3458A - Page 1
Forum section: electronics / repair

author: drhex
author_rank: Regular Contributor
timestamp: December 08, 2024, 04:09:01 pm
content: Hi all,acquired a new (to me) 3458A, looking at the date codes on the housings it is a 1997 unit, it does have the newer style A5 with one EPROM though. Haven't really done a lot with it yet, it runs ACAL ALL as well as the extended self test without errors, CAL constants seem to make sense. No RAM errors so far, will do the FRAM conversion at some point. Don't have good enough reference points to really test it properly, next step would be a calibration - before I shell out for that I'd like to know it works reasonably well though. One immediately noticable thing is it totting up voltages - up to 11V on open inputs, -.0039 mV on shorted inputs - that can't be right?Thanks for thoughts,Florian

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 08, 2024, 05:05:10 pm
content: Hello Florian,congratulations on acquiring this wonderful instrument. 1997 is a good vintage.Please make sure to immediately extract the content of the CAL RAM to avoid calibration loss.You will find tons of information here, how to check your instrument for sanity, especially the U180 drift problem.If you already infected by the the volt-nuts virus, you might already own a ~ 10V voltage and a ~ 10kOhm resistance reference, for checking and maybe calibration of your 3458A.Several well equipped volt-nuts are living in UK, watch out for them to exchange amateur grade references, otherwise.Your 3458A has very high impedance inputs, and very low bias / leakage currents.Leaving the input open will show a slow rise or decrease of the readings, that's completely normal, and will then cause an overflow, or automatic up-ranging.I usually put a short on the input jacks, to avoid this self-charging effect. 3.9µV is fine, when the instrument is cold and you're using a simple short.With a proper, low e.m.f. short and fully warmed up, zero reading should be somewhere @ several tens of nVFrank

author: drhex
author_rank: Regular Contributor
timestamp: December 08, 2024, 05:33:25 pm
content: Hallo Frank,Vielen Dank für die schnelle Antwort! Will continue posting here on progress. Have ordered the usual suspects (Caps, line filter, etc.). Don’t have good references (yet) so may have to Resort to a professional calibration. Then again, I don’t have an air conditioned lab so let’s see…

author: Grandchuck
author_rank: Super Contributor
timestamp: December 08, 2024, 07:37:28 pm
content: One place to look:https://xdevs.com/fix/hp3458hfl/(you were perhaps already aware of this)

author: coromonadalix
author_rank: Super Contributor
timestamp: December 08, 2024, 10:39:27 pm
content: do some tests to be sure the a3 board does not have drifts problems ... talk about that here ..normally if im not mistaken and not to alarm you, you don't have problems if all the tests are fine ...

author: drhex
author_rank: Regular Contributor
timestamp: December 09, 2024, 09:32:07 am
content: @coromondalix: Will do - my understanding is that I need a stable voltage source to test against? I have a Keithley 7510 which I could connect in parallel but that will probably not suffice?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2024, 10:10:10 am
content: The test for the feared A3 drift does not need any external reference. The point is calling ACAL (likely just the DC part is enough) and than look at the parameter CAL72 that reflects the ADC gain. Depending on how stable the temperature and the use history it takes a few weeks to see the real ADC drift without too much temperature effect.

author: IanJ
author_rank: Unknown
timestamp: December 09, 2024, 05:26:28 pm
content: Here's a Excel spreadsheet I knocked up to record CAL?72 readings and give you the drift relative to Day1 and drift relative to last reading.I think the formula is correct.Just fill in the values in the white cells.PS. Dummy readings on it to give you an idea.Ian.

author: drhex
author_rank: Regular Contributor
timestamp: December 09, 2024, 10:53:17 pm
content: Thanks, so testing the internal reference against the AD. Will execute & post

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 09, 2024, 11:26:24 pm
content: Great job, Ian.To express this more clearly, what SN18 is lacking: You need to measure the temperature coefficient of U180, to separate its temperature and timely drifts.This "goal" of < 0.43ppm/d is randomly set by HP, to give an upper limit for practical usability.Working U180's usually have much lower timely drifts, mine has a calculated < 0.001ppm/day, or so.For long term (24h) stability measurements of external 10V references with the 3458A, it's of course important to have such a low timely drift U180, to detect < 0.1ppm / 24h stabilities.Much more important is, to have a low overall T.C. (U180 + internal LTZ reference) AND a very constant room temperature over this period of time. The usual T.C. is typically on the order or lesser than 0.5ppm/°C, so you need < 0.2°C stable room temperature.Latter is as well recommended, if you use the 3458A as a transfer instrument for different reference values ( i.e. 10.00V vs. 7.15000V) .. I compare my 11 references within about 40 minutes, achieving 0.1ppm transfer uncertainty over this timely enhanced period (above those specified 10minutes), if R.T. changes not more than about 0.3°C.I hope, you've got a nice basement as well, where you can build your lab.Frank

author: drhex
author_rank: Regular Contributor
timestamp: December 10, 2024, 10:37:01 am
content: @Frank: I did have a nice basement lab when I was still in Germany, incl a 3458A in really nice nick, but sold everything when moving to UK - here, I don't have meaningful temp control sadly. Will measure and document.

author: drhex
author_rank: Regular Contributor
timestamp: December 17, 2024, 12:42:54 am
content: First three days:Day Date Time TEMP? CAL? 72 PPM rel day 1 Av drift Drift day by day1 14/12/2024 9:00:00 PM 38.35 1.0008595402 15/12/2024 4:05:00 AM 38.1 1.000859590 1 0.049957060 0.02498 -0.0499570603 16/12/2024 11:00:00 PM 37.6 1.000859560 1 0.019982824 0.00666 0.029974234So much better than spec so far (.43 ppm/day) even in a non temperature controlled environment, 3458A has been running continously.Voltage at rest fluctuates between -0.71 to -0.55 µV which seems fine. Open input still goes up to about 11V then changes range which drops it down to about 7V again. This is with a shorted shielded cable, nothing fancy.Outside of the instrument looks s***, have ordered parts from Keysight. Also have caps etc here to get the power supplies sorted.Seems to me I made a decent buy?And no, it isn't warped, just a pic from the auction...

author: drhex
author_rank: Regular Contributor
timestamp: December 19, 2024, 06:58:55 pm
content: So given my temperature is all over the place is it even worthwhile continuing to measure day by day change? I assume you can't cancel TEMP changes out somehow?1 14/12/2024 9:00:00 PM 38.35 1.000859542 15/12/2024 4:05:00 AM 38.10 1.00085959 1 0.049957060 0.02498 -0.0499570603 16/12/2024 11:00:00 PM 37.60 1.00085956 1 0.019982824 0.00666 0.0299742344 17/12/2024 9:50:00 PM 37.85 1.00085942 1 -0.119896944 0.02997 0.1398797655 18/12/2024 7:54:00 PM 38.45 1.00085932 1 -0.219811064 0.04396 0.0999141326 19/12/2024 12:00:00 AM 37.45 1.00085914 1 -0.399656479 0.06661 0.179845455

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 20, 2024, 01:32:11 pm
content: Quote from: drhex on December 19, 2024, 06:58:55 pmAs said, you can cancel TEMP variations by using a stable environment, or you can determine the T.C. of U180 and subtract this effect from the CAL? 72 parameter, hopefully chosing the correct sign. I guess, latter is automatically done by Ians program.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2024, 01:43:23 pm
content: Quote from: drhex on December 19, 2024, 06:58:55 pmTo a certain degree one can correct the temperature effect. With data for a longer time window the individual measurements also get less critical.After a long time not using the meter, there and be some additional drift anyway.So far the drift data look good, though still a bit short to be really sure. The limit should be seen more as 3 ppm per week instead of 0.43 ppm per day.

author: drhex
author_rank: Regular Contributor
timestamp: December 21, 2024, 12:51:46 am
content: Thanks, I will go on monitoring until Monday then (I am off to Munich for Xmas after and don't want to let it run unsupervised as I still have to recap). Have a lot of parts on my desk including a new front which should restore it to its former glory. Project for after Xmas. Have also modified WinGPIB (thanks Ian!) for NI GBIP-232CT-A use, will share that back in case anybody needs it.Frohe Weihnachten and many thanks for the support so far!Florian

author: drhex
author_rank: Regular Contributor
timestamp: December 21, 2024, 01:56:16 am
content: On another note, is it possible to get the nuts off the binding posts and/or change out the red rings? I bought two of the fused ones fixing one of my damaged ones in the process but the guard terminal is still missing its ring. A new terminal block is a whopping 473 GBP plus VAT from Keysight, bit steep for a red ring :-)...

author: IanJ
author_rank: Unknown
timestamp: December 21, 2024, 08:34:11 am
content: Quote from: drhex on December 21, 2024, 12:51:46 amLooking forward to integrating your mods back to my master, just send me your source and details when you can.Ian

author: drhex
author_rank: Regular Contributor
timestamp: December 22, 2024, 12:42:00 am
content: So recapped outgard today (C1/11 were leaking) and socketed the chips on A5 - direct read of the CALRAM yielded something different than WINGPIB - need to research that a bit. Still waiting for the boards to do the FRAM conversion. Next step is to give it a good scrubdown and recap the inguard PS, internals are surprisingly clean. Sadly only got three feet from KS, still wating for five more :-).@Ian: will mail the changed files later on.

author: IanJ
author_rank: Unknown
timestamp: December 22, 2024, 03:48:07 am
content: Interested in the Calram differences, can you email me both hex files?When I did my 3458A I used the WinGPIB generated one (which matched the DOS app one also) and it was fine……..so am interested in why your direct read is different.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 22, 2024, 08:12:19 am
content: If you have used the 3458A between GPIB and direct read-out, esp. if you did any ACAL in between, it's normal that you get different data.In the CALRAM, a lot of temporary constants are stored as well, not only the basic, fixed gain constants for DCV, OHM and AC.Please check the Calibration Manual, which constants these are are. There's a list somewhere, at which location you can find these.Frank

author: drhex
author_rank: Regular Contributor
timestamp: December 22, 2024, 10:12:29 pm
content: @Ian: Have shared a folder with the files. Think the results handling for multi byte returns is not working correctly with the NI GPIB-232CT-A - May give that another look, but it is painfully slow with this interface in the first place...@Dr.Frank: This could be, did at least one ACAL DCV inbetween the reads. It is massively different though - rhink it is more the above.

author: drhex
author_rank: Regular Contributor
timestamp: December 29, 2024, 10:10:12 pm
content: Trying to run @Tin's X18 script via pyvisa using a NI USBGPIB getting VI_ERROR_IO (-1073807298): Could not perform operation because of I/O error. This is intermittently happening on any request sent - the instrument starts executing and shows no error so must be the response not arriving as expected. Funnily works if executed manually from the python console. Has anyone come across this?

author: drhex
author_rank: Regular Contributor
timestamp: January 11, 2025, 05:36:43 pm
content: And another one: Is ACAL supposed to fail if I have 10V connected?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2025, 05:44:09 pm
content: ACAL is not supposed to fail with an external voltage. However it may have a small effect on the accuracy. Here 10 V DC may not be that bad. AC could be more of an issue, especially if not mains frequency. So ideally there is no significant external voltage.

author: drhex
author_rank: Regular Contributor
timestamp: January 11, 2025, 06:05:29 pm
content: Then I have an issue with the box. Meh.205, "HARDWARE FAILURE -- CAL VALUE OUT OF RANGE 101"210, "HARDWARE FAILURE -- CALIBRATION REQUIRED"101, "CALIBRATION ERROR"101 is precharge DAC, assume this is A3?

author: drhex
author_rank: Regular Contributor
timestamp: January 12, 2025, 08:15:15 pm
content: Strange thing it is working pretty well when it is just sitting there. Average drift over the last 30 days is 0.12156 in a completely temperature uncontrolled environment with TEMP? varying from 37.4 to 39.1 degrees. So good enough by KS standards.@ianj thinks it might be a weak relay - how do I find that?

author: drhex
author_rank: Regular Contributor
timestamp: January 12, 2025, 08:49:21 pm
content: Another interesting finding, stepping up through the ranges I get materially different indications between mA and A i.e 0.0000984 A and 0.0999915 mA for a .01 mA input from a 3245A.

author: IanJ
author_rank: Unknown
timestamp: January 13, 2025, 10:22:29 am
content: Weak relay or iffy contacts for the issue whereby it wasn’t letting you ACAL with the bananas populated……if I read you correctly.I’d be looking at this issue first before any drift problem.Ian