Edit (awade, Mon Sep 18 13:39:59 2017): These are modifications to the North path field box

Craig has been working on active probing the FSS boxes to work out what is going on there.  He came up against some unexplainably low gains and stuff that didn't quite match up with what we should expect when measuring transfer functions between TEST2 EXC (the common path excitation point) and TP5 and later test points in the box.  

A while ago I had found that measuring and tuning out1/out2 OLG with the Agilent plugged in would get to a stable loop, but as soon as I unplugged the Agilent it would be unstable.  I assumed that the monitor points needed to be 50 Ω loaded when not being measured: which is what I did as a default.  

The open loop gain from common EXC to TP5 on the servo board was -3.15 dB.  With common OUT1 terminated with 50 Ω this TP5 dropped to -17.8 dB or with OUT2 it dropped to -25.75 dB. Bad. This was a clue that maybe the series resistance to the monitor outputs was selected too low.

Now looking at the changes that Frank made to the schematic (PSL:893), he had changed the series resistors R1 and R2 from 453 Ω to 50 Ω saying that the monitors were limited by thermal noise.  I don't know if I believe that. We should have plenty signal to noise. Also, with 50 Ω to monitors a 50 Ω load of the network analyzer will have 100 Ω to ground. I have a feeling that this is causing the op amp to be over drawn in current. The AD829 datasheet says max current 32 mA.  I don't know if the servo signal at the point gets to 3.2 V offset but it might. It seems like it would be better to have higher load impedance to avoid overdrawing current out of the op amp output. 

I switched the R1 and R2 50 Ω (on D040105-C) out for something higher again, this time 470 Ω.  I could only find 470 Ω in the EE workshop (which is close enough to the design value of 453 Ω).

Now plugging 50 Ω loads into the OUT1 and OUT1 common monitor points lowers the common path OLG by 7.8 dB and 10.8 dB respectively.  This still seems like a big change to me but I'm not sure if I should increase series resistance any higher.

Let me know if this is dumb and I'll change it back.