AN EMB120 FLC OVERSHOOTS THEIR ASSIGNED ALT WHEN THE ALT CAPTURE FAILS TO OPERATE AS EXPECTED 40 NM W OF JHW; NY.

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WE WERE ISSUED A CLB TO 17000 FT. I SET THE ALT ALERTER. ACFT WAS CLBING ON THE AUTOPLT. AUTOPLT DID NOT CAPTURE AND THUS FAILED TO LEVEL OFF. FO AND I WERE TALKING AND HE WAS LOOKING AT AN ENRTE CHART. CTR ADVISED US OF OUR ERROR AS WE WERE CLBING THROUGH 17700 FT. WE DSNDED BACK TO 17000 FT. NO CONFLICT WITH OTHER TFC WAS CREATED OR MENTIONED BY ATC. I FEEL THAT I LEARNED A VALUABLE LESSON. BEING TOO RELIANT UPON THE AUTOPLT LEADS TO COMPLACENCY. THIS IN TURN LED TO MY MISTAKEN ALTDEV. SUPPLEMENTAL INFO FROM ACN 476062: IT WAS VERIFIED THAT THE AUTOPLT WAS ON AND SELECTED TO THE CAPT'S SIDE SINCE IT HAD BEEN USED IMMEDIATELY BEFORE OUR ALT CHANGE. ONE OF THE DIGITS IN THE ANALOG ALT SELECTOR SEEMS TO HAVE NOT BEEN QUITE CTRED IN THE ALT WINDOW; EVEN THOUGH IT WAS CLRLY SET TO 17000 FT. THE ALT WARNING NEVER SOUNDED AND THE ALT WAS OBVIOUSLY NOT CAPTURED. WE WERE MOMENTARILY DISTR LOOKING AT OUR LOW ENRTE CHARTS TO VERIFY OUR OUTBOUND RADIAL AT JHW VOR WHEN WE GOT THE QUERY FROM ZOB CTR. NO OTHER ACFT APPEARED TO BE PRESENT WITHIN A 15 MI RING BASED ON TCASII; AND ZOB DIDN'T INDICATE ANY SEPARATION CONFLICTS.