What was found
BOTH CPT AND FO UNABLE TO TRANSMIT ON HF1 AND HF2. CAN TUNE BOTH HF 1 AND HF2 AND CLEARLY HEAR TRANSMISSIONS FROM ATC AND OTHER A/C. WHEN TUNING FROM 112.82 TO 5.547, COULD TRANSMIT AS LONG AS R/T MIKE SWITCH WAS NOT LET GO. ONCE LET GO BACK TO NEUTRAL POSITION, TRANSMISSION NO LONGER AVAILABLE. GROUNDED - YES. PERFORMED HF COMM SYSTEM TEST ON #2 HF REF. B737-800 MM 23-11-00. GROUND VERIFICATION TEST GOOD. FOUND #1 HF SYSTEM WILL NOT TUNE. OPENED AND TAGGED (2) C/B'S. REF B737-800 AMM 23-11-61. REMOVED AND REPLACED #1 HF COUPLER. RESET (2) C/B'S. GROUND VERIFICATION GOOD. OPS TEST NORMAL.