Alright, problem solved! Actually in a slightly odd way... ?
?
I moved all easily swapped modules from my working unit into this unit with the Squelch diag error.
Still same problem in RF_Diags receiver test where the test "Squelch Fixed, High Level" shows actual value of 0.001 VDC when the range allowed is 0.754 - 1.131 VDC.
So, before I was going to attempt to swap the input module as well I was reading through the Assembly Level documentation which describes the Receiver Assembly test and especially the Squelch Test-part. I could not figure out why I would get fail on this test while all other tests were ok since the input signal for the squelch test is the same for other tests as well.
I was wondering if this could be a firmware issue since the firmware in my failing unit is quite old, version 6.16. I did have the complete set of firmware upgrade chips to firmware 14.14 so I decided to pull out the controller board, signal board and memory board and completed the firmware upgrade (as well as the U401 chip on the signal board). I inserted back the three cards and powered up the 8920A again and waited anxiously for the boot up sequence to complete. Screen was black but I could hear beeps (normal beeps) so I thought that the controller board was not re-fitted back properly so I powered off the unit and re-inserted the modules once again and made sure the were fully seated into their slots. Powered up the unit again and voilaaaaa!
Screen visible and normal beeps and then "All self tests passed"!
Ok, time for the RF_Diag tests....
And guess what, the unit now passes the "Squelch Fixed, High Level" test with proper value!
Wohooo! Now my 2nd unit passes AF_Diags test, RF_Diags test and MS_DIag test. Wonderful!
Now my next step is to replace the U4 amp on my 2 extra receiver cards and see if I can get them to pass the RF_Diags as well.
