FLC INADVERTENTLY SQUAWKED XPONDER CODE 7500 IN ERROR AFTER EXPERIENCING A LOSS OF COMS WHILE DOWNWIND FOR LNDG. FLC WERE ABLE TO RE-ESTABLISH COM AND CANCEL THE ALERT ERROR.

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LOCATION: DOWNWIND DFW. ENRTE; CAPT COM TROUBLE LIT-DFW. CAPT NOT RECEIVING. SPEAKER OK; XMIT OK; FO COM OK. DURING FREQ CHANGE DFW BY DFW APCH LOST BOTH FO AND CAPT XMIT AND RECEIVE. RESET CIRCUIT BREAKER NO HELP. AT 6000 FT ON DOWNWIND; NOSE-TO-NOSE WITH THE PATTERN; IFR VERY BUSY. DIRTIED ACFT SO AS NOT TO RUN OVER TFC; TRYING TWR FREQS; APCH FREQS; CIRCUIT BREAKER RESET; ETC. VERY BUSY IN THE COCKPIT WITH OUR PROB AND OTHER TFC. THE DOWNWIND IN THE MIDDLE OF AN IFR ARR IS NOT A GOOD PLACE TO GO NO- COM. THE CAPT; IN A VERY BUSY STATE; SAID WE HAVE TO LET ATC KNOW WE HAVE A NO-COM; GO AHEAD AND ENTER THE NO-COM XPONDER CODE 7500. INSERT SPURIOUS BRAINTRON HERE. ALTHOUGH A SUBTLE DIFFERENCE BTWN 7600 AND 7500 IT IS NEVERTHELESS A LARGE TACTICAL ERROR. WE FINALLY GOT FO COM BACK; ESTABLISHED COM WITH DFW TWR; AND THEY ASKED TO VERIFY 7500 XPONDER. ACKNOWLEDGED AND WE WERE NOW VFR AND COULD SQUAWK 1200. AT THE SAME TIME THE CAPT AND FO REALIZED THE MISTAKE AND ADVISED TWR WE WERE ALL OK AND REQUIRED NO SPECIAL HANDLING. THE WHEELS OF PROGRESS WERE ALREADY IN MOTION FOR 7500. ADVISED GND AND DISPATCH AND TWR ALL OK AND CALLED DFW TWR SYS OP CTR WHO ADVISED MOD ALL OK 7500 ENTERED FOR A NO-COM SIT. CAPT TAKES FULL RESPONSIBILITY FOR ERROR. IT WAS AN HONEST MISTAKE IN A VERY BUSY ENVIRONMENT; BUT CAUSED QUITE A STIR. MUCH LEARNED BY FO; AND CAPT LIKELY WILL NOT MAKE AGAIN.