still unclear to me, comparing, with the original schematic J and F are connected together on the probe side, they are used to guard the sampling section of the probe, this guard must be floating and connected to ground only on one point to avoid any ground loop, in your schematic J is tied with E (signal ground) and with F trough C2.
If i remember right the magnitude of DC signal at -50 dB is around 9 uV then any issue on loops, layout, etc. will reflect in a severe degradation of the performance.
It's true, there is an error in the marking. Currently, the cable shield is connected to J. From the analysis of the PCB photos in the early versions of the HP8484A, there is probably a capacitor between J, F, M and the probe housing. Similarly, A is connected to J, F, M. In newer diagrams this connection is no longer present. In any case, in the HP437B socket, pins A, J, M and F are fastened together with a cable and connected to the housing screen.
But there is also new good news
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
Replacing Q1/2 with MMBFJ113 significantly improved the situation. The background level is -60.5dBm. However, when we choose measurement range 5, i.e. -50dBm, the reading is approximately -51. In auto mode, the reading does not always go to a lower range (6 or -60dBm). Zeroing is still not possible. Similarly, calibration for -30dBm (correction of the reference signal to -27dBm already allows calibration). But the measurements are already as follows for signal/reading:
for a -50dBm signal, the reading is -48.5
-40dBm gives -42.5
-30dBm gives -33.1
-20dBm gives -23
I may experiment with MMBFJ309 and MMBFJ201 when I assemble the second PCB.