNOW OF ANY POSSIBLE PROBS EVEN IF THEY ARE NOT TALKING DIRECTLY WITH THEM. I ALWAYS MONITOR CTR RADIO WHENEVER IN THE VICINITY OF CTLED AIRSPACE.