Here's a picture of HY460:
Uhmm... It looks like a signle balanced mixer, but the outcoming
signal from T450 should have a great shape, it must have harmonics
over the GHz limit to work with that couplers.
Do you have tried to see the signal on R450 with the scope?

Last time I tried to do that the signal was visible only to few hundred
kHz and then the amplitude decreases rapidly. But maybe with preamp...
Never use x10 probes (if you did) since they highly reduce the
bandwidth of the scope. Anyways the T450 transformer output seems to
be insulated from ground and have very low impedance, so your x1 probe
should be used without any problem (doens't matter where you put the
ground terminal).
If your scope has at least 40MHz bandwidth you should see a discrete
square wave there (I guess).
I wonder reading that you read a so low signal at J430 because looking
to the schematic of the following stage, you can see just two opamp
having low gains (about 10) connected there.

For now I haven't touch it at all. If I understood correctly it will
only adjust the 1st mixer injection and has no effect on HY460 input?
Yes, it is. I asked just to know if the YIG has fall down of some dB
there.

Also I'm not sure if I lose the amplitude measurement calibration if I
adjust that? Of course it's possible to recalibrate the analyzer after
that, but may be it's wise to do some reference measurements before
adjustment...
Do not adjust any trimmer if you don't know the procedure for do it, I
asked just to know if the service manual said something about it.
Have a nice weekend.

Massimo