Tektronix 177 not working in nA ranges

[rodd]

dear all,
I am trying to fix a TEK 577/177 Curve tracer I was able to put my hands on.
The step generator was finally fixed, so I made my first attempt to calibrate the instrument.
In order to adjust the Vertical Gain, the 177 has to operate in the 50nA range.
When trying to follow the procedure, the dot on the display goes all the way down instead of staying around the 8th division.
There are two suspects capable of producing this behavior:
 1- The Vertical amplifier or
 2- The Vertical Preamp Section that is located on the 177.
I am able to produce a "normal " display in all other ranges of the Vertical Current/Div switch (in the 177), except in the nA range.
So I disassembled the 177 and checked for open or weird value resistors connected to S630, but I could not find any.
However when I measure the resistance between TP617 (point "A") and the lower end of R640 (point "B", I get the expected values up to the nA range.
In the nA range, the resistance is about 600k-800k (instead of 1M and 10M) and increases slowly with time (as charging a capacitor).
The same happens when measuring the resistance between points "A" and "C" .
Please refer to the attached schematics.
I tried to remove Q604, butthe behavior is the same.
Any ideas?
Thanks