Looking for Keithley/Tektronix triax cable for 486 (237-ALG-2) - Page 1
Forum section: electronics / general

author: ZGoode
author_rank: Frequent Contributor
timestamp: February 20, 2025, 11:18:53 pm
content: As the title says, I am looking for the 237-ALG-2 cable for use with my Keithly 486. I'm not sure if the usual chinese sources should be trusted when it comes to a low noise cable like this one. If anyone has one they wouldn't mind parting with I would appreciate it

author: jonpaul
author_rank: Super Contributor
timestamp: February 21, 2025, 01:15:53 pm
content: Hello I have the geuine Pomona 4725 triax, 36" long, 2 lug like new.I am unsure if this is the exact Keithley equivalent.https://www.pomonaelectronics.com/sites/default/files/d4725_5342_1_01_0.pdfAm in USA, open to a swap or offer.Please use a PM.Jon

author: Hydron
author_rank: Super Contributor
timestamp: February 21, 2025, 01:45:54 pm
content: In this case the OP needs the more-standard 3-lug version. I think the Pomona triax is not low-noise either.The best way might be to pickup a low-noise 3-lug to 3-lug cable off ebay (generally cheaper than the 237-ALG-2), chop one end off and add the alligator clips (leaving the guard braid + insulation running all the way up to the 'HI' clip - just strip a bit off to make the guard connection at the breakout). If you start with a long enough cable you can make 2 leads.Huber&Suhner and Lemo also do low-noise cable but these days I'd pay more money to not need to put triax connectors on where possible, making cables is a chore and triax makes it worse.

author: ZGoode
author_rank: Frequent Contributor
timestamp: February 22, 2025, 06:03:10 am
content: Hi Jon,Thanks for the offer, but Hydron is right, the 237-alg-2 is a three lug as far as I'm aware and is the "low noise" version. I'm not sure if there is a directly compatible 3rd party cable. I think the only one is the one direct from Keithly/Tek (please correct me if I'm wrong here). I'm not normally against crimping my own coax cable, I've done that a fair bit for all kinds of custom LCR fixtures and for my spectrum analyzer, but I've never done triax cable.

author: Hydron
author_rank: Super Contributor
timestamp: February 22, 2025, 01:16:05 pm
content: I'm suggesting getting a cable with a triax connector on both ends, cutting it in half and adding the clips. Most of the triax connectors I've seen (trompeter, AliExpress specials) have been screw/solder rather than crimp btw.

author: jogri
author_rank: Frequent Contributor
timestamp: February 22, 2025, 02:51:31 pm
content: Wouldn't recommend that option, i made my fair amount of Triax to banana cables at this point and honestly, they are all significantly worse than Keithley cables when it comes to resistance between the conductors. Probably not ideal or worth it if you're after low noise, and genuine low noise triax cables with two connectors aren't that much cheaper than Triax to banana ones to begin with. Just a waste of a perfectly good cable imo

author: Hydron
author_rank: Super Contributor
timestamp: February 23, 2025, 11:06:38 am
content: I've seen a few original Keithley cables recently at decent prices, which is why I floated it as an option.Personally I use a shielded box with a triax connector on it and some clips inside; now that I have a second SMU and an electrometer I'm going to build an upgraded box with a lot more connection options.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 23, 2025, 11:10:42 am
content: I made a few Triax cables to banana myself and after a while you know how to make them right.Here was my starting point a few years ago:https://www.eevblog.com/forum/metrology/my-triax-cable-shielded-box-project-for-keithley-instruments/

author: neverendingstudent
author_rank: Regular Contributor
timestamp: April 08, 2025, 01:52:19 am
content: So, this is exactly what you asked to NOT get, but I wanted to mention it here if for no other reason than to ask if anyone else has tried one of these cables.https://www.ebay.com/itm/286274641488- the eBay seller's name is NDT2009 in case that link goes bad however far in the future.I'm chiming in because I just got a Keithley 617 on eBay for cheap, but as is common, no triax cable with it. Since I don't even know for certain if the Electrometer I just bought even works, I am loathe to pay as much (or more) for the cable as I did for the instrument. I'll likely be getting one of those el-cheapo cables to at least try it - ~$25 is a fairly low risk of loss if the cable is crap.So, has anyone bought one of these cables I linked before? Or bought from that seller? I'm curious if they are any good. The specs they claim in the pictured / translated spec sheet *seem* ok, although I'm not seeing beryllium copper or tellurium copper clip options, the 'better' ones give options between pure copper or gold-plated copper.What do you all think? Any reasonable chance they will work acceptably if I'm taking measurements in a shielded box? Or am I more likely to get crap readings when trying to get down to nanovolts / picoamps and below?

author: Hydron
author_rank: Super Contributor
timestamp: April 08, 2025, 04:44:39 am
content: I don't trust those terminations (would like to see guard braid continue down the "HI" connection to close to the clip) and the cable will NOT be low-noise, so cannot be moved during measurement. The cable itself (during static use) and triax connector could be pretty good by themselves though (My cheap triax cables and connectors test pretty good).

author: neverendingstudent
author_rank: Regular Contributor
timestamp: April 08, 2025, 05:14:34 am
content: Quote from: Hydron on April 08, 2025, 04:44:39 amThank you for the information. I'll be curious to see how it performs.For what it's worth, the diagram at least *claims* the guard continues down the center cable, and the center cable does look thicker than ground and guard in their pictures.I'd probably have to destructively deconstruct it to know for sure.

author: voltsandjolts
author_rank: Unknown
timestamp: April 08, 2025, 07:51:35 am
content: Just a few weeks ago I bought one of those yellow "low noise" chinese triax cables, wasn't cheap at about £90 but it was total crap.Three alligator clips at the end, one of which wasn't connected to anything.Pure copper alligrator clips were some sort of non-conductive copper colour shit.Luckily the Ali-x seller refunded after my complaint - they likely know they are selling crap.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 08, 2025, 12:25:45 pm
content: Same here.These are NOT quality cables by any standard!

author: neverendingstudent
author_rank: Regular Contributor
timestamp: April 08, 2025, 02:42:45 pm
content: Unfortunate. But then, not really surprising. I knew going into this that there are specific categories that I just should not expect to get cheap out of China and also expect to get something good, or even 'good enough'.I'll see what testing and characterization I can do on the cable when I have it in hand. Thank you everyone.

author: neverendingstudent
author_rank: Regular Contributor
timestamp: April 22, 2025, 05:07:38 am
content: 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. EDIT: er, I'm dumb. This would be the thickness increase due to the extension of the black guard wire sheathing the inner conductor. So long as the cable is made to spec of course.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/eAsZ0imEDIT: 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.

author: Kean
author_rank: Unknown
timestamp: April 23, 2025, 04:22:20 am
content: The resistance of the conductors isn't so critical compared to the insulation resistance.Poor insulation will lead to higher leakage which will affect measurement precision. This can be partially offset by the driven guard inner shield in triax cables, but cheaper triax will generally have higher leakage making it less desirable.What is the outer diameter of that triax? It looks a little small.Any manufacturer markings on the insulation? I didn't see anything in your photos.I have a couple of Keithley electrometers and several triax cables/adapters, but I admittedly have not used them in quite a while. I did some photodiode detector designs over 10 years ago for underwater sunlight and fluorescence measurements.

author: neverendingstudent
author_rank: Regular Contributor
timestamp: April 23, 2025, 04:28:34 am
content: OD of the combined triax cable is 3.52mm after fiddling around to try to measure with no bend radius. Cable is fairly stiff.OD of the black and green are 2.1mm, OD of the red is 2.48mm.Ironically, I think the Electrometer is exactly what I would use to test the shielding resistance. I don't have an insulation tester / Megohmmeter on hand at the moment. I've heard before from other people repairing Electrometers that the tool you need to diagnose issues with an Electrometer is another Electrometer... didn't expect to run into that immediately.No markings or print of any kind found anywhere besides the '10A' stamp on the alligator clips.