AN ACR MDT LOST ALL XMITTING CAPABILITY.

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WHILE IN CRUISE FLT; #1 RADIO BEGAN HAVING XMITTING PROBLEMS. WITHIN 5 MINS; #1 WAS UNUSABLE. WE INFORMED ATC OF OUR PROB AND ASKED FOR FREQ CHANGE TO CONTACT COMPANY FOR FURTHER INSTRUCTIONS ON WHETHER TO PROCEED TO GRB. COMPANY SAID AFFIRMATIVE. UPON RECONTACTING ATC; #2 RADIO XMITTER BEGAN TO FAIL. WE WERE UNABLE TO TELL ATC BEFORE TOTAL FAILURE. AT THIS TIME WE RAN THE COM FAILURE CHKLIST. WE ALSO BEGAN TRYING DIFFERENT MIKES AND THE ADDITIONAL CREW MEMBER AUDIO PANEL TO NO AVAIL. ALL THINGS THAT WERE TRIED HAD NO EFFECT ON OUR ABILITY TO XMIT; THOUGH RECEIVE CAPABILITY WAS NEVER INTERRUPTED. WE THEN DECIDED IT WAS TIME TO SQUAWK 7700 AND IDENT. THERE WAS NO RESPONSE FROM CTR! AFTER 1 MIN WE SQUAWKED 7600 WITH NO RESPONSE. GRB WX WAS IFR AND OUR DEP ARPT OF MQT WAS CLR AND VISIBILITY UNLIMITED. AFTER A FEW MINS WE REVERSED COURSE AND STARTED FLYING THE REVERSE BACK TO MQT. AFTER 5-10 MINS; CTR CONTACTED US TO CONFIRM OUR SIT AND INTENTIONS THROUGH SQUAWK CODES AND IDENT. WE THEN CONTINUED SAFELY AND LANDED AT MQT WITHOUT INCIDENT. THE PROB WAS A FAULTY RCAU UNIT WHICH PROCESSES ALL 3 AUDIO PANELS XMITTERS. THIS SEEMS TO BE UNSAFE AND A DESIGN FLAW WHICH MAY WARRANT CORRECTIVE ACTION ON THE MDT ACFT. BECAUSE OF THIS FLAW THERE IS NO BACKUP XMIT CAPABILITY WHEN THE RCAU FAILS. A GOOD LESSON WAS LEARNED THAT BY FOLLOWING PROCS SET FORTH IN THE AIM AND GOOD CREW TEAM WORK; WE RETURNED TO OUR DEP POINT WITHOUT INCIDENT.