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The receiver for the earlier 8920A units is a bit different but will still work in the 8920B. It may give errors because you can't transfer the cal data too. What you need to do is just swap the receiver module and see if your 8920B spectrum analyzer level is in the "ballpark" of where it should be. If so, You have a bad signal path through the receiver module.

Your next step is to check U4 (MSA0786) amp. Check the dc voltages (4V and 1.6 -input) on the U4. This is generally the problem with low levels to the spectrum. I have never seen a bad downconverter. Rick