A LOW-TIME CFI SET THE HIJACK SQUAWK CODE; INSTEAD OF THE RADIOS OUT CODE; WHILE PENETRATING MLB CLASS D AIRSPACE.

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ELECTRICAL FAILURE OCCURRED ON THE WAY INTO MLB. NO INDICATIONS ON THE ALTERNATOR INDICATOR LIGHT OR THE GAUGE WARNED OF A PROB. THE FIRST RADIO FAILED; SO A SECOND RADIO WAS TO BE USED. BEFORE MELBOURNE TWR COULD BE REACHED; THE SECOND RADIO FAILED. 7500 WAS MISTAKENLY PLACED IN THE XPONDER INSTEAD OF 7600. THE RADIOS CONTINUED TO GO ON AND OFF. WE COULD STILL HEAR THE TWR AND OTHER ACFT; BUT COULD NOT XMIT. WE HAD TFC FOR RWY 4 IN SIGHT; SO WE DECIDED TO APCH AND LAND. THE EMER CHKLIST WAS READ OFF FOR EMER GEAR EXTENSION. WHILE THIS WAS OCCURRING; WE RECEIVED AN ALTERNATING RED/GREEN LIGHT GUN SIGNAL; SO A GAR WAS INITIATED. AT THIS TIME THE GEAR WAS DOWN; AND WE PROCEEDED FOR R TFC RWY 4 WITH A FLASHING GREEN LIGHT. ONCE THE LIGHT GUN BECAME SOLID GREEN; THE ACFT LANDED. WE TAXIED WITH LIGHT GUN SIGNALS BACK TO THE RAMP. CLASS D AIRSPACE WAS ENTERED WITHOUT ACKNOWLEDGEMENT FROM THE MELBOURNE TWR; AND 7500 WAS MISTAKENLY PLACED IN THE XPONDER. THERE WAS NO WAY TO AVOID ENTERING CLASS D AIRSPACE WITH 2-WAY RADIO COMS; BUT I SHOULD HAVE NOT ENGROSSED MYSELF IN THE ELECTRICAL FAILURE AND GEAR PROB TO THE POINT THAT I DID NOT NOTICE THE INCORRECT XPONDER SQUAWK CODE.