AN INSTRUCTOR PLT DIALS IN THE WRONG XPONDER CODE WHEN HE HAS A PARTIAL RADIO FAILURE AND DIVERTS BACK TO NZY. THE POLICE MEET THE ACFT AT THE RAMP. HE HAD TAXIED IN TO THE RAMP WITH WHAT WAS PERCEIVED AS A WHITE LIGHT. IT WAS RED.

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WE DEPARTED NZY ENRTE TO BROWN FIELD. LEAVING NZY'S AIRSPACE; I CHANGED FREQS AND OUR HEADSETS WENT DEAD. I HAD THE STUDENT CIRCLE OUTSIDE CLASS D AIRSPACE WHILE I TROUBLESHOT THE RADIOS; TURNING OTHER ELECTRICAL EQUIP OFF; RECYCLING THE RADIO; AND SWITCHING TO OVERHEAD SPEAKER AND HAND MIKE. THE OVERHEAD SPEAKER PRODUCED A LOUD SQUEAL; BUT I WAS ABLE TO INTERMITTENTLY HEAR NZY TWR OVER THE SQUEAL. I TRIED XMITTING ON 121.5 AND NZY TWR FREQS. TWR FINALLY RESPONDED; CLRING ME TO LAND VIA A STRAIGHT-IN TO RWY 29 BEFORE THE SQUEAL AGAIN DROWNED OUT THEIR XMISSIONS. I ENTERED THE LOST COMS XPONDER CODE AND TOLD TWR I WAS LOSING MY RADIO. I RECEIVED 1 XMISSION GARBLED AND BARELY READABLE; APPARENTLY ASKING ME IF I WAS SQUAWKING THE LOST COMS CODE. I REPLIED 'AFFIRM.' INBOUND TO THE RWY; I WAS BUSY SEARCHING FOR OTHER TFC; INCLUDING THE S3 I'D SEEN ENTER THE PATTERN MINS BEFORE; AND TRYING TO ADJUST THE VOLUME ON THE RADIO TO LISTEN OVER THE SQUEAL. I WAS ALSO MONITORING THE VOR FREQ FOR VOICE XMISSIONS. I MADE A STRAIGHT-IN TO RWY 29 AS CLRED; MAKING GEAR DOWN AND POS CALLS IN THE BLIND. AFTER LNDG; I LOOKED FOR A LIGHT SIGNAL TO TAXI. BOTH MYSELF AND MY STUDENT OBSERVED A FAINT FLASHING WHITE LIGHT; SO WE TAXIED TO PARKING. UPON REACHING PARKING; I WAS MET BY THE BASE POLICE; AND TOLD TO CONTACT TWR ON THE LAND LINE. TWR INFORMED ME THAT I'D BEEN SQUAWKING THE HIJACK CODE; AND I EXPLAINED I THOUGHT I'D BEEN SQUAWKING THE LOST COMS CODE. RECOMMENDATIONS: THIS WAS AN EASY MISTAKE TO MAKE; ESPECIALLY IN LIGHT OF THE FACT THE LOST COMS AND HIJACK CODES ARE ONLY 1 DIGIT APART. A HIJACK CODE OF 7555 WOULD MAKE IT EASIER TO KEEP THEM APART. I WAS PRETTY BUSY; BTWN TRYING TO LISTEN TO TWR OVER THE SQUEAL; TROUBLESHOOT THE RADIO; CLR FOR OTHER TFC; AND MONITOR MY PRE-SOLO STUDENT WHO WAS FLYING THE ACFT WHILE I TROUBLESHOT THE RADIOS. I NEVER LOOKED AT MY XPONDER AFTER INITIALLY SETTING THE CODE. A CALL FROM TWR; CLARIFYING THAT I WAS SQUAWKING CODE 7500 AND NOT LOST COM WOULD HAVE ALLEVIATED THE PROB; BUT THEY ARE VERY RESTR IN WHAT TO SAY IN THAT SIT. TWR ALSO INFORMED ME THEY GAVE ME A FLASHING RED LIGHT; NOT A FLASHING WHITE LIGHT. MORE TRAINING FROM BOTH SIDES OF THE SIGNAL LIGHTS MIGHT HELP ALLEVIATE THAT SIT. IT'S VERY HARD TO SEE ATC LIGHT SIGNALS FROM THE COCKPIT; EVEN ON THE GND WHEN THE ACFT IS FAR FROM THE TWR.