What was found
AT CRUISE, UNABLE TO TRANSMIT ON HF. WHEN PUSH TO TALK SWITCH IN R/T POSITION, NO TRANSMISSION OCCURS WITH TRANSMITTER SELECTOR SWITCH ON HF. GROUNDED - YES. UNABLE TO DUPLICATE DEFECT. ACCOMPLISHED T/S PER B737-800 FIM 23-11 TASK 806 - INITIAL EVALUATION DID NOT CONFIRM FAULT BOTH HF 1 AND HF 2 TRANSMIT AND RECEIVE NORMALLY ON GROUND. DUE TO HISTORY OF 1 HF PROBLEMS, ACCOMPLISHED ENGINEERING REQUEST TSR701324, INCLUDING REPLACEMENT OF 1 HF COUPLER. ACCOMPLISHED HF OPERATIONAL TEST PER AMM 23-11-00-710-801 AND HF SYSTEM TEST PER AMM 23-11-00-730-801 - ALL TESTS PASS.