Keithley 181 - Nanovoltmeter

[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.