Triaxial connector for Keithley Electrometer

[inse]

I am trying to refurbish a vintage Keithley 601 Electrometer from the 60‘s.
It is a instrument which measures wide ranges of dc voltages, currents, resistances and charges. Therefore it has an extremely high input resistance and uses a triaxial input connector “Dage Electric Co. 33050-1“.
Unfortunately the meter comes without input cables.
Original cables are traded at ridiculous prices so I was hoping to be able to make one myself.
The manufacturer mentioned in the service manual does not exist anymore and the search for the part number does not lead anywhere.
Google and eBay return different looking types of connectors  tagged “triaxial“.
Is there a standard like BNC for triaxial connectors?
At least the one used here reminds of BNC…

[coppercone2]

there is 2 lug and 3 lug traxial. 3 lug is cheaper but you probobly have two lug.

they look like BNC connectors.

https://www.eevblog.com/forum/projects/two-vs-three-lug-triaxial-connectors/

[inse]

Thanks, maybe the three lug variant makes an affordable combination…

[coppercone2]

I think its more modern but again your likely to come into problems with say test probes or test sets if you actually go further with the electrometer that need to be retrofitted if you go for a 3. Alot of the old stuff is 2.

I will say, god damn electrometers are expensive! I really slowed down my low current research because its expensive, finicky and quite frankly a little boring.

At low voltage something might be happening. Because you know the impedance can be low.

At low currents nothing is ever happening, because for something to happen at that current the power supply would be totally ridiculous!

Maybe if you find particles interesting then its a different story... because I can just put a bigger battery for everything else!

IMO electrometer to typical electronics is like fog in a water pipe!!! Maybe interesting to see leakage of input stages or something, and high resistance measurement is useful, but I would say is quite limited.


But, if you are planning to use the electrometer for more electrometry things like particles, materials testing, etc... then you might run into problems down the line connecting things if you 'modernize' the connector (although all the electrometer connected devices are usually really fucking expensive too. Like material holder or whatever). Prepare to modernize more stuff. If its just for rare measurements that come across in your circuits your probobly fine to replace it with 3 lug so long you get a probe (and more then likely you will end up needing a single unterminated cable that you can solder into the circuit in addition to the probe)...

if you plan on building your own accessories and not using expensive old commercial ones, then just stick to 3 lug.

i.e. https://www.axiomtest.com/Meters/Accessories-and-Other/Keithley/8009/Resistivity-Test-Fixture/

https://www.testequipmenthq.com/product/keithley-8002a

I bet you will come across something like that with 2 lugs when you decide to mod for 3!!!

[inse]

I have a weakness for vintage (though I hate the term) equipment like Nixie counters or VTVM and all kinds of strange devices.
I put them back into service (or at least try to) and have my fun with it, keep some parts and give away others.
I once had a HP425 microvolt microammeter, it had an amazing opto-mechanical chopper in the input section.

[Jwillis]

Try Triaxial/Twinaxial . Cinch Connectivity Solutions/ Trompeter is a little cheaper. Example As far as I know there are two variants TRS and TRB

[donlisms]

I think the think with twinaxial is that the two conductors are parallel inside the shield, and with triaxial, they are concentric.  The connectors look the same at first glance, but expect different kinds of cables.  It matters because you wouldn't be able to use the same guarding with twinax.

It is, however, possible to put a triaxial cable into a twinaxial connector with certain effort and... compromises.

[inse]

lemo ffa.0s.650 looks neat as well and I’ve seen an offer with attached teflon triax cable.
Edit: I found this thread, will dig into it
https://www.eevblog.com/forum/metrology/my-triax-cable-shielded-box-project-for-keithley-instruments/

[alm]

Cables for two lug triax connectors are offered occasionally on eBay, but something like $50-75 for a cable is a good price. For example Keithley 6011. There are also cheaper cables from Pomona and the blue noname cables from China, but when doing low current measurements with those, expect the measurements to change every time you blink due to triboelectric and piezoelectric effects. The cable they use is optimized for RF, not low current. The Keithley Low Level Measurements handbook is a great resource about this.

There is also the hack of filing off one lug so the connector becomes a 1-lug triax connector that will fit both 2-lug and 3-lug cables, but contact can be intermittent. In general, expect everything about triax to be expensive.

[coppercone2]

yes there is two realm of triax cable

1) high speed for data etc (i.e. the superbowl stadiums happen to use triaxial cabling for camera/networking). It has its benefits. 4GHz +
2) stable kind for electrometry

[inse]

Thanks for the clarification.
I also came across the Pomona cable and had a look at the specs of the cable - PVC isolation and PE dielectric, so not too fancy.
I hope to get at least the same performance from the all Teflon cable I got hold of on ebay for 50 euro bucks with fancy LEMO triaxial connectors already mounted.
Possibly this is camera stuff (though I wonder why they should use all Teflon wiring)- let’s see how it works out…
I will share a photo when it’s adapted to the meter and measurement experience once the meter is back in service.
Another roadblock to be removed is the power supply- actually nine batteries are needed, five of them now obsolete mercury types…

[alm]

Polyethylene dielectric is not a problem, the Keithley SC-22 cable also uses PE dieletric. The main advantage of PTFE is lower leakage (used in the Keithley SC-181 cable). What makes a cable low noise is graphite lubrication between the inner dielectric and inner shield to reduce friction when the cable moves. See this post by a Keithley employee.

[coppercone2]

I think we need a cable that uses ceramic bearing slides

[Swake]

Good an simple explanation of triaxial wiring and information about how to connect 3 wires triaxial to 2 wires coaxial can be found in the pages 21; 22 and 23 of "The Parametric Measurement Handbook" from Agilent.
This PDF is over 10MB, If you can't find it online PM me your email.

Attached is another doc about Low Level Measurements from Keithley.

[inse]

In the meantime I got the meter back in working condition.
A little bit of metalwork was necessary to fit the LEMO connector to the front panel while keeping full fallback compatibility.
I made some coarse tests with the meter, measuring down to 100pA and up to 100MOhm.
So far the input wiring seems to be fine, resistance measurement going into OL even at 10^13Ohms range :-)
Let’s see how far I can verify the functionality, though I doubt I will get anywhere close to the limits.
Now it’s time to dig into the document provided by Swake (thanks)
In the coulomb mode you seem to see the ants farting on the other side of the globe…