MD80 CREW HAD VHF RADIO FAILURE ARRIVING AT ATL. CREW ACCIDENTALLY SQUAWKED THE HIJACK CODE INSTEAD OF THE COM FAILURE CODE.

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NORDO. FLT WAS PROCEEDING AS PLANNED. CHKED IN WITH APCH CTL FLYING RMG 2. AS WE GOT ABNORMALLY CLOSE TO ATL; I DIRECTED THE FO TO REQUEST DSCNT. AFTER NUMEROUS CALLS; WE ATTEMPTED ALTERNATE MEANS; IE; PREVIOUS FREQ (TWR; GND; GUARD; VHF #1 AND #2 AND FROM CAPT AND FO POS; BOTH HEADSET AND HAND MIKE. AS WE PASSED OVERHEAD ATL; I DIRECTED FO TO LOOK UP APPROPRIATE CODE FOR NORDO; WHICH HE PUT IN THE XPONDER. CAPT STATED HIS INTENTIONS WERE TO ESTABLISH A DOWNWIND FOR FINAL TO LNDG RWY AND IN ABSENCE OF A RED LIGHT; LAND. VERY INTERMITTENT COMS WITH APCH CTL WERE ESTABLISHED AND WE WERE CLRED TO LAND AND TOLD TO RETURN TO ORIGINAL SQUAWK. DIFFICULT RADIO PROB PERSISTED TO ENG SHUTDOWN. ON TAXI IN; CAPT WAS ASKED TO CALL TRACON. I DID SO AND LEARNED WE WERE SQUAWKING 7500 NOT 7600. THE FO AND I DISCUSSED THE NEED FOR PROPER XPONDER CODES IN THE CURRENT ENVIRONMENT. POSSIBLE CONTRIBUTING FACTORS: FATIGUE. 3 LEGS; 3 DIFFERENT ACFT; FINAL LNDG AT XA30. ALSO; POSSIBLE PAX ELECTRONICS PAGER/CELL PHONE.