Keithley 616 on its way

[cncjerry]

I don't know why, but I've had my eye on electrometers for a while and found a Keithley 616 on ebay for what I thought was a deal for a piece of equipment I might never use again after playing around.  It looks like it is working.

I need a triax cable with alligators or other connectors, actually I plan to use it to check a KV divider under power so I will change it to pin types no matter what I find.  So before it gets here, does anyone know if they are 2-lug or 3-lug triax connectors on most if not all of the 616 meters?  From the pictures it looks like two.  The I went onto Pomoma and saw that the cables are pretty expensive, like $125 for triax to alligators, more than I paid for the meter, of course.  I also found what looks like a cable on a another so called electrometer that I can get for like $25 including shipping.

So 2-lug or 4-lug?  What do you recommend to be the least expensive route to maintain the guard up to the alligators?  I saw where one person who used a triax to BNC to clips but couldn't tell if that was a triax cable in the middle or BNC coax?  If it is a 2-lug, then I would probably do triax -> cable -> triax -> BNC -> clips.  This would still cost about $100 but if the meter works, then I am ok with it.

Any other low cost options?

Thanks

[technogeeky]

It is 2-lug.

I discovered this by correlating the Keithley 616 manual which (in two locations) specifies the connector as CS-141. And then this Keithley connector literature says that CS-141 is a 2-lug connector.

Make sure to give us a teardown!

[Assafl]

It is a 2-lug. These are mostly pretty expensive but the guy there (from Israel) sells them for $9 a piece and I offered $12 for 2.

http://www.ebay.com/itm/272445116225

These are Kings type and good quality.

This is the cable I used - but he seems to be all out of it: http://www.ebay.com/itm/201706863349

Altogether came out to $20 per real Triax cable.

You cannot use a coax for guarded ("Fast") mode (you need a sense line, a guard and a return ground) - but perhaps you can use a combination coax and external ground wire.

Since the input impedance is so high, you need the entire chain to the clips (or pins) to be PTFE or similar. I tried Teflon and PVC pigtails to croc clips and touching them would send the 616 berserk. Strip, solder and don't forget to clean. I used Teflon tubing to encase the joints.

Also, the 616 I got needed massive cleaning and drying or it ran all over the place. It looked clean inside but still had leakage. After cleaning - if I put a voltage on the input and disconnect it - It will take it hours for the charge to dissipate.

It has quite a bit of tempco and replacing the caps reduced it considerably.

Note that there are pretty high voltages inside - the current source rides on the 1:1 voltage stage which spans +-200 VDC. So inside you have ~+-270VDC that "floats" the current source. Touching it (IMHO) will fry that part of the circuit (it is the area between the selector switch and the shielded transformer).

If you use it in "fast" mode, the guard will be at 200V for any of the resistance ranges. It is low current though - but you feel it well...

[kridri]

There is an teardown of an Keithley 616. On the first picture you will see a two lug triax connector..

[cncjerry]

I saw the teardown and I wasn't sure of the lugs.  I remember seeeing a comment that they changed it to 3 lug but that must have meant on the newer equipment.

Thanks for the pointers on eBay sources.

[cncjerry]

I offered him 15 as well and asked if he has cable.  What size cable do they fit?

[pmcouto]

It is a 2-lug. These are mostly pretty expensive but the guy there (from Israel) sells them for $9 a piece and I offered $12 for 2.

http://www.ebay.com/itm/272445116225

These are Kings type and good quality.

This is the cable I used - but he seems to be all out of it: http://www.ebay.com/itm/201706863349

Hi Assafl,

Is that 2-lug connector adequate for the cable diameter?

All 2-lug triax connectors I could find were designed for a much larger (thicker) cable and using them with that thin cable is not easy…

[guenthert]

Well, the quick and dirty hack is to change the 2 lug connector to a 1 lug one by filing down one of those lugs.  Then either 2 lug or 3 lug connectors fit.  I found it easier to acquire the latter kind.  Works better than it sounds and has the advantage of avoiding to have to solder at the (heat-, dirt-) sensitive input stage -- this is important to me as I'm much better with a file than with a soldering iron 

[cncjerry]

I thought about filing down one lug but I didn't want to modify it the first day I get it.

I sent the guy with the connectors an offer of 15 for two, he turned it down, so I am going to wait to get the unit and see if it works before investing in a cable.

[vindoline]

Well, the quick and dirty hack is to change the 2 lug connector to a 1 lug one by filing down one of those lugs.  Then either 2 lug or 3 lug connectors fit.  I found it easier to acquire the latter kind.  Works better than it sounds and has the advantage of avoiding to have to solder at the (heat-, dirt-) sensitive input stage -- this is important to me as I'm much better with a file than with a soldering iron 

I know your pain. I picked up a working 616 a couple of years ago for well under $100. I was shocked to find out that a simple cable to connect to it cost more than the meter! Do yourself a favor and file off a lug. I had the same reticence to "hack" a nice old piece of test gear. But if you do it carefully you can't really see it and now both 2- and 3-lug connectors will fit. They're all expensive, but the 3-lug are much more common.

[Assafl]

It is a 2-lug. These are mostly pretty expensive but the guy there (from Israel) sells them for $9 a piece and I offered $12 for 2.

http://www.ebay.com/itm/272445116225

These are Kings type and good quality.

This is the cable I used - but he seems to be all out of it: http://www.ebay.com/itm/201706863349

Hi Assafl,

Is that 2-lug connector adequate for the cable diameter?

All 2-lug triax connectors I could find were designed for a much larger (thicker) cable and using them with that thin cable is not easy…

Attached are two pictures with the measurement in Metric as well as Imperial.

It is a wee bit too wide for the cable (not very wide like the ones intended for the Video triax cables). So it holds tight - but since the outside is round and smooth like Teflon - a yank is sufficient to pull the cable out - but I just used a heatshrink to make the cable a bit wider and as a strain relief of sorts.   

Building these cables however - is tricky. If your 616 is clean, any leakage is measurable (and I mean 1 fingerprint is enough). Use lots of IPA. I ran into a slight issue building the cable actually at the 3 cable splitter at the measuring side. Seems the polyolefin shrink is leaky. On the higher end of the ohms range I get very low readings.  So now I keep it in air. I know some put that junction in Teflon blocks. I ordered PTFE shrink (didn't know these even existed) and will try that.

On the positive side - this is not a nano voltmeter - so once you get to the measuring side of the cable cleanly - any cheap ass croc or rusty pin will do.   

BTW - My 616 (also about 100$) is in pristine shape (came from Texas Instruments). I have it hooked to the back of the HP34401A so basically I use it as an ultra high buffer for the 34401 whenever I sense that I am affecting the reading. Albeit for most purposes the hi input R mode is sufficiently high resistance - and more accurate and precise displaying less drift than I get with the 616. But it should make for a very good DC probe for high impedance circuits when guarded.   

[muvideo]

Hello, this thread remembered me that I have an unused 616 in my lab!
I ordered the connectors from the ebay seller (thank you Assafl) and now
I will search for a suitable cable.

cncjerry I will look forward for update on your 616.

[cncjerry]

Mine arrived today in what I would call 'very good' condition. Great shape, it zero'ed perfectly with lots of range.  I don't have a cable so I did the charged cap trick to watch the bleed-down, of which there isn't much, from 20V.  I think I got a shock trying to shove a 10M .01% resistor in there so I guess I have to be careful.

I'm thinking about making up an adapter.  I'll use the center pin of a bnc (if it fits), some nylon, a split retainer, more nylon and then a bnc outer shell.  I'll turn the parts on a lathe.  The concept is this will connect onto the front without a cable to allow measurement of very high value resistors without using a cable, about as direct as possible.  I can also put some other adapters on it for measurements, maybe some banana jacks or posts.

[Assafl]

AFAIK - the reason they switched from 2 to 3 lugs is that the BNC center pin is thick and will ruin your 616's triax connector.

I use a 3 wire adapter with single pin DuPont at the end so I can use grabbers and any other connector. To measure resistors either directly or in the feedback loop (fast mode).

Any insulation I tried other than PTFE cases the 10^12 and usually the 10^11 ohms range to show low numbers.

I made a needle probe and needed to make a PTFE insert to hold the probe itself.

[cncjerry]

Looked at it yesterday prior to ordering some triax and connectors.  Someone had ground off the lug.  Pissed me off to no end.

[cncjerry]

Where do I get PTFE in round bulk rods?

AFAIK - the reason they switched from 2 to 3 lugs is that the BNC center pin is thick and will ruin your 616's triax connector.

I use a 3 wire adapter with single pin DuPont at the end so I can use grabbers and any other connector. To measure resistors either directly or in the feedback loop (fast mode).

Any insulation I tried other than PTFE cases the 10^12 and usually the 10^11 ohms range to show low numbers.

I made a needle probe and needed to make a PTFE insert to hold the probe itself.

[TimFox]

Try
https://www.mcmaster.com/#standard-plastic-rods/=160xxo0
in the US

[Assafl]

McMaster Carr also have them in tubes - which might be easier to use as a probe.

Too bad for us though as they won't ship internationally (probably serve regulated industries and find it easier to not deal with export issues for single orders)....

[cncjerry]

Been playing around with my 616 in the spare time.  I've been shocked by it about five times and this thing doesn't realize how fast it will end up in the garbage if it bites me one more time.

Anyway, I hooked a spare 3478 up to the back thinking the integration would be improved but I found that the output doesn't vary with any degree to the display.  So for instance, if I measure a 899.5k vs a 899.875k the front display and rear going to the 3478 are pretty much the same, actually the first four digits atleast reading 9.000 if I have all the digits right, but you get my drift.  It's almost like the output is post integration and DAC'ed back to analog?  I haven't looked at the schematic to see where the output is coming from.

Overall, pretty interesting unit, still fighting with cables and connectors but I was able to make a triax to clips that works well. I want another that would be triax to bananas and a triax to clips and a probe.