This is the second converter. I think, it takes in the 4231.4MHz from the 1st mixer, through a bandpass filter (The 2 Golden Screws). Then it gets mixed with 3810MHz, from the LO (Bottom Golden Screw). The Mixer Diode is marked "2R".
Then it puts out the 421.4MHz 2nd IF.
The coax cable on the bottom left takes a "sample" from the LO, for the PLL.
Looks very familiar to my R3273.
Better dont open the Filter, or turn the Golden Screws!!
If im understanding correct, the analyzer doesnt show a applied signal?
And you injected 421MHz into the 2nd converter? Thats wrong...the 2nd converter needs 4231.4MHz input.
Did you test the input step attenuator? Maybe its broken...