I have got a faulty 8753D network analyzer with 6 GHz option here. It shows the "pase lock lost" message when operating at frequencies above 3.000 GHz.
So I traced the signal path with a 54120A scope until I found a failing 5086-7934 YIG oszillator. In source tuning mode it has no output in the range of 3 GHz to 3.1 GHz when it is cold. With warming up the signal fades away and can be activated again with higher frequency setting.
I found this thread here:
It describes a similar behaviour due to failing transistors in th 8753B. But according to this thread the newer versions use MMICs.
Does it mean that the MMIC is dead? Is it somehow replaceable?
Or is there a problem somewhere in the biasing circuit?
Thank you in advance.