Keithley 616 Electrometer Cables

[JJ]

Hello, I'm new to the forum. I've read through all the existing threads on triax cables and connectors. I was wondering if anyone has tried using Trompeter PL74-7 (2 lug connector, TRB series) triax connector with the recommended TRC-50-2 triax cable for the input to the Keithley 616. The PL74-7 appears identical in specs to the PL74-24A. The connector ($31 at Mouser) and cable is readily available (albeit the cable in large quantities).

[alm]

My guess is that it will be similar to Belden 9222, which has been discussed here. So mediocre performance with regarding to triboelectric effect due to lack of lubrication between inner conductor and inner shield, compared to low noise cables such as the Keithley cables. It will probably perform okay as long as the cable is fixed in one position.

[JJ]

OK Alm, understood. Thanks

[graybeard]

Trompeter makes the best triax connectors.   They are the only ones I use for many test stations I have designed that use both Keithley and Keysight Source-Measure units and Keithley triax switch matrices.  If you need new professionally made cables Tropmeter has a division that will make the cables to any length you need for about 1/2 the price you pay from Keithley or Keysight.

Avoid triax connectors from Pamona Electronics.  I used to use them, but have had many failures since they moved their production to China.

[JJ]

Thanks for the heads up! I had also found that the Pomona 5655056 connector with Belden 9222 Triax cable would work as well. So, I'll take your advice and stick with the Trompeter PL74-7 connector and the TRC-50-2 Triax cable.

[neverendingstudent]

As an update to anyone who finds this thread looking for an electrometer / triax cable that won't break the bank, I found a seller on eBay that sells them - they are out of China and I'm pretty sure they are making them.  No idea about if the materials or ratings are truth or lie, but the connector at least looks right.

https://www.ebay.com/itm/286274641488 - right there is a 2-lug triax connector for the old brown Keithley electrometers with copper aligator clip ends for anywhere between $22 and $39 depending on how long a cable you want: 2ft to 9ft.

Seller name is NDT2009.

I just recently won an auction for a Keithley 617 Electrometer and will be getting one of these cables so I can test it out.  We'll see how it goes.

Edit 2025-Apr-8: as a follow-on related to this and other Chinese-origin low-cost Electrometer cables, please see this thread: https://www.eevblog.com/forum/buysellwanted/looking-for-keithleytektronix-triax-cable-for-486-(237-alg-2)/
I asked about it in that thread and other users are telling me that it will likely be a crap cable, to at best expect something that only work with no movement on the cable, and at worst possibly fake materials and leads that aren't even connected.  I'll see what I can test when I have the cable in hand.

[neverendingstudent]

Update 2025-Apr-21:

Cable has arrived.  Looks like I won't be able to make *meaningful* analysis of it just yet as I have confirmed that the Electrometer itself is in need of repair and does not take a reading.  This was not unexpected as the Electrometer itself was a gamble as it was listed for parts/repair - but I got it for just under $170 so I figure that's acceptable.  It will be a project meter.

I have at least confirmed that the cable has the correct wiring - center to red, middle to black, outer lug to guard.  I should be able to get resistance readings on each for what that may be worth.  Going to have to figure out if and how I can test the 'low noise' characterization without having a working Electrometer to use it.

I can at least report resistance values.  For reference, these were taken with a slightly past cal 34465A (which has been pretty spot-on against my sources) using non-abused Probemaster 8017S 48" leads.  The leads were touched together, pinched firmly and gave ~60 milliohms for the whole loop of 2 leads, then the meter value nulled for a compensation of 60.53 milliohms.

Guard/green clip to lug body: 56 milliohms with about half a milliohm of fluctuation.
Middle/black clip to middle triax ring: 55 milliohms, about same flux, had 1 value as low as 52 milliohms but there is variance with position and pressure given I'm touching a lead to a surface instead of using a more stable contact.
Center/red clip to center triax pin: 444 milliohms.  That's a pretty big increase compared to the other connections.

The measured resistance from the tip of a clip to the back when making best possible contact came in at under 1 milliohm, so I do think it is at least flashed with real copper or a copper alloy.

The alligator clips are not reactive to magnets except at the pivot where the spring is.  The springs in the clips are pretty stiff - more so than I prefer for my leads.

The wires are soldered into the clips pretty well.  The center/red wire (and ONLY that wire) appears to have some heat shrink or other additional material placed in between the outside of the wire and the inside of the wire-accepting portion of the alligator clip.  An attempt to improve signal quality?  The red wire is thicker than the others as well.

I have scraped at the rear surface of the guard clip and under the yellow/copper color there is silver color metal.  Not strongly enough magnetically interactive to have any perceptible 'stick' to a magnet, but clearly not copper underneath the copper/copper alloy coating or flashing.

I have not gone so far as to attempt disassembly of the triax connector head since I've made my last purchases out of China that can arrive before the De Minimis rule is ended and everything becomes prohibitively expensive - I still want to use this if I can.  Even if it's crap, if I can use it to tell the difference between 'yep, the Electrometer doesn't read anything at all' and 'ok, the electrometer will recognize a voltage source with at least some minimal accuracy'.

Attaching pictures.
Hmm.  Was going to attach more, but max 8MB.  Limited to 2.... resolving with Imgur link: https://imgur.com/a/eAsZ0im

EDIT: a few minutes later.  Kind of confused now about my Electrometer (Keithley 617).  I DO get readings, moderately accurate on voltage and yeah-there's-a-value-there plus/minus 20% of real on ohms, at least *low value* ohms... if I measure with red/center and green/guard instead of red/center and black/low.  I have double and triple checked that the cable IS wired correctly.  I have tried switching the GUARD ON/OFF switch on the back of the K617 to either position with no change.  Weird.  I'm wondering if someone modified the internal wiring to use normal 2-wire BNC instead of triax.

[MathWizard]

So how did the Electrometer and ebay triax cables work out after ? I see some still for sale, and no way would I pay for any original cables, they cost up near what the old electrometers themselves cost, and yeah I don't much trust decades old cables anyways.