Keithley 181 - Nanovoltmeter

[branadic]

Hi,

we have a Keithley 181 Nanovoltmeter at work, which was turned off for a while. I didn't even know that this gear is in one of our lockers. So when I found it I went to my desk and turned it on, still functioning 
But after a few weeks of working without problems I noticed, that the 7 segment display turned of. Powering it on and of showed only a short sign of life. So what is your suggestion, what could be faulty? Only some caps are dry and recapping is necessary?
Hints are welcome.
I already downloaded the manual with the schematics.

-branadic-

[TiN]

I'd start with usuals, PSU checks, recap and contact checks on cables/switches.

[branadic]

Had the time today to take the thing appart and indeed I have a capacitor issue. Time for some complete recapping.
Original caps are from Richey any suggestions what to replace it with? Nichicon, Rubycon or are Yageo enough the effort?

-branadic-

[pelule]

I recommend a good quality brand like Nichicon to get long lifetime.
Absolute value is not critical, as they hav usually ~10% higher values anyhow to keep them in spec during aging.
/PeLuLe

[branadic]

Maybe someone can help, what brand is this capacitor inside the Keithley 181?

-branadic-

[GreggD]

That yellow cap is metalized poly(something).
It is unlikely to ever go bad. Should last forever.

[TimFox]

That film capacitor is from Electrocube.  The 650B series is still in production, but I don't know the exact meaning of the second line on the package.

[branadic]

Thanks TimFox, no I do know where the capacitors on my LT-LTZ1000 board are coming from. Seems like these capacitors are not available at digikey or any other distributor in germany

-branadic-

[pelule]

Hi branadic,
I have checked the distributors list of Electrocube (www.electrocube.com).
In the US they use:
   FUTURE ELECTRONICS
   www.futureelectronics.com
Future sells also in Germany and offers caps from Electrocube:
http://de.futureelectronics.com/de/Technologies/Product.aspx?ProductID=650D1A473JELECTROCUBE1063365&IM=0
/PeLuLe

[Shock]

1.0 MA = 1uF "A" 0.1% tolerance?
What does it say on the parts list?

[RobK_NL]

1.0 MA = 1uF "A" 0.1% tolerance?
What does it say on the parts list?

"A" denotes 50V working voltage.

[branadic]

As I said, I had some capacitor issue, some big 10.000µF inside was blown, probably due to many years without any power supplied to it. However, I ordered caps to replace all of them. I received some new Nichicon caps today, soldered everything to its place and what to say, the kit is now working again. 

To make it easier for all of you out there planing to replace the caps on a Keithley 181 here is a list of caps that are installed and the Digikey number I replaced it with:

• 1x 10.000µF/25V: 493-1072-ND
• 3x 1.500µF/25V: 493-1833-ND
• 2x 100µF/35V: 493-4501-1-ND
• 2x 6.8µF/25V: 493-13850-1-ND

As you can see, they perfectly fit their place. The 10.000µF is somewhat bigger in diameter, but no problem whatsoever.

-branadic-

[branadic]

The sad thing about this repair is, that Keithley part of Tektronix does not longer support this beautiful piece of gear, neither repair nor calibration. That's why it is now on my privat desk.
I don't have the possibilities to check whether it is still within spec or if it needs some calibration. And I don't have the gear to calibrate it

-branadic-

[Neomys Sapiens]

There are other calibration services that support it over here, such as
https://www.esz-ag.de

Helmut Singer might do it as well.

[branadic]

Thank you for the hint. I'm currently asking myself if I should sell the Nanovoltmeter with its accessories (original manuals and original test cable).

-branadic-

[Johnny10]

I have used mine twice in three years, I wonder the same as you.

Makes you wonder just how long to keep items you aren't using???
I believe my HP3457A which I use daily is accurate enough for my needs.

[branadic]

There are other calibration services that support it over here, such as
https://www.esz-ag.de

Helmut Singer might do it as well.

Contacted both of them and was told that HS uses an external service for calibration and esz doesn't offer their service for privat persons and the last time they calibrated such a unit was in 2009.
However, someone on this board will now help. The good thing is, I have the maintenance kit 1815 and he seems to have most of the necessary equipment to do the calibration. On the other hand he will use the unit for some own measurements. Quid pro quo. 

-branadic-

[Neomys Sapiens]

Yep, ESZ requires a bit of posing.
Make up a 'Meier GbR' or whatever is sufficient. Its about liability toward private end customers. They don't insist on fiscal documentation or anything like that.

[branadic]

Hi,

after the caps have been replaced I found, that the meter shows quiet high noise in 2mV range with input shorted.
The unit was rarely used before and powered of for at least 10 years. Dr. Frank adjusted the ranges with equipment he had at hand, but noise is still high.

I've heard that the matched input FET transistors pair could be the reason, but as mentioned the unit was rarely used and turned off for a while. My imagination is that this matched pair can only break when unit is often used, not when turned off.
I don't have the 100:1 divider, which is less of a problem as I can copy that with some Hammond aluminium die cast, some selected PWW (10k : 100R) and Pomona connectors.

But I don't have the low noise preamp (Ithaco 1201) too, that's needed for troubleshooting compared to 181 service manual.
What I could use for first indication is a Keithley 2002 togehter with a live data plotter software as a noise analyser to locate the source of noise. On the other hand I found the manual of the low noise preamp and the first stage looks like some INA103 + DSO could do the job as well.

What is your suggestion on that and does anyone of you have any experience on what to look at? Thanks for your help.

EDIT: Here is a video I got from CalMachine how the noise can look like on a 181, but I do have intermittent jumps of 500nV in the 2mV range.

-branadic-

[Dr. Frank]

I simply checked  the  noise on your 181A with the 3458A, in its 10mV A.C. range.
This did the job very well , if I remember right, it was about 0.5 mV rms, at the 1000x amplifier output
Frank


P.S.: That measurement was the 'pump-out', as Keithley used to call it, it's nothing else but the chopper spike suppression, which has to be trimmed.
These 520µV, measured easily with the 3458A, were well within  the limits, but that has nothing to do with the amplifier noise, and also a pre-amplifier is not necessary for fault-finding.

The error is definitely located in the pre-amp, as the 2V .. 1kV ranges were ok.

What's really strange is that the excessive and sporadic noise, of 500nV up to several mV is in the 2mV range only (resolves 1nV or 6 digits), but it's not visible at all in the 20mV range.
As the 20mV resolves 10nV, it should be visible on the 2nd last digit, but there's nothing.
And the only difference in the setting is a reed relay, which is switched off (inactive) for the 2mV / 1000x range of the pre-amp.
Therefore, all semiconductors might probably be ok.

Either this reed relay is not ok ( so de-solder it in first place).
Or the whole pre-amp might oscillate at 1000x, which I didn't check with an oscilloscope, only with my 34465As digitizing function, i.e. at 50kHz sampling rate / bandwidth.

[Kleinstein]

The low noise amplifier (Ithaco 1201) is not that special. It is a low noise high impedance amplifier. The input of the K2002 might be about as good. Not many places in the circuit would actually need the high impedance.

There are more parts than the expensive input FETs that can get damaged or turn bad over time.

Quite some test could be done without an extra meter. One of the first and more obvious test would be to check if the noise is coming from the preamplifier or the rest of the instrument. So check for noise in the 2 V range  - this should be reasonably low. One could also measure the preamplifier output  with an external meter, so only use the preamplifier of the 181, but not the ADC.

With the preamplifier, one of the first parts to check would be the supply, including the bootstrapped one.

One could also check the input amplifier before the output buffer.

[VintageNut]

The 181 and 182 are one of the few KE vintage pieces that I have not had the itch to buy. I have two of the 147 which have a 30nV full-scale range. I have a 148 which has a 10 nV range but it is difficult to actually make a stable <10 nV phenomenon in open air.  I have had to measure 20nV drop across a tetrajunction. None of the digital brown boxes can do this measurement.

I would use a KE 260 to calibrate the 181. I have two of the 260 nanovolt source boxes. One I re-capped and checked it against one of my 147 nanovoltmeters.

I checked the claimed <=4 nano ohms of my Ohm Labs 100-0 with the 147 nanovolmeter.

Personally,I would call the 181 a microvoltmeter. You can actually measure a microvolt with it.

Good luck with the troubleshooting.

[Echo88]

I own both a K181 and K148 and while the 148 is indeed more stable and has less noise, i value the K181 for its digital zero button instead of the finicky K148-potentiometer, the faster thermal equilibrium and the silence compared to the buzzing of the K148. With averaging in my software i get about 6nVpp noise with the K181, so it is indeed a nanovoltmeter.

But its really an excursion in the world of thermal stabilization, patience and error source avoidance.   

[VintageNut]

I own both a K181 and K148 and while the 148 is indeed more stable and has less noise, i value the K181 for its digital zero button instead of the finicky K148-potentiometer, the faster thermal equilibrium and the silence compared to the buzzing of the K148. With averaging in my software i get about 6nVpp noise with the K181, so it is indeed a nanovoltmeter.

But its really an excursion in the world of thermal stabilization, patience and error source avoidance.   

I agree that it takes patience and knowledge to measure below uV. I have spent days learning how to measure small resistances and small voltages.

Your 6nV p-p noise is only one source of error. There is also the range error and/or offsets and/or nonlinearities.

My point remains. I do not have a compelling reason to acquire a 181 or 182. If I needed to measure small voltages or small resistances for daily work with lots of digits of precision, I would either acquire a KE 1801 or create a similar solution from an EM Electronics A10 or A20. The A10 is what Keithley used as basis for the 1801.

I had occasion to use a KE 2001 + 1801 for a couple of hours. If I could buy one, I would snap it up in a moment.

[branadic]

I own both a K181 and K148 and while the 148 is indeed more stable and has less noise, i value the K181 for its digital zero button instead of the finicky K148-potentiometer, the faster thermal equilibrium and the silence compared to the buzzing of the K148. With averaging in my software i get about 6nVpp noise with the K181, so it is indeed a nanovoltmeter.

But its really an excursion in the world of thermal stabilization, patience and error source avoidance.   

I agree that it takes patience and knowledge to measure below uV. I have spent days learning how to measure small resistances and small voltages.

Your 6nV p-p noise is only one source of error. There is also the range error and/or offsets and/or nonlinearities.

My point remains. I do not have a compelling reason to acquire a 181 or 182. If I needed to measure small voltages or small resistances for daily work with lots of digits of precision, I would either acquire a KE 1801 or create a similar solution from an EM Electronics A10 or A20. The A10 is what Keithley used as basis for the 1801.

I had occasion to use a KE 2001 + 1801 for a couple of hours. If I could buy one, I would snap it up in a moment.

Well, for precise and low noise measurements we do have a Keithley 2182A at work, no problem whatsoever. But the linked video above indicates, that you can make stable and low noise measurements with Keithley 181 as well if everything is in perfect condition.

Keithley 2002 is now warming up to start some poking around to find the source of noise and thus the faulty part.

-branadic-

[Kleinstein]

I simply checked  the  noise on your 181A with the 3458A, in its 10mV A.C. range.
This did the job very well , if I remember right, it was about 0.5 mV rms, at the 1000x amplifier output
Frank


P.S.: That measurement was the 'pump-out', as Keithley used to call it, it's nothing else but the chopper spike suppression, which has to be trimmed.
These 520µV, measured easily with the 3458A, were well within  the limits, but that has nothing to do with the amplifier noise, and also a pre-amplifier is not necessary for fault-finding.

The error is definitely located in the pre-amp, as the 2V .. 1kV ranges were ok.

What's really strange is that the excessive and sporadic noise, of 500nV up to several mV is in the 2mV range only (resolves 1nV or 6 digits), but it's not visible at all in the 20mV range.
As the 20mV resolves 10nV, it should be visible on the 2nd last digit, but there's nothing.
And the only difference in the setting is a reed relay, which is switched off (inactive) for the 2mV / 1000x range of the pre-amp.
Therefore, all semiconductors might probably be ok.

Either this reed relay is not ok ( so de-solder it in first place).
Or the whole pre-amp might oscillate at 1000x, which I didn't check with an oscilloscope, only with my 34465As digitizing function, i.e. at 50kHz sampling rate / bandwidth.

This description already strongly points towards the relay used to switch between the 2 mV and 20 mV ranges.  The pot R409 might be a potential problem too.
The good thing it, the input FETs are very likely not the culprits.

[Echo88]

Indeed the modules from E.M. Electronics leave the standard nanovoltmeters in the dust. A pity that they are pretty much unobtanium on the used market. Seems theres only a real small market for excellent nanovoltmeters.

[branadic]

I will work through the nanovolt preamp step by step: I have the newer version with minor changes within the circuit. This starts with U404 which is a LF351 with output at pin 6 instead of the original pin 7.

1. Checks of Preamp Zero (pin 6 of LF351N with 2mV range selected and input shorted)
As found: ++7.964V --> this is way out of spec given a maximum of +/-2V

Time to desolder R423 and R424, insert some 20k pot, find the new ratio where pin 6 is within +/-2V, measure it and replace R423 and R424 with the new ratio. I did it the fast way and paralleled some decade resistor box to R423.

original ratio: R423:R424 = 9.5458k : 8.5707k --> +7.964V
new ratio: R423:R424 = 9.5458k|| 8.2k : 8.5707k --> +425mV or R423:R424 = 9.5458k|| 6.8k : 8.5707k --> -356mV

So we need R424 = 2* R423 to get close to zero. So something like 6.8k and 12k could do the job quite well.

[branadic]

Either this reed relay is not ok ( so de-solder it in first place).
Or the whole pre-amp might oscillate at 1000x, which I didn't check with an oscilloscope, only with my 34465As digitizing function, i.e. at 50kHz sampling rate / bandwidth.

Meanwhile I'm sure it has nothing to do with the relay, which is only active in 20mV range and off in 2mV and 200mV range. I guess I need to power the preamp by a lab power supply to make sure it's not a power supply issue.
By the way, the preamp is based on of the shelf parts with original manufacturer and not Keithley labeling. 

-branadic-

[Alex Nikitin]

I'm slowly restoring my Keithley 181. I've replaced all electrolytic caps as some of them already failed, and Volts ranges are working fine with the exception of 20V range which had a very high offset drift (about 1mV over 20 min after switching to the range). I've replaced the relay K302 with a small 5V REMtech reed relay I had in a draw, the drift now 10 times better (under 100uV over 20 or so min) but still quite significant    . I am waiting now for some low thermal Pickering reed relays, hopefully these will resolve the problem more or less completely. On the mV ranges I have a sort of a popcorn noise with random shifts so have to investigate the preamp section later. The ADC in the K181 is quite interesting, simple 6.5 digits design with 10ppm linearity and reasonably low noise (below 5ppm p-p on 2V range). The reference is LM399. One interesting feature is an isolated (referenced to the GPIB ground) analogue output with a 12bit DAC, which can be set to show the top 3.5 digits or the bottom 3.5 digits.

Cheers

Alex

[branadic]

I have a similar problem on the mV range. I probed around today and the frist thing broken is in the bootstrap part of the nanovolt preamp. Either the 6.4V zener diode VR404 = 1N4571 or the fet CR406 = J505 is broken. I could find 1N4571 at Mouser, but any advise for replacement of the fet?

-branadic-

[BU508A]

Hi branadic,

I have a similar problem on the mV range. I probed around today and the frist thing broken is in the bootstrap part of the nanovolt preamp. Either the 6.4V zener diode VR404 = 1N4571 or the fet CR406 = J505 is broken. I could find 1N4571 at Mouser, but any advise for replacement of the fet?

How about the LS845?
http://www.linearsystems.com/product-search-result.html?type=products&partnumber=LS845

This site is saying, the LS845 could be a replacement for the J505
http://www.allxref.com/search.htm?part=J505

Update/edit:
possible source in Germany:
http://ib-fluck.de/produktlinien/

HTH

Andreas

[Alex Nikitin]

I have a similar problem on the mV range. I probed around today and the frist thing broken is in the bootstrap part of the nanovolt preamp. Either the 6.4V zener diode VR404 = 1N4571 or the fet CR406 = J505 is broken. I could find 1N4571 at Mouser, but any advise for replacement of the fet?

-branadic-

J505 is specified at 1mA, so most JFETs with ~1mA current will do, of a JFET with a higher current and a resistor to get 1mA.  VR404 can be replaced by 6.9V LM329 I suppose.

Cheers

Alex

[branadic]

I found some J505 and ordered them. Replacing the temperature compensated zener 1N4571 (6.4V, 1mA, 50ppm/K) by LM329 needs some further tweeking of the resitors too, as the output of the bootstrap circuit should deliver 10V ±10%.
If someone on this board has a few 1N4571(A)/1N4572(A)/1N4573(A)/1N4574(A) laying around I would be happy to buy them. Otherwise I have to order at Mouser. They stock them fabricated by Microsemi.

-branadic-

[Alex Nikitin]

I found some J505 and ordered them. Replacing the temperature compensated zener 1N4571 (6.4V, 1mA, 50ppm/K) by LM329 needs some further tweeking of the resitors too, as the output of the bootstrap circuit should deliver 10V ±10%.
If someone on this board has a few 1N4571(A)/1N4572(A)/1N4573(A)/1N4574(A) laying around I would be happy to buy them. Otherwise I have to order at Mouser. They stock them fabricated by Microsemi.

-branadic-

With 7.0V from LM329 you'll get about +/-11V without changing the resistors, so should be OK.

Cheers

Alex

[branadic]

Received some 1N4571A and NOS J505 that I replaced today, giving 9.77V and -9.70V, which is well within 10V +/-10%. This replacement also requires an offset adjustment of U404, thus a replacement of R423 and R424. Currently I have a 20k pot at the Berg pins installed and observe, if I still have large jumps.

-branadic-

[branadic]

I have put everything together, even though the board isn't cleaned yet. I've put a short directly at the screw terminals of the nanovolt preamp input under the shielding and put everything together with the 20k pot installed back and closed the chassis. The meter is now running a while and all jumps in reading have vanished. Instead the display is running up and down a view nanovolts around an offset of a few 10nV. Looks like a decent repair by now 

The behaviour is now similar to what is shown here:

https://xdevs.com/cm/RandomPics/K181_Cable/20171009_194206.mp4

-branadic-

[branadic]

It's done.
After binning some 9k and 11k 1% resistors today I found a good combination to replace the temporary 20k pot at the Berg pins. Pin 6 of U404 now delivers an offset close to zero in the 2mV range, which is well below the required +/-2V.
I carefully cleaned the whole preamp with Aceton and IPA with some cotton buds, removed the temporary short at the preamp input, reconnected the cable at the input, installed the shielding box and put everything back together.

Is someone interested in a working Keithley Model 181 Digital Nanovoltmeter with Model 1506 input cable, Model 1815 Maintenance Kit and another set of spare parts for CR406, CR407 (2x J505) and VR403, VR404 (2x 1N4571A)?

-branadic-

[Neomys Sapiens]

Is some interested in a working Keithley Model 181 Digital Nanovoltmeter with Model 1506 input cable, Model 1815 Maintenance Kit and another set of spare parts for CR406, CR407 (2x J505) and VR403, VR404 (2x 1N4571A)?

-branadic-

Hmmm, what's your asking?

[Alex Nikitin]

At last I've had a chance to fix my Keithley 181. At the end the main reason for the excessive noise turned out to be an incorrect resistors ratio in the offset compensation network. It looks like at some point input JFETs were replaced and the offset resistors were not changed. After re-setting the offset everything is quiet, here is the 100s trace from the analogue output , a shorting plug on the input and zero ON (without zeroing the offset is about 25nV). 4nV per division vertical scale.

Cheers

Alex

[HighVoltage]

I bought a Keithley 181 for a low price and it arrived dead with lots of loose parts inside.
It must have been thrown around during shipping.

I have not started the trouble shooting. Probably best to start with the PSU and electrolytics.
The analog output binding posts are broken.
And a loose brass nut was inside that does not have a place of origin as far as I have seen, hmmm...

Interestingly, the loose bindings posts have no threads on them.
But the two loose cables have lugs on them.
Any advise before I start?

Here are some pictures:

[Shock]

A good inspection to check safety and see if any physical problems are going to cause more damage. If it's been worked on, what may have been changed. Then confirm what the no power issue is.

I generally work on the fault before reconditioning anything unless it's dirt or bad controls. These type of models are known for stuck button issues and the 170 series is dirty ganged switches. Anyway the service manual is the way to go as it should have a test/check procedure.

Any tantalums and the electrolytics are probably mandatory replacements if it's a keeper. 

[Neomys Sapiens]

That red jack (not a binding post) doesn't seem original. The original had binding posts and the brass nut would fit there. That other part I cannot recognize.

[lowimpedance]

Those leads go to standard looking 4mm binding posts (gold plated ?). Any way google images turn up a picture on TiN's archive showing an inside photo which includes the binding posts on the rear.
As stated above the only original bit appears to be the nut  .

[tooki]

I bought a Keithley 181 for a low price and it arrived dead with lots of loose parts inside.
It must have been thrown around during shipping.

I have not started the trouble shooting. Probably best to start with the PSU and electrolytics.
The analog output binding posts are broken.
And a loose brass nut was inside that does not have a place of origin as far as I have seen, hmmm...

Interestingly, the loose bindings posts have no threads on them.
But the two loose cables have lugs on them.
Any advise before I start?

Here are some pictures:

This model shipped with proper binding posts (Polklemmen) on the back, not the press-in banana jacks (Bananenbuchsen) shown here. So definitely an aftermarket modification.

[HighVoltage]

Thank you all for the feedback.

It is powering ON now.
The mains switch was broken and I just fixed it.
The instrument is working now and not far off in the 10V calibration.

Ok, now I will be looking for some fitting binding posts.

Any idea of where to get a fitting shorting block for the zero calibration?

[HighVoltage]

Pomona 3770 binding posts fit perfectly
Instrument is running perfectly now.

[Dr. Frank]

Hello,
maybe you also need these Amphenol input connectors ?
I bought them quite cheap at TME, silver surface only, but working.
That's 97-3106A-16-11P and 97-3057-1008-1
You might use one of those to build a zero connector.

For shorting, simply get a thicker copper wire, from a stiff power cable, isolate, bend and plug it into the holes. Add a small, thermally isolated box around, ready.

Frank

[HighVoltage]

Hello,
maybe you also need these Amphenol input connectors ?
I bought them quite cheap at TME, silver surface only, but working.
That's 97-3106A-16-11P and 97-3057-1008-1
You might use one of those to build a zero connector.

For shorting, simply get a thicker copper wire, from a stiff power cable, isolate, bend and plug it into the holes. Add a small, thermally isolated box around, ready.

Frank

Thank you, Frank.

I made my shorting block from one of these Amphenol connectors with a little custom aluminium cap fitting the end of the connector for shielding purpose.

The copper wire I used is 99.999 % pure copper at 2.7 mm diameter, fitting snugly in to the openings of the Amphenol connector and then soldered tight.

[Kosmic]

Nice work, look really good 

I also need to make one of those for my 181.

[Kosmic]

BTW if the volt ranges are flaky it's probably the 2 big relays being a little bit dirty. Normally I clean them with some deoxit and a piece of paper.

I fixed 3 x Keithley 181 and they all had that problem.

[HighVoltage]

BTW if the volt ranges are flaky it's probably the 2 big relays being a little bit dirty. Normally I clean them with some deoxit and a piece of paper.

I fixed 3 x Keithley 181 and they all had that problem.

Thanks for this info.
So far my 181 performs very well in the V and mV rage.

[HighVoltage]

For Christmas I got one more Keithley 181 from a friend .... he gave up on the repair.

Broken and taken apart, all in one big cardboard box.
Many plastic parts are broken.
Allegedly showed garbage on the display.

I am looking forward to this repair.

[guenthert]

[..]
I've put a short directly at the screw terminals of the nanovolt preamp input under the shielding
[..]

  I didn't think of that.  Thanks for the hint.  With that short in place, readings in the most sensitive range (2mV at 6.5 digit, i.e. 1nV, resolution) with default filtering (filter 1 on, filter 2 off, damping off, are the defaults I believe) I get some 20nVpp.  This is within spec as I read it and good enough for my purpose.  Earlier, with a short using a simple copper cable on the external input, I saw noise in excess of 140nVpp, but that seems to have been largely varying thermal EMF.  Now I do need to get a proper connector ...

[HighVoltage]

The opportunity presented itself, to buy one more of these great Keithley 181 and it arrived today. This one also needs repair and I took the cover off.

To my total surprise, this one has a complete shielding installed, all the way around the case, including the top. I have never seen this shielding in any of the pictures from other 181s.

Does anyone know, if this was standard in the later versions or was this shielding always an option from Keithley?

[MikeK]

Is it electrically connected to the rest of the shielding?  My Keithley 175 has only a bottom shielding, but it is electrically connected to ground via a spring.

[MikeK]

Also, the factory would have used the same thin material for both shields.

[HighVoltage]

Is it electrically connected to the rest of the shielding?  My Keithley 175 has only a bottom shielding, but it is electrically connected to ground via a spring.

Yes.
The bottom shield is the heavy aluminum that is grounded with bolts to the back panel.
The top shield is grounded trough the spring loaded hook that is mounted together with the green ground cable to the back panel.
(See pictures above)

From the looks, this is definitely from the factory.
Maybe this was a special edition of the 181 for usage in areas with high frequency pollution?

[Kleinstein]

There should be 2 layers of shielding: one outer connected to PE and one inner shield connected to the input low side (or maybe a separate guard terminal - but AFAIK not with this meter). The extra to shield should be connected to the outer one.  It looks reasonable well made, though I don't see a ground connection.

Different materials make sense, as the innter shield needs to hold up, while the top outer shield can be glued to the case. So only a thin foil is sufficient. I would not expect the extra top shield to be very important in most cases, though it can help sometimes. I don't think it would really help much with RF, more like a thing against mains hum.

[HighVoltage]

In addition to the extra heavy shielding, I also found this

- Complete front panel shielding with an aluminum plate.
- Ferrite core at the output terminals
- Ferrite core at the input terminals
- Mains input filter

Maybe this was a military edition?

 

[guenthert]

[..]
Does anyone know, if this was standard in the later versions or was this shielding always an option from Keithley?

    Forgot now where, but I've seen hints at models with additional shield before.  I thought it was the other way 'round, early versions came with a shield which has been removed from later ones (cost reducing measurement).

[HighVoltage]

This shielded version is from 1987, the two un-shielded units I have, are about 6 years older.

Now I have:
- All electrolytics replaced.
- Schaffner input filter (time bomb) replaced.
All works perfect.

With my self built short installed it sits stable at 0 to 1 nV without filter or damping.
Just very impressive after so many years and without any calibration.

[Dr. Frank]

Hello HighVoltage,
are you sure that you have connected everything properly?
Does your 181 now measure correctly a 1mV input?

As the noise is < 30nV_pp, it can't stay on a 1nV display especially w/o any filtering.
Frank

[HighVoltage]

Hello Frank

After installing the short, it starts at around 100-200 nV on the screen and then slowly moves down to zero within 10 to 15 min.

Once reached zero it switches between 0 - 1 - 2 nV and once in a while 3 or 4 nV then back to 1 nV, where it stays most the time.
This is watched from a 3 m distance.
I tried to take a video but as soon as I get near the instrument it jumps to 10 nV or higher and needs a few minutes to settle again.

The 1 mV and 10mV range seem to be ok, measured with the Advantest R6144 as source.
I know, not the best connection with the alligator clips.

[alm]

This shielded version is from 1987, the two un-shielded units I have, are about 6 years older.

I have two units, a shielded one from ~1989 (S/N 391xxx) with a yellow label "RFI SHIELDING INSTALLED" with Schaffner mains filter, and a 1984 model (S/N 267xxx) without shield or mains filter. I couldn't find anything in the late-eighties catalogs and price lists about an optional shielding option, so my guess was it was some later development either to improve performance or deal with the increasing number of digital systems like computers. Does your shielded unit have the label?

[HighVoltage]

Very interesting, alm.

Yes, my shielded unit also had that sticker. And it seems this sticker was original from Keithley. First I thought that the previous owner had this sticker applied.

Since we can not find any reference to this version of the 181 in any of the Keithley catalogs, I would think that this was a special batch for a special customer. It must have cost a lot to get this unit modified and someone was willing to pay that price.

What kind of noise reading do you get on yours with an installed short?

[alm]

It must have been a large batch given that we three units with shielding out of 6? Maybe something like TEMPEST compliance (emission security)? Can't have the Soviets know the voltage of our JJR 😂

Do we know of late eighties units without the shielding? It could still be an improvement to all later units, right?

Actually, I checked my paper service manual, and in the addendum section in the back are the shielding changes described as an improvement, so not a modification for a specific customer. So probably all unit after about 1985 will have the shielding. I attached a scan of the two pages.

I haven't found or made a proper short yet, so I don't know what my units would read.

[HighVoltage]

That PDF was a nice find.
* Keithley 181 shielding addendum.pdf

Based on this it seems this shielding was a later addition to all 181 and not just a special batch.

Interestingly, I have been watching the 181 on ebay for many years and never had seen one before with this yellow shielding sticker.
 

[HighVoltage]

BTW, most dates on chips of my shielded unit are from ~1987.
But S/N starts with 407xxx

Yours is from ~1989 (S/N 391xxx)
Weird!

[alm]

The digital board had mostly 1987 date codes, and some older. But the NatSemi ICs on the nanovolt preamp board had what I figured were 1989 date codes (/937). But maybe that's wrong, or maybe the preamp board in this unit has been replaced.

[Swainster]

S/N 402xxx checking in here. IC date codes mostly 87 or ealier and yellow RFI label and shielding present. This is actually the first time I've had the lid off... the schaffner inlet is looking a bit hairy.

[BU508A]

Very interesting, alm.

Yes, my shielded unit also had that sticker. And it seems this sticker was original from Keithley. First I thought that the previous owner had this sticker applied.

Since we can not find any reference to this version of the 181 in any of the Keithley catalogs, I would think that this was a special batch for a special customer. It must have cost a lot to get this unit modified and someone was willing to pay that price.

What kind of noise reading do you get on yours with an installed short?

I've bought two weeks ago two 181 with that very same sticker on it. See pictures below (pictures are from the seller).

[HighVoltage]

Nice find, congratulations, these are very special units with that sticker.
Most likely you found them in the USA?

Would be nice to find out, how stable yours are with a short installed.

[BU508A]

Nice find, congratulations, these are very special units.
Most likely you found the in the USA?

Would be nice to find out, how stable yours are with a short installed.

Thanks. 

I've found them in Switzerland, not USA.
Have bought the same connector as you have but I need to assemble the short and the Aluminium shield.
Will update here then.
I'll keep only one of the units, the other one is for an EEVBlog forum fellow.