OK, I've invested some effort and wired the unknown probe as below..
As usual the most troubles made me the reed relay.. I almost resigned with my attempt to run this 40+ years old device, until I recognized the relay's contacts are switched "on" (shorted) when the coil is not powered. Sure, this was the great idea - as thanks the input shorted with the output (actually via 39k to opamp's output) helped the HDIG1030 survive rough handling and the transistor still WORKS!
I added inside the module an 0.47u capacitor (tantalum drop) across the opamp's power and a diode at the relay coil.
With the relay not powered (the contacts shorted) I can set zero (50k 10t) at the opamp's output (OUT1), the zero then fluctuates slowly some 500uV up and down in time. The voltage at the HDIG1030's source (IN-) is +2.855V with the 13k source resistor and 15V Vcc, when zeroed.
With the relay powered (the contacts disconnected) the OUT1 voltage settles from initial jump after about 20secs and fluctuates then some 80mV peak-peak from +300mV to +380mV in "quiet periods" (from 1.5m distance, while watching it I am interacting with the electrons at the input - the wave functions collapse, when not watching it could be much less
![Evil >:D](https://www.eevblog.com/forum/Smileys/default/xevil.gif.pagespeed.ic.tdrs7Uj5gu.png)
), and much more based how you move yourself around. While messing in vicinity of the input it sometimes jumps into volts level, thus it seems the transistor works.
I have to investigate the role of the R3=39.2k, on the pcb there is written an old remark "R75=39.19" and the board is populated with the "39.2k" one.. Any idea??
![Tweak :-/O](https://www.eevblog.com/forum/Smileys/default/smiliey_cal.gif.pagespeed.ce.yYMcAvxBJk.gif)
PS: ok, the shorted part of the FB is outside the 39k, thus the 39k resistor creates the +300mV offset, it seems.
PPS: Correction - the capacitor is 10pF/500V Porcelain Vitramon