AN HS125 800 FLT CREW DEPARTING TEB ON THE TEB 5 DEP WAS QUESTIONED BY ATC REGARDING THEIR ALT BECAUSE XPONDER #1 ALT ENCODING WAS INOP.

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DEPARTING FROM RWY 24 AT TEB WE WERE GIVEN THE TEB 5 DEP. WE DEPARTED TEB AS PUBLISHED IN THE SID WHEN I WENT TO INITIATE A CLB TO 2000 FT AFTER REACHING 4.5 DME THAT NEW YORK APCH CTLR ASKED US IF WE HAD STOPPED AT 1500 FT. WE REPLIED TO NEW YORK THAT WE DID STOP AT 1500 FT UNTIL 4.5 DME. NO MORE WAS SAID UNTIL WE WERE HANDED OFF TO THE NEXT CTLR. WE CHKED IN WITH THE NEXT CTLR AND ASKED IF THERE WAS AN ISSUE WITH OUR ALT READOUT. THE CTLR RESPONDED THAT HE HAD NO ALT READOUT ON US AT ALL. WE SWITCHED TO THE #2 XPONDER AND HAD OUR ALT CHKED AGAIN AND THE CTLR CONFIRMED HE HAD A POSITIVE ALT READOUT. WE INFORMED THE CTLR THAT WE HAD BEEN QUESTIONED EARLIER IN THE FLT AND THAT WE WOULD HAVE IT LOOKED IT. WE SWITCHED BACK TO THE #1 XPONDER AGAIN LATER IN THE FLT AND IT WAS FINE. THE CORRECTIVE ACTIONS WERE TO CONFIRM THE ALT READOUT WITH THE CTLR AND TO SWITCH TO THE SECOND XPONDER AFTER BELIEVING THAT OUR #1 XPONDER WAS PRODUCING AN INTERMITTENT ALT READOUT. THE TEB 5 DEP IS NOT A COMPLEX DEP; HOWEVER; IT DOES REQUIRE THE CREW TO BE VIGILANT IN ENSURING THAT IT IS FLOWN PRECISELY DUE TO THE COMPLEX AND HIGH DENSITY NATURE OF THE AIRSPACE AROUND IT. IT ONLY TAKES ONE SLIGHT DISTR IN THE COCKPIT TO HAVE AN ERROR.