SMA ON IFR FLT HAS RADIO FAILURE. SQUAWKS 7700; THEN 7600. CONTINUES TO VMC; SQUAWKS 1200.

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ENRTE TO SBM FROM LDM; EXPERIENCED LOSS OF COMS WITH ATC. AFTER TRYING TO REACH ATC VIA MINNEAPOLIS AND CHICAGO CTRS WITHOUT SUCCESS; SET SQUAWK CODE 7700 INTO TRANSPONDER FOR 1 MIN THEN 7600 FOR THE NEXT 15 MIN. CONTINUED TO SBM VIA LAST RTE AND ALT CLRNC RECEIVED. WHEN WITHIN MSA SECTOR; INITIATED DSCNT TO 2600 FT (MSA). BROKE INTO VFR CONDITIONS AT APPROX 3200 FT. UPON ENCOUNTERING VFR CONDITIONS CHANGED TRANSPONDER SQUAWK CODE TO 1200; ENTERED L TFC PATTERN UPON ARRIVING AT SBM. CONTACTED FSS (GRB) VIA TELEPHONE AFTER LNDG. ATC RPTED THAT THEY DID NOT RECEIVE 1200 TRANSPONDER SQUAWK CODE. RPTED TO THEM THE PROC WE USED INCLUDING SQUAWKING 1200 UPON REACHING VFR CONDITIONS. ATC EXPLAINED NO RPT WOULD NEED TO BE SUBMITTED BECAUSE WE INDEED DID SQUAWK 1200; THEY JUST DID NOT SEE IT ON THEIR RADAR. POSSIBLE REASON FOR UNSEEN SQUAWK CODE WAS DISTANCE FROM FACILITY AND ALT BEING FLOWN. TRANSPONDER WAS OPERATIONAL; SET TO MODE 'C;' AND SET TO 1200 UPON DOUBLE CHKING IT. RADIO FAILURE CAUSE IS BEING CHKED INTO.