Failures when the source frequency is less than or equal to 1.5 GHz are usually a low band path problem. Suspect the A20 Lowband Assembly or something between the input connector and the Lowband assembly.
Failures when the source frequency is greater than 1.5 GHz are usually a high band problem. Suspect the A19 RYTHM or something between the input connector and the RHYTHM assembly.
See the PSA Service Guide for troubleshooting information.