As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
HF #1 RADIO INTERMITTENTLY + AUTOMATICALLY CHANGES THE " ACTIVE" FREQ FROM WHAT WAS MANUALLY SET(8843) TO 3254 WITH NO PILOT INPUT. HF#1 CONFIRMED TO WORK FINE + SELCAL FINE WHEN TESTED BUT KEEPS CHANGING THE ACTIVE FREQ FROM WHAT IS SET TO "3254" ON ITS OWN ABOUT EVERY 20 MINUTES OR SO. GROUNDED - YES. PERFORMED B737-900 FIM FOR HF TRANSCEIVER PER 23-11 NO FAULTS FOUND. REMOVED AND REPLACED TUNING PANEL PER B737-900 AMM 23-12-41. PERFORMED HF & VHF CALLS. OPS CHECK GOOD. NO AUTOMATIC FREQ CHANGES AFTER 20 MIN.