V7-49 electrometer produced in Minsk, Belarus.

[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.