V7-49 electrometer produced in Minsk, Belarus.

[bsw_m]

Some weeks ago I'm bought new 30 year old-stock  electrometer V7-49. This instrument designed by MNIPI and produced by BELVAR. Start production year is 1986. End production - I don't known.
This instrument is lowest end electrometer by MNIPI, but not bad at all.
This instrument has all sapphire insulation in input circuits.
By official documentation this instrument can measure current range 1fA - 10mA and voltage range 10uV - 200V with input impedance greater than 100TOhm (Measured by me input impedance has 1.3POhm, but I think this is can be more. At this time  I'm don't have methanol to properly wash the insulators).

After some repair (replace all electrolytic capacitors, cleaning insulators and some other work) I'm take the performance verification. (I'm not calibrate instrument)
For voltage I'm use V1-18 as source and cannot calculate error in all ranges (in relation to V1-18)
For current verification I'm use V1-18 as voltage source and 1TOhm resistor.

Rated current 500aA:
0.0005pA.JPEG на ixbt.photo:


Rated current 1fA:
0.001pA.JPEG на ixbt.photo:


Rated current 1pA:
1pa.jpg на ixbt.photo:


And rated current 10pA:
10pa.jpg на ixbt.photo:


Do you accept me to the ampnuts club? 

P.S. Teardown of this instrument: https://yadi.sk/d/uUZoiU154grgFg

For 1TOhm resistor, big thanks to Shodan_x

[ramon]

accepted !

Is that a KPC104A with Voskhod/Kaluga logo?
Who has datasheet? Generic FET or secret weapon component from cold war?

(Saving part number on ebay in case it someday it pops there.)

[schmitt trigger]

I Attempted to open the provided hyperlink, and while it opens a Yandex website, all of the image links are broken. 

https://yadi.sk/d/uUZoiU154grgFg

[bsw_m]

From home I'm do reload photos to other hosting.

[bsw_m]

I Attempted to open the provided hyperlink, and while it opens a Yandex website, all of the image links are broken. 

So, I'm reloaded photos to google drive: https://drive.google.com/drive/folders/1XB6oVjcwRhqEmF3nmEniEFkNRFCoNIB6?usp=sharing

[bsw_m]

accepted !

Is that a KPC104A with Voskhod/Kaluga logo?
Who has datasheet? Generic FET or secret weapon component from cold war?

(Saving part number on ebay in case it someday it pops there.)

Here is

[guenthert]

Neat pictures of an outstanding instrument, thanks for sharing.

I'm a bit puzzled by the large number of what I presume are TTL logic chips and even some very specialized chips on the (again, I presume) logic board.  I guess those were due to constraints of the market in soviet times.  The analogue board is (of course) just magic -- are circuit diagrams still available?

Kudos on the calibration.  Creating a stable current of precisely 1pA is no small feat. 

[bsw_m]

Upper board is digital board, based on 580VM80 cpu (Russian analog to i8080)
Lower board is analog board, and power supply regulators.
Schematic is here, but sorry for low quality.

Big metal case IC on center of analog board - is high voltage hybride ampifier IC. In schematic is DA15.

[TimFox]

I can see the pdf file for the schematic, but I can't open the jpg file for the specs.
I assume VT8, the dual FET, is the critical component--do you know any details about it?
In the close-up photo of the hybrid IC, the socket resembles a ceramic socket for a transmitting tube.  How large is the IC?

[RoGeorge]

accepted !

Is that a KPC104A with Voskhod/Kaluga logo?
Who has datasheet? Generic FET or secret weapon component from cold war?

(Saving part number on ebay in case it someday it pops there.)

What's that 8 petals component?

[bsw_m]

I can see the pdf file for the schematic, but I can't open the jpg file for the specs.
I assume VT8, the dual FET, is the critical component--do you know any details about it?

This is selected KPS104A dual jfet (very old from 197x). Selection procedure present in instrument manual.

In the close-up photo of the hybrid IC, the socket resembles a ceramic socket for a transmitting tube.  How large is the IC?

In this instrument socket is plastic. Ic have long side is about 40-45mm.

[bsw_m]

What's that 8 petals component?

This is heart of this instrument - electrometric commutator (8 relays). Sapphire insulators and AuNi alloy for contacts with percentage 95Au 5Ni.

[schmitt trigger]

Upper board is digital board, based on 580VM80 cpu (Russian analog to i8080)
Lower board is analog board, and power supply regulators.

The board material for the digital and analog boards appears to be different?
Or perhaps the digital board does not have a solder mask? That would explain the color difference.

Quite a beauty! 

[bsw_m]

Or perhaps the digital board does not have a solder mask? That would explain the color difference.

Right!

[jaromir]

Thank you for sharing the pictures and schematics, very intriguing! It's refreshing to take a look at this instrument, totally unknown to me 

The schematics has no part numbers, I can mostly guess the ICs functions, but there is mysterious DA21, in two logical blocks DA21.1 and DA21.2 Could you tell me what is this part, please, or perhaps share datasheet?
Also, what parts are DA11-DA13 and DA5/DA7? That looks like a part of integrating or mark-space ADC, along with DA21.2 (I guess resistor network?) and DA5/DA7 (reference switches).
Reference circuit looks to be build around DA6 and DA10 and resistor network (?) in DA21.1. What part is VD29?

[bsw_m]

Thank you for sharing the pictures and schematics, very intriguing! It's refreshing to take a look at this instrument, totally unknown to me 

The schematics has no part numbers, I can mostly guess the ICs functions, but there is mysterious DA21, in two logical blocks DA21.1 and DA21.2 Could you tell me what is this part, please, or perhaps share datasheet?
Also, what parts are DA11-DA13 and DA5/DA7? That looks like a part of integrating or mark-space ADC, along with DA21.2 (I guess resistor network?) and DA5/DA7 (reference switches).
Reference circuit looks to be build around DA6 and DA10 and resistor network (?) in DA21.1. What part is VD29?

You right DA21- this is resistor network 313NR220  http://www.155la3.ru/datafiles/k313nr2xx.pdf
DA11 - KR140UD1408A (Lm308)
DA12,13 - comparators - K554SA3A (LM311)
DA5 - KR590KN4 (HI5043)
DA6 - KR140UD1408A
VD29 - KS190D 9V at 10mA Temperature coefficient ±0,0005% / degree  and 0,02% / 5000h

[bsw_m]

I'm reloaded specs for KPS104. The first time, for unknown reasons, the file was attached with an error.
Now all it's ok.

[coppercone2]

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An optional additional explanation is:

[exe]

Wow, that's impressive. How much does it cost? I found it on sale in one place, they want ~$1200 or thereabout... Other shops don't tell the price, or don't work with individuals.

[HighVoltage]

Interesting comparison with the Keysight electrometer.

[Echo88]

The following document contains B2985A current reversal settling time measurements/noise. Maybe its of interest in regard to comparison against other electrometers.
https://arxiv.org/pdf/1808.08881 Page 3

[bsw_m]

Real cost is about 450$. If attempt deep searching (waste time), can buy this instrument about 300$ "NOS" condition.
Used unit without kit can buy 100$

[RoGeorge]

I've scanned books. Unfortunately quality of paper is too bad. 

After some post processing i create more reliability images, but it's still bad...

V7-49 user manual: https://ampnuts.com/data_pub/V7-49_book1.djvu
V7-49 Schematic, Bill of material, diagnostics procedures: https://ampnuts.com/data_pub/V7-49_sch_bom.djvu

Djvu reader: https://windjview.sourceforge.io
Raw data from scanner: https://ampnuts.com/data_pub/V7-49_books_source.rar

Thank you!

I was just looking for specs, because the first pic from the OP is so intriguing.
500 attoamps will mean slightly less than 10 electrons each millisecond.   

[TimFox]

When measuring such low currents, the bandwidth of the electrometer will be correspondingly low, far below 1 kHz.
Of course, current in a conductor is not discrete flow of discrete electrons as in a vacuum tube (current in a conductor does not exhibit shot noise).
However, if we were measuring the shot current in a "saturated" diode (no space-charge limit), and wanted 1% statistical error, (Poisson statistics) we would need 10,000 electrons per measurement.
At 500 x 10^-18 A = 3125 Q_e/sec, that would require a measurement time of 3.2 sec, which seems reasonable for an electrometer at such a miniscule current.
Another poster here quoted a settling time of 12 to 15 sec for 1 fA full-scale on a Keysight electrometer, while another measured about 1 sec at 0.1 to 1 fA on a Russian unit.

[TimFox]

Oops, my typo!  The Keysight measurement was done at 1 pA, not 1 fA, and that was not full-scale.  The settling-time values quoted for the Russian unit are surprisingly short.

[bsw_m]

As You can see, schematic of this instrument is classical, without any secrets.
I think many of the strengths of this tool come from its low input capacitance and low leakage.
All this is achieved thanks to well-thought-out counter-input circuits and sapphire insulators.
All this is achieved thanks to the thoughtful design of the input circuits and sapphire insulators. Which in the device a total of 14 pieces. 6 large (10mm diameter and 4mm thickness) and 8 small.
You could see small insulators in the petals of the switch. Large ones are used in connectors and also in the center of the switch.
Large insulators look like this:

[TimFox]

Sapphire is a wonderful insulator.  For capacitance, however, its dielectric constant is a bit large (9.3 or 11.5, depending on direction in the crystal).  It's dielectric loss, 0.01%, is about as low as it gets.
PTFE has a dielectric constant of only 2, with < 0.02% dielectric loss.  The bulk resistivity of both materials is so large as to be unimportant;  the resistance of an insulator component will depend on the surface cleanliness.
PTFE does have some interesting mechanical problems, however, while sapphire is extremely hard.

[RoGeorge]

From the video, it looked like the sapphire based instrument was doing better than the Teflon based one.
Wasn't that so?

[bsw_m]

Sapphire is a wonderful insulator.  For capacitance, however, its dielectric constant is a bit large (9.3 or 11.5, depending on direction in the crystal).  It's dielectric loss, 0.01%, is about as low as it gets.
PTFE has a dielectric constant of only 2, with < 0.02% dielectric loss.  The bulk resistivity of both materials is so large as to be unimportant;  the resistance of an insulator component will depend on the surface cleanliness.
PTFE does have some interesting mechanical problems, however, while sapphire is extremely hard.

You totally right!
But good input commutator with low capacitance is better solution, that plastic relays in B2985A (may be I'm wrong).
So schematic of V7-49 is really simple. Where is magic?  I think - only in input circuits.

[bsw_m]

So what about hundred attoamps and this old electrometer?
I'm using very dirty setup for this low currents, but...
To get this small current I'm use V1-18 calibrator as voltage source with 500uV on output and 1TOhm resistor in small black box.



Noisy ... but probably if using averaging is acceptable.
Manual said this instrument is capable to measure 1fA and have resolution of 0.1fA this is absolutely truth.  But 1*10^-16A is semi acceptable due noise.
This simplest instrument from MNIPI has warmed up my interest in small currents and now I get a "monster" electrometer from MNIPI, which is capable of measuring 1aA (though I can’t imagine how to generate such a current at home). We'll figure out.
After receiving a new electrometer - I will open a new topic for it 

[jaromir]

I noticed another interesting device here - V1-18

Majority of those instruments are totally unknown to me and it's difficult to find much of an information about it outside russian speaking part of internet. I think it's interesting to observe how engineers "on the other side" approached the same problems, often bringing surprising solutions. Though I can barely read cyrillic, and google translate can do some half-useful job at translating websites like radiokot, parsing the needed information is very slow. Having analysis and insight from somebody native to the language, culture background and history of those instruments is very helpful.

Don't know about other users of this forum, but I would welcome any other threads about those mostly unknown T&M devices

[2N3055]

I noticed another interesting device here - V1-18

Majority of those instruments are totally unknown to me and it's difficult to find much of an information about it outside russian speaking part of internet. I think it's interesting to observe how engineers "on the other side" approached the same problems, often bringing surprising solutions. Though I can barely read cyrillic, and google translate can do some half-useful job at translating websites like radiokot, parsing the needed information is very slow. Having analysis and insight from somebody native to the language, culture background and history of those instruments is very helpful.

Don't know about other users of this forum, but I would welcome any other threads about those mostly unknown T&M devices

I agree with Jaromir. Thank you for very educational posts and please don't stop!!

[Gyro]

...
This simplest instrument from MNIPI has warmed up my interest in small currents and now I get a "monster" electrometer from MNIPI, which is capable of measuring 1aA (though I can’t imagine how to generate such a current at home). We'll figure out.
After receiving a new electrometer - I will open a new topic for it 

Irrc 1aA is around 63 electrons/sec, so some form of sub-atomic catapult might be in order. 

[TimFox]

...
This simplest instrument from MNIPI has warmed up my interest in small currents and now I get a "monster" electrometer from MNIPI, which is capable of measuring 1aA (though I can’t imagine how to generate such a current at home). We'll figure out.
After receiving a new electrometer - I will open a new topic for it 

Irrc 1aA is around 63 electrons/sec, so some form of sub-atomic catapult might be in order. 

Still, remember that current flow through a conductor is not like through a vacuum diode where electrons hit the anode one by one.  Instead, it is a very slight tendency of a gazillion electrons to move to the right, superimposed on their natural random motion (Johnson noise) due to thermal agitation.

Back in graduate school, I tried to resuscitate an ancient Cary vibrating-reed electrometer.  The vacuum-tube electronics units (30, 31, and 32) in the 1950s were later replaced by solid-state electronics after Varian got the company (model 401, ca. 1970).  The magnetically-driven vibrating capacitor produced an AC voltage (450 Hz) for the low-noise amplifiers from the charge on its plates.  Manufacturer's literature from that time claimed that approximately 10 aA (from a high-Z source) could be measured.  The construction quality of the sensing head was comparable to the quality of these modern Russian units.

[TimFox]

By the way, can you post a photo of the input connector for the Russian electrometers?  Is it compatible with "standard" coaxial connectors or is it proprietary?  I have an ancient -hp- 425A unit, ca. 1958 (with a motor-driven optical chopper wheel) with an input connector that is no longer available, but can mate with a standard "N" connector (without the extra guard cylinder).  Available concentric triaxial connectors have different dimensions.  In current mode, its lowest full-scale current is +/- 10 pA.

[strannik2039]

Hello. In a parallel branch
https://www.eevblog.com/forum/metrology/circuit-design-of-branded-electrometers/75/
I gave a link to a Google drive where I shared a photo of the input device of the same В7-45 (V7-45). The device was introduced to the world in 1986.
If we talk about 1aA - 10aA

[doktor pyta]

Below a nice teardown of Keithley 642. Also saphire and interesting relays.

http://bbs.1ppm.cn/topic/398/%E6%9C%80%E9%AB%98%E7%AD%89%E7%BA%A7%E7%9A%84%E9%9D%99%E7%94%B5%E8%AE%A1-keithley-642-%E6%8B%86%E8%A7%A3%E5%88%86%E6%9E%90

[TimFox]

The RX-1M seems to be an improved version of the classic glass-encased resistors used in Keithley electrometers.  I believe they were made by Victoreen.

[bsw_m]

I've check some caps... Yup! that is true! Allmost all K50-35 electrolytic caps has died

After replace dead caps, You get about 3*10^-16 noise floor.
Temperature stabilization is more complex. First step is fix error with trimmer in TIA.
Second step is replace OPAMP by new more stable, like OP177, OPA140 e.t.c. After replace need shunt 22Ohm trimmer by 1.5..2.2Ohm resistor.
Thrid step is replace 1k trimmer by 10k trimmer and place in parallel 510Ohm resistor.
So, 10k trimmer (SP5-24) in current set KPS104  I highly recommend replace by more stable trimmer.
These modification will give you good temperature zero reading stability.
So now I have rock solid 1*10^-15 with T change about 6Celsius and for time about week, after these modifycations.

[guenthert]

  Not sure where you've seen inadequate parts or workmanship (I don't think I'd be able to tell), but have you kept in mind that the 642 is a remote electrometer, i.e. the sensitive pre-amplifier is physically separated from the end amplifier, ADC, display unit?  The latter one presumably doesn't need exotic parts or manual assembly by expert crafts(wo)men. 

  I always wondered why not more sensitive instruments go that route.

[guenthert]

  Others will be more qualified to comment, but my understanding is that for an electrometer (measuring feeble currents) EMF (due e.g. to the solder at the relays) is, unlike for e.g. nanovoltmeters, of little consequence.  The burden voltage is measured in milli, not micro, Volt anyhow.

  As has been mentioned before, PTFE is sufficient, if there is enough space.  Using cheapest materials to get the job done is these days (compare instruments from the Victorian age) considered good engineering and those were engineers, not jewelers, at Keithley.

  As for the geometry:  this looks indeed a bit like a low-volume, hand-assembly production unit as scientific instruments often do.  But as as long as it works ...

[bsw_m]

K642 have external TIA module this is a very good decision. But this module also has certain disadvantages. First of all, in the input switch. Here I will not describe them, all the same this topic is devoted to the B7-49 electrometer and his metrology. And not a comparison, which is better. In the development of any tool, tasks are set to ensure certain characteristics, and if the tool began to be produced, then these tasks were completed.

[MegaVolt]

Also zenner in +-15V power source has been replaced to much stable. (dear voltnuts, please forgive me)

Can I have the details? Scheme?

[bsw_m]

Also zenner in +-15V power source has been replaced to much stable. (dear voltnuts, please forgive me)

Can I have the details? Scheme?

Due LM399 and KS170A have very little difference in zenner voltage, this mod need only change current resistor (KS170A - 10mA, LM399 - 1mA).
Installing the LM399 in 15V regulator - was my crazy idea that was more like a joke.
This modification gives a slight improvement in the temperature stability of the TIA input stage. And it is not really required.

[exe]

Wait, is that the price at the bottom? OMG...

[bsw_m]

Yup!
My device work fine without any modifications... but i want to replace some parts... a little-bit

You forgot to add to the calculation a new custom display with backlight. 

[Vgkid]

That must have been a rather expensive piece of gear back in the day.

[MegaVolt]

All stock electrolytic cap is replaced.

What is their condition? They all lost much capacity or only part? Were there any measurements?

[MegaVolt]

You really want to known??? >:Dght way.

Yes. It was interesting to me. I came across old equipment and sinned on capacitors. After spot checking a couple of them, I did not find any problems. So it’s interesting how truly this total replacement is really justified. How big is the percentage of bad compared to the total. Is it 100%? 10%? 1%.

[MegaVolt]

Maybe he right, but ESR=4R is bad condition for reduce noise in power line.

I agree.

[exe]

How big is the percentage of bad compared to the total. Is it 100%? 10%? 1%.

I think even one bad cap can be enough to ruin measurements. Caps are there for a reason...

[2N3055]

That's dead alright..
4R ESR is only good for 10uF/63V elko...
2200uF elko must be in tens of milliohms...

[MegaVolt]

I think even one bad cap can be enough to ruin measurements. Caps are there for a reason...

Here is more a question of the search method. Or we change all capacitors. Or we turn on the device and see where the ripple is more than necessary and change only there. It all depends on the percentage of marriage. If 1 capacitor per device, then the second way is easier. If 90%, then by itself the first.
I think the storage and operating conditions of the device before this are still strongly affected.

[MegaVolt]

Storage conditions of my devices looks good. All packages come in very good conditions, no oxidize into device or accessories. There is no sign of damp.
When i check 5-7 caps, all of it is died, then i make decision - do not waste time for check and replace it all.

I understood

[asis]

Hi
I read this exciting story with great interest.
I remember with pride our instrumentation.
Keep pouring balm on your soul, please.

Good luck.

[Vgkid]

Thanks for the link, keep it up.

[exe]

Don't they have different proximity to the speakers? Also the sound was too quiet, if one wants to listen to Lordi, crank up the volume to 140dBa or more (so your lovely neighborhood can enjoy it too).

[bsw_m]

So what about noise of this instrument?
I'm was collect  data  from instrument in 1*10^-12А range and settling time 10s. Time for data collection is ~1Hour (Instrument in stock condition, no modifications, only a bug fixed with resistors in TIA)
For graph I'm use the Stable32 program.

So now time for modding this instrument. 

RAW data from instrument attached. Sampling time 0.1s

Program for working with this instrument trough GPIB written in C and use linux-gpib library. Later I will post the code.

[bsw_m]

Some new data and new graph

[Kleinstein]

There are regular jumps, like some periodic adjustment steps to null the amplifier. those steps seem to be quite large in the beginning. Maybe the meter was not yet fully warmed up. Otherwise jump of up to 5E-16 A are quite large compared to the noise level shown before (5 E-17 range for a few minutes).

[bsw_m]

You absolutely right!

[bsw_m]

Second attempt to plot noise floor for this instrument with capped input.
Environmental temperature is quite stable: not more that 1Celsius change. Instrument is properly warmed-up.
Sampling rate in file is 10sps. Data collection time about 11hours.

[bsw_m]

My idea with REF200 has fail. i'm found some interesting things - that JFET's need a programmable current source to set extremely low leakage mode, REF200 can't be programmed.

You right. For low gate current (less than 1*10^-14) input jfet in this instrument work on special combination with drain voltage/source current.
This interesting mode is described in more detail in this patent

[bsw_m]

10 fA... hmmm... that is too big leakage current for me. If i can't drop down current to less 1fA, i will try ADA4530-1 or some similar.
But now i need stabilize zero at first, because zero stability is affect to leakage current directly.

Less that 10fA, in real world less that 1fA ;-)

[bsw_m]

Right!

[MegaVolt]

My TC test of Vbe change is accomplished, i'm select 3 NPN's (2 as 50-150uA driver, 1 as temp.co. stabilizer) to create dual channel JFET current driver, calculations shows possibility to create programmable  50-150uA current source with up to 100ppm/C temp.co.

Can I sketch a diagram ?? At least on a piece of paper to understand what is at stake?

[MegaVolt]

And what parameter of the device is planned to be increased by such revision?

[bsw_m]

Irrc 1aA is around 63 electrons/sec, so some form of sub-atomic catapult might be in order. 

1aA is around 6.3 electrons/sec.
And I'm got the "sub-atomic" catapult  This is NK4-1. Soon i will open the topic for this tool

[bsw_m]

I'm got the "sub-atomic" catapult

Yup, that NK4-1 catapult use for calibration all Keysight/Keithley electrometers equipment in our country.
I go to your's lab with my devices to calibrate my 2985.

So, this is not problem, Andrey! I will be glad to talk live.   

[Kleinstein]

To adjust the drift of the FETs it may actually help to independently adjust the currents. Via the current ratio one should be able to adjust the differential temperature drift of the offset. Just adjusting the current would alow to adjust the bias current - though likely no big gain of reduces current noise by actually getting it down to zero, as there will likely be some compensation of a positive and negative current from different parts of the fet.

For calibrating very small currents, there would be single electron turn styles, so counting electrons, one at a time. I don't remember calling them catapult. This is still quite new and may require low temperature.

[MegaVolt]

I have to buy the most sensitive electrometer on planet Earth to test these OpAmp's and select one OpAmp with lowest leakage.

To measure small currents, you can watch the charging time of a small capacitor connected to the input.

[bsw_m]

So, I performed a rapid test on the lower ranges.
Used current source: NK4-1 calibrator.
For 100fA using differentiator №1, for lower current using differentiator №2
IMHO I see very good results for this simple electrometer.

[Kleinstein]

The low bias OPs have a relatively high offset drift and at higher temperature there can be drift in the bias current.
Still those OPs are often better than the single FET solutions, unless the TC is really well trimmed.

The circuit shown looks odd: As a follower for a low bias TIA input the ADA4530 should get away with a even lower supply. It would be only for higher voltage readings that the higher supply is needed.  In combination with the ADA4530 there is no real need to trim the offset of the OP177. Adjusting the offset would also change the TC. So with trim the OP177 has very limited advantage compared to a 741 in the given circuit. As a low temperature would help, I would consider more like a low power OP.

[Kleinstein]

There is nothing wrong with the 2 OP configuration. If only use as a TIA it would be more like a lower supply to the ADA4530, more like +-5 V or +-3 V.  The +- 7 V mA be needed for use in voltmeter mode however. You may have to check if this is enough - some electrometers use am extra bootstrapped supply or driving the opposite side, so it depends on the rest of the circuit if the +-7 V are sufficient and needed.

Both the ADA4530 and OP177 are supposed to have a rather low offset - so I doubt a trim would be needed. There is still the option to use the lower power LT1097 if wanted. Noise would be dominated by the ADA4530 anyway.

[Kleinstein]

Is the V7-45 OK with working horizontally ? The mechanical part may be effected from working in a different orientation.

I would guess the V7-49 is working in voltage reading mode. In current mode it would not make much sense to connect 2 TIA like instruments.

What type of offset compensation is used, that it effects the input current ? I would not expect the voltage offset adjustment to effect the current so much.

[Kleinstein]

Connecting both meters in current mode is tricky and not measuring the real offset of the current. Both meters can have a small input voltage offset. They are not made to work with a low impedance source, but a high impedance current source.
With both meters in current mode would measure the difference in offset divided by the protection / isolation resistors of both meters. Adjusting this offset to zero would be better with a simple short than a 2 nd meter.

I know the V7-45 is using a kind of integrator, but similar to an TIA it is a relatively low impedance (may be still in the 100 M range) input. With those small currents 100 M is considered low impedance .

The more relevant test would be having the V7-49 in voltage input mode, which I assume the meter would also support.

[Kleinstein]

The meters do have an offset, the troublesome point is the offset voltage. If one connects a resistor of let's say 10 M from ground to the input of a TIA / integrator type amp-meter it would read a  different current from the open circuit case. This is due to the internal offset voltage. I don't know how much this would be with the vibrating capacitor, but there is a good chance to have some 10 µV, possibly more.

Turning off the feedback at the V7-49 is kind of setting the meter to voltage mode, though the 2nd OP may still run into saturation. Because it is separate from the input stage it would still no effect the input current.
The output of the program 10 should ideally be the voltage. So the V49 would read the offset voltage of the V45 + it's own offset voltage and the V45 would read the bias current of the V49 (ADA4530) + it's own bias current (rather small).

[bsw_m]

Historical information.
I'm found price for this electrometer in 1990 year: 2975 rubles.
The official exchange rate in the USSR was 1.64 USD per ruble.
So this device cost $4879 in 1990.

[exe]

So this device cost $4879 in 1990.

Wow, that's a lot of money... Well, makes sense, I just expected it to be much more... affordable.

[bsw_m]

I did a home metrology check on my B7-49. The results can be found in the attached file.

[bsw_m]

This device has a strange GPIB, I had to spend a lot of time to get the device to work smoothly via GPIB.
As result I'm share dirty, but smoothly working code for using this electrometer over GPIB.

[bsw_m]

Little and may be last upgrade to my V7-49.
New full custom display with backlight from the daughter of MNIPI: unitary enterprise "Display"

[harrimansat]

accepted !

Is that a KPC104A with Voskhod/Kaluga logo?
Who has datasheet? Generic FET or secret weapon component from cold war?

(Saving part number on ebay in case it someday it pops there.)

Here i

Can you please share datasheet again?   I have two of these pair fets

[bsw_m]

Can you please share datasheet again?