This forum exists for the purposes for discussing service monitors (This includes but is not limited to Motorola, HP, Aeroflex, GD, etc). Additional topics allowed include test procedures, interpretation of test results, where to find information about specific tests, antenna VSWR, return loss testing, duplexer and filter alignment, etc.
If you search the board you will see previous posts on COM-120 self tests failures that provide some details on troubleshooting.
The short version is that this part of the selftest loops the audio gen out ( this signal varies for the distortion test ) to the signal gen to the rx and finally to the distortion meter.
Any issue in that loop can cause failures. Does not always mean your distortion meter is bad ...that's just what that test is looking at to verify the entire loop.
You can try re-capping and re-calibrating and see if that clears up the error.
That test should really be re-named "something failed" or "test everything " given the extent of what can cause a failure.
I usually re-cap the front panel as well. may not have anything to do with this issue though.
You also have to clean the boards after the old cap is replaced since the leakage may cause issues either with an unintended DC path across that cap or surrounding circuits.
There are some details in other posts about how to setup the unit to simulate that selftest and troubleshoot. Basically you need to verify the RX, Signal generator and distortion meter.
You may have to scope the signals on the Baseband tray but the easiest way is to swap modules to isolate the failure
This is one of the more problematic errors to resolve since as I mentioned since there are a number of assemblies involved.
