What was found
FLT 1703 BOS-BDA, AIR TURNBACK TO BOS DUE UNABLE TO TRANSMIT ON HF FREQUENCY WHEN PTT IS SELECTED, AN AUDIBLE TONE IS GENERATED AND STAYS ON FOR AN EXTENDED TIME, RANGING FROM 5-30 SECONDS AND ON THE HF RADIO PAGE, AN AMBER "TUNING" MESSAGE WOULD ILLUMINATE AND STAY ON ALONG WITH THE TUNING TONE. WE COULD HEAR CONVERSATIONS ON HF, BUT THE TUNING PROBLEM PREVENTED ANY TRANSMISSIONS. T/S PER TSM 23-11-00. REPLACED MAU 1 I/O MODULE IAW AMM 31-41-05.