FLC OF ACR LGT ACFT LOST COM DURING HIGH ALT CRUISE AND INADVERTENTLY SQUAWKED THE HIJACK CODE ON THE TRANSPONDER INSTEAD OF LOST COM.

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ENRTE BTWN ATL AND RIC THE RADIO (ATC) SEEMED TOO QUIET. WE TRIED A RADIO CHK ON VHF #1 AND GOT NO ANSWER; SO TRIED SEVERAL FREQS OFF THE LO ALT CHART. WE THOUGHT WE HAD MISSED THE CALL TO GO TO ZDC FROM ZTL. WE ALSO TRIED VHF #2. FINALLY; WE GOT A DC SECTOR ON VHF #2 AND THEY ANSWERED 'STANDBY.' AFTER ABOUT 2 MIN OF ABSOLUTE SILENCE; I CALLED AGAIN. SHE ANSWERED; 'DID YOU GET YOUR DSCNT FROM FL290 TO FL270' AND SHE SAID A DIFFERENT FREQ. WHEN I TRIED TO CONFIRM; I COULD GET NO ANSWER. THE CAPT STAYED AT FL290 BECAUSE HE DIDN'T HEAR THE CALL AND I COULDN'T CONFIRM. WE TIRED THE NEW FREQ ON BOTH RADIOS AND AFTER SEVERAL TRIES; HEARD ATC SAYING TO ANOTHER ACFT THAT '(WE) HAD A STUCK MIKE. I ESTABLISHED COM WITH ZDC ON THIS FREQ ON VHF #2 AND ATC GAVE US A BOX PATTERN DSCNT. WE TOLD ATC WE THOUGHT VHF #1 WAS BAD. THEN; ON A HDG OF 090; WITH RICHMOND ARPT PASSING OFF THE L; IT WAS AGAIN SILENT FOR TOO LONG. WE COULD NOT GET THE ATC FACILITY ON EITHER HEAD OF EITHER RADIO FROM ANY POS. THE SO PULLED AND RESET SEVERAL CIRCUIT BREAKERS; WE ALL CHKED FOR DEPRESSED SWITCHES (STUCK MIKES); ETC. WE DECIDED WE WERE REALLY LOST COM AND THE CAPT ANNOUNCED HIS PLAN TO TURN TO RIC AND SHOOT THE PUBLISHED APCH. AT THIS TIME; I MADE A MISTAKE WHICH CAUSED GREAT CONSTERNATION ON THE GND. INTENDING TO SET 7600 (THE LOST COM SQUAWK) IN THE IFF; I INSTEAD SET 7500 (HIJACK). THEN; I REMEMBERED I SHOULD HAVE SET 7700 FOR 1 MIN PRIOR TO LOST COM SQUAWK; SO I SET 7700 THEN WENT BACK TO 7500. THIS REALLY CAUSED CONSTERNATION ON THE GND. DURING THIS TIME; WE WERE ALSO TRYING APCH CTL AND RIC TWR ON THE RADIOS. FINALLY; WE COULD HEAR RICHMOND TWR CLRING US TO LAND; BUT THEY EVIDENTLY COULD NOT HEAR OUR RESPONSES. RICHMOND TWR THEN ASKED US TO CONFIRM SQUAWK 7500. AT THIS TIME; I REALIZED I HAD SET THE WRONG SQUAWK AND SWITCHED TO 7600. SHORTLY AFTER; RICHMOND TWR STARTED HEARING OUR TRANSMISSIONS; WE VERBALLY CONFIRMED THE 7500 SQUAWK HAD BEEN IN ERROR; AND WE WERE CLRED VFR APCH. BOTH HEADS OF BOTH RADIOS CHKED OK FROM ALL POS AFTER LNDG. I MET WITH THE PERSON IN CHARGE OF HIJACKING RESPONSE AFTER LNDG AND EXPLAINED IT WAS MY ERROR ON THE WRONG SQUAWK. IN THINKING THIS OVER; WE SHOULD HAVE BEEN QUICK TO RECOGNIZE RADIO FAILURE; AND WHEN WE STARTED BEING VECTORED; SHOULD HAVE ESTABLISHED FURTHER LOST COM PROCS. I SHOULD HAVE BEEN MORE CAREFUL WITH THE IFF SETTING AND; EVEN THOUGH WE WERE ALL QUITE BUSY TRYING RADIO CALLS AND PLANNING A VFR DSCNT; SHOULD HAVE REFRESHED MY MEMORY ON THE PROPER SQUAWK WITH THE REST OF THE CREW OR THE ON-BOARD PUBLICATIONS. COMPLETE RADIO FAILURE IS RELATIVELY UNCOMMON; BUT ONE OF THE THINGS ALWAYS IN THE BACK OF MY MIND FROM NOW ON WILL BE A PLAN; JUST IN CASE.