AN ATP GA PLT PENETRATES THE CLASS D OF BJC; CO.

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I REQUESTED A CLRNC TO TRANSIT BJC'S CLASS D AIRSPACE FROM N TO S. TWR CTLR ADVISED HE WAS IN THE MIDDLE OF A RWY CHANGE; WAS VERY BUSY; TO MAINTAIN ABOVE 7500 FT MSL AND GAVE ME A FREQ TO CONTACT DENVER APCH. I CONTACTED DENVER APCH AND WAS GIVEN A XPONDER CODE. I PROCEEDED S THINKING I HAD APPROVAL TO ENTER HIS AIRSPACE. I CALLED HIM AGAIN; WAS TOLD I WAS IN RADAR CONTACT BUT THAT I WAS 'IN CLASS D AIRSPACE;' WITH AN ALT READOUT OF 6500 FT. I STATED THAT MY ALTIMETER WAS READING 7600 FT. HE THEN TOLD ME TO TURN OFF MY MODE C XPONDER. I PROCEEDED S; CONFIDENT THAT MY ALTIMETER WAS READING CORRECTLY AND THAT THE PROB WAS WITH MY XPONDER. VISUALLY I WAS CERTAIN THAT I WAS AT 7600 FT SINCE I AM VERY FAMILIAR WITH THE AREA AND BJC ARPT. THE PROB WITH THE XPONDER HAS BEEN RPTED TO THE FLT CLUB MAINT DIRECTOR. I WAS RUSHED AND SHOULD HAVE REMAINED ABSOLUTELY CLR OF CLASS D AND CLASS B AIRSPACE UNTIL CONFIRMED IN RADAR CONTACT; AT LEAST THEN I WOULD NOT HAVE COMPROMISED DENVER'S OR BJC'S AIRSPACE EVEN IF THE READOUT ON ALT WAS MISTAKEN. MY LESSON ON THIS -- SLOW DOWN AND DON'T ASSUME ANYTHING.