WRONG XPONDER CODE GIVES ATC CONCERN ABOUT THE STATUS OF FLT ON DEP FROM FLL; FL.

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APPROX AT XA35 AM (LCL) ACFT WAS CLRED FOR TKOF FROM FT LAUDERDALE HOLLYWOOD INTL ARPT. DURING THE CLBOUT THE TWR CTLR REQUESTED US TO CHK 'XPONDER ON;' WHILE WE WERE PERFORMING THE APPROPRIATE CLB CHKLIST; WE REPLIED 'XPONDER IS ON.' FOLLOWING THAT; WE WERE REQUESTED TO CONTACT MIAMI DEP CTL. UPON OUR CHK-IN WITH DEP CTL; WE WERE ADVISED TO CHK 'XPONDER ON.' WE REPLIED: 'ARE YOU RECEIVING OUR XPONDER?' AND CTL RESPONDED AFFIRMATIVELY. MIAMI CTLR REQUESTED 'SAY INTENTIONS.' AT THIS POINT WE REALIZE WE ARE NOT MAINTAINING THE NORMAL COM WITH CTL. AT THIS TIME; WE RECOGNIZE PROB AS THE WRONG CODE BEING SET ON THE XPONDER (7500). IMMEDIATELY PROCEEDED TO RESET THE XPONDER TO THE CODE GIVEN ON THE CLRNC (XXXX). THEN; WE INFORMED CTL THAT THE PREVIOUS CODE WAS WRONG; AND IT HAD BEEN RESET TO THE CORRECT ONE. WHEN WE ESTABLISH COMS WITH CTR; WE WERE QUESTIONED IF SIT WAS NORMAL; WE REPLIED OUR SIT WAS NORMAL AND CLARIFIED TO CTLR ABOUT THE MISTAKE AND THAT IT HAD BEEN FIXED. CTLR GAVE US A NAME AND PHONE NUMBER TO CALL UPON LNDG. PERSON WAS CONTACTED; AND INFORMED THAT SIT WAS NORMAL. WE CONSIDERED POSSIBLE CAUSES: 7500 CODE MIGHT HAVE BEEN PREPROGRAMMED IN XPONDER MEMORY; AND ACCIDENTALLY SWITCHED FROM CORRECT CODE TO MEMORY. XPONDER MALFUNCTION: TURNING OFF THE AVIONICS IN THE ACFT MIGHT HAVE RESET XPONDER TO 7500 CODE.