Ok I wrote up about four questions before I hit post. I thought about each then rewrote this lol.
I make the assumption that your black cable is serviceable.
During cal the first thing it does is a user test, look for the cal signal at about the right level and frequency, make sure the user actually plugged in the cal signal
It will look for a tone at exactly 30MHz and within a power range about -20dBm. I don't know the exact ranges but -74 is not close enough.
-First I would like to confirm the measured frequency. Does it dance around at all?
-Can you run the frequency counter on the unit and measure the frequency of the tone. I want to see it come back as exactly 30.000000MHz. Since it generates its own cal signal from the same low frequency reference there should not be any variance if all the PLLs are locked and operating correctly.
-Is the RF still passing self test? I would bet $$$$ that the self test does not do much more than check PLL lock detect signals and some other simple checks, that is why it happens so fast. If you have all the lock detects and pass self test then I would say you do have a 3840MHz Lo.
Assuming that you do indeed have a stable exactly 30MHz cal signal then I am inclined to suspect the MMICs. My thoughts here need confirmation from an expert as I am only familiar in passing with RF. Here goes:
There are three MMICs in the 2nd Lo, one that drives the oscillator, one that amplifies a take off from the VCO and drives out to the mixer (also drives the test port), and the third that drives the frequency divider part of the PLL from the output of the 2nd MMIC. If for example the middle guy is damaged, the Lo drive to the mixer would be low which would show up as a low or missing amplitude on the trace, however the 3rd MMIC could still drag up the signal enough from the noise for the divider circuit to do its job and bang you get a functioning Lo with insufficient output drive into the mixer.
I do recall reading that Fraser had replaced the MMICs in one of the units he worked on, perhaps this may be needed.
-Ty