If you look at the RF I/O you should see an MMIC or two used for the higher ranges.
Those may be weak in both units.
You could look at the RF I/O output? using single step on the RX diag section and a external SA.? ?The mixer assemblies are pretty reliable ...have maybe replaced one or two over the years.? Not much to break.
I usually loop the Antenna to Duplex in the SA screen and use the tracking gen across the full 1Ghz to check the response ...should be pretty flat across the range? ...like may +- 4 db ripple max IIRC
Chances are without the SA both units pass all the RF diags because the tests for the absolute levels are skipped ...three tests.? Usually the low frequency passes.
