SMA ON IFR FLT PLAN TO OSH HAS RADIO FAILURE. CAN RECEIVE BUT NOT XMIT. FOLLOWS IFR RADIO OUT PROCS.

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AFTER BEING HANDED OFF FROM CHICAGO CENTER TO OSHKOSH APCH; APCH SAID OUR XMISSION WAS GARBLED AND ASKED US TO CHANGE FREQS. IN GOING FROM ONE RADIO TO THE OTHER THE RADIO SELECTOR KNOB ON THE AUDIO PANEL BROKE IN THE #3 POS; WHERE NO RADIO EXISTS. WE COULD HEAR APCH AND TWR OK BUT WE COULD NOT XMIT. DURING THE CONVENTION (JUST PRIOR) OSHKOSH AIRSPACE IS PANDEMONIUM - I ELECTED TO FOLLOW IFR RADIO OUT PROCS AND LAND VFR WITHOUT A LNDG CLRNC. NO CONFLICT OCCURRED. I COULD HEAR TWR ADVISING OTHER ACFT OF OUR NORDO SITUATION (BECAUSE WE WERE NOT ANSWERING THEM) AND THAT THEY HOPED WE WOULD 'LAND SHORT' SO OTHERS COULD LAND LONG. AT THAT TIME I COULD NOT TELL IF TWR EVEN KNEW WE WERE IFR TFC. AFTER LNDG (VERY SHORT) BUT WELL ON THE APPROPRIATE SECTION OF THE RWY; WE IMMEDIATELY EXITED THE RWY ONTO THE GRASS AND I CALLED (AFTER TYING DOWN) GREEN BAY FSS TO CLOSE THE FLT PLAN (SINCE I DIDN'T THINK TWR WOULD DO IT) AND ASKED GREEN BAY TO CALL OSHKOSH TWR AND EXPLAIN WHAT HAPPENED - HE SAID HE WOULD. TENSE MOMENTS!