What was found
HF COMMS CHECKED OK ON GROUND. HF 1 & 2 DATA, VHF 3 DATA, ALL SET CORRECTLY, ENTERING OVERWATER ROUTE. HF 1 THEN WOULD NOT TRANSMIT, UNABLE TO TRANSMIT ON HF 2. PLACED HF 1 TO DATA, HF 2 RADIO, VHF 3 DATA FLIGHT CONTINUED. TRIED TO TRANSMIT ON HF 1 JUST A CONTINUOUS TONE. ACFT GROUNDED. R & R THE NR 1 HF COUPLER AND CHECKED OK IAW AMM 23-11-61, PB400, AND 23-11-21, PB 400.