A CESSNA 425 PLT RPTS THAT HE FLEW THROUGH HIS ASSIGNED ALT WHILE MANEUVERING FOR AN APCH AT STL. ERROR ADMITTED.

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WHILE BEING VECTORED FOR THE ILS RWY 30R APCH TO STL; I INADVERTENTLY DSNDED THROUGH MY ASSIGNED ALT OF 3500 FT. I CAUGHT THE MISTAKE AND INITIATED A CLB AT 2700 FT. I CALLED APCH TO VERIFY MY ASSIGNED ALT; TO WHICH HE REPLIED; '3500 FT.' I CONTINUED THE CLB TO 3500 FT AS ASSIGNED; AND COMPLETED THE REMAINDER OF THE APCH UNEVENTFULLY. STL APCH ASSIGNS UNUSUAL ALTS WHILE VECTORING ACFT FOR INST APCHS. ASSIGNED ALTS OF 5500 FT; 4500 FT; 3500 FT; 2500 FT; ETC; ARE NOT UNCOMMON ALTS TO BE ASSIGNED BY STL APCH ON DOWNWIND AS WELL AS BASE AND FINAL. I WAS TOLD TO 'MAINTAIN 180 KTS;' TO 'EXPECT A CLOSE IN BASE TURN;' AND 'CAUTION WAKE TURB FOLLOWING B747' TO SAME RWY. I COULD HEAR ACFT AHEAD OF ME BEING ASSIGNED 2500 FT AND WAS EXPECTING THE SAME MYSELF. WITH THIS IN MIND; AS WELL AS WAKE TURB CONCERNS; PREPARATIONS FOR A CLOSE IN BASE TURN AND PRE-LNDG CHKS; I INADVERTENTLY FLEW THROUGH MY ASSIGNED ALT. WHEN I BEGAN CORRECTION; I NOTICED THE AUTOPLT WAS DISCONNECTED. I DO NOT RECALL DISCONNECTING IT MYSELF OR HEARING IT DISCONNECT. THIS AUTOPLT HAS DISCONNECTED WITHOUT AUDIBLE WARNING IN THE PAST. I DO NOT RECALL HEARING A WARNING FROM THE ALT ALERTER EITHER. THIS ITEM HAS BEEN INTERMITTENTLY INOP IN THE PAST. PERHAPS A COPLT ON BOARD; BETTER MAINT OR MORE VIGILANCE ON MY PART WOULD HAVE PREVENTED THIS ALTDEV.