Agilent 34401A front inputs reading 0.5 Ohms with shorted test leads - Page 1
Forum section: electronics / general

author: cskilbeck
author_rank: Contributor
timestamp: August 11, 2023, 02:51:18 pm
content: Hi, I recently purchased a used HP34401A. When I put the device in 2W resistance mode and plug test leads into the front inputs I get about 0.5 Ohms when the leads are shorted together. The same test leads measured through the rear inputs give me a more reasonable ~0.02 Ohms. Does this indicate a fault or is it possibly just a calibration issue?Thanks,Charlie.

author: mower
author_rank: Contributor
timestamp: August 11, 2023, 03:29:44 pm
content: It may be an issue with the panel selection switch, it has many contacts and perhaps one of them is not making good contact?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 11, 2023, 07:50:57 pm
content: The front / rear switch is the prime suspect in this case. Sometimes it is as easy as operating the switch a few times to clean the contracts.

author: bdunham7
author_rank: Super Contributor
timestamp: August 11, 2023, 09:03:49 pm
content: Quote from: cskilbeck on August 11, 2023, 02:51:18 pmYou need to use a high quality copper shorting bar for this test, not test leads. You may like that 0.02R reading, but it is pretty unlikely that you actually have 20mR test leads.The zero calibration can be done separately on these and all you need is a high quality 4 wire short. The front and rear terminals are calibrated and zeroed separately, so a calibration error could cause them to read differently.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 12, 2023, 07:37:02 am
content: In theory a calibration error could cause the error, but it is rather unlikely that a wrong zero calibration was done with a -0.5 ohm resistor. A bad contacts gives to high a reading and if this is during the zero adjust, that following reading would read to low (e.g. neg resistance with a short).

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 12, 2023, 08:25:31 am
content: 2W will normally do this, that's why 4W mode exist, to compensate the wiring resistance.Though 0.5ohms suggest a problem somewhere, either bad connection or crappy/damaged probes.A workaround is to short the probes and toggle relative mode, so it measures the delta over the existing resitance.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 12, 2023, 11:31:46 am
content: Quote from: cskilbeck on August 11, 2023, 02:51:18 pmHi Charlie,Please provide a more detailed description, or a picture of your short, so one can estimate its real resistance.At first, the difference in reading indicates a problem in the front / rear switch, i.e. contamination on the contacts.Put an identical short on both jacks, actuate the switch several times, and observe the change in reading for front and rear.0.02 Ω is quite a low value, depending on your kind of short, and might indicate that somebody has made an erroneous zero calibration, which overrules too high readings caused by excessive resistance of the switch. (As bdunham7 already described correctly)That means, the contacts might even have much higher values, like like several 10 Ω, which would become evident, if these readings vary greatly during the actuations.Therefore, I do not recommend at all to make any calibration attempts in this current status.A few years ago, I provided a detailed description of desoldering the switch, carefully disassembling, cleaning the interior contacts and reassembling.I had to be careful with this zero calibration as wellFrankHere's the elaborate thread:https://www.eevblog.com/forum/testgear/hpagilentkeysight-34401a-2w-problem/

author: cskilbeck
author_rank: Contributor
timestamp: August 12, 2023, 06:42:05 pm
content: So, I'm using these two leadsEach measures zero on the Brymen handheldHere's the difference on the 34401A, the results are the basically the same if I swap the leads between front and rearFrontRear[ Specified attachment is not available ]

author: cskilbeck
author_rank: Contributor
timestamp: August 12, 2023, 09:48:20 pm
content: Sorry, I can't make the pictures work properly, but hopefully you get the idea

author: bdunham7
author_rank: Super Contributor
timestamp: August 12, 2023, 11:26:02 pm
content: Quote from: cskilbeck on August 12, 2023, 09:48:20 pmThe question remains--what happens if you switch the F/R switch repeatedly? Do you go back and forth between those two exact numbers or is there a variance? If so, how much?

author: coppercone2
author_rank: Super Contributor
timestamp: August 13, 2023, 09:16:15 pm
content: if you want a cheap short standard, and easy, get a double banana jack solder post thing, solder a bare piece of 12 gauge wire across it. its pretty good

author: mawyatt
author_rank: Super Contributor
timestamp: August 13, 2023, 11:11:53 pm
content: Quote from: coppercone2 on August 13, 2023, 09:16:15 pmSomething like this works well, we use #12 Cu wire and Dual Banana Jacks.Best.

author: coppercone2
author_rank: Super Contributor
timestamp: August 13, 2023, 11:15:02 pm
content: oh yeah a 4 wire version. but having a 2 wire version is great for hand held DMM verification and shorting stuff

author: mawyatt
author_rank: Super Contributor
timestamp: August 13, 2023, 11:22:50 pm
content: Yep, like this.Best,

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 14, 2023, 08:42:35 pm
content: follow my link. there's another working 4W short, which is required for this zero calibration

author: Nanitamuscen
author_rank: Contributor
timestamp: August 15, 2023, 04:24:50 am
content: To close all 4 wires, it is worth making such a jumper.

author: cskilbeck
author_rank: Contributor
timestamp: August 15, 2023, 10:16:07 pm
content: OK, I took out the front/rear terminal switch and there were a couple of slightly high resistances (mostly they were 0.02 or 0.03 Ohms, measured on the Brymen, a couple were around 0.2 to 0.25 Ohms). I sprayed some IPA into the body and jiggled the switch a bunch. After letting it dry the resistances are all close at around 0.02 Ohms. After soldering the terminal switch back in, I still get very similar results - the rear terminals are measuring 0.002 Ohms now (presumably the IPA improved the contacts in the terminal switch a little, this result is slightly closer to zero than before), and the front terminals are still measuring 0.51 to 0.53 Ohms. Toggling the switch repeatedly doesn't affect the result.I think it's _probably_ not the terminal switch. Although I didn't take it apart and inspect it, the resistances measured out of circuit all seem reasonable.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 15, 2023, 10:20:38 pm
content: Are there separate overvoltage protections on the front and back connections?Just a shot in the dark, I would think there would not be.

author: mawyatt
author_rank: Super Contributor
timestamp: August 15, 2023, 10:53:21 pm
content: Seems you could short the 34401A input, set it to 2 wire ohms and place the Brymen DVM one lead on the negative 34401A input terminal and begin tracing backwards to find where the voltage rises significantly, this might hint to where the problem is.Best,

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 16, 2023, 06:29:42 am
content: Quote from: cskilbeck on August 15, 2023, 10:16:07 pmWhich of the contacts have you measured to be 0.25 Ohm? Those for the front terminal?That's pretty high, and might sum up to 0.5 Ohm.A 2W Ohm measurement is not appropriate, you have to measure those contacts with 4W method.Again, please read the complete thread I have provided, try to understand what was going on, and then you may need to disassemble the switch to properly clean it. Just spraying IPA into the switch does not do the job!!As you get stable readings, and as the only difference is the setting of the switch, then there is no other component left than the switch itself, or its calibration.The zero readings might only be caused by an erroneous zero calibration, so get a proper 4W short and do this zero calibration only.That should solve your problem.Frank

author: Swake
author_rank: Frequent Contributor
timestamp: August 16, 2023, 03:02:20 pm
content: Interesting information here.Is it possible to read out the calibration settings?Would those readings be of any information about the difference front/rear in this specific case or would these be very off one from the other anyhow?

author: tszaboo
author_rank: Super Contributor
timestamp: August 16, 2023, 03:37:04 pm
content: Maybe it's the front terminal which needs cleaning, though that's gold plated, so unlikely.I just shorted a 34465A with it's original probes, and got 0.05Ohm.Otherwise I always used the 2W mode by shorting it out and then pressing zero...

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 16, 2023, 09:46:56 pm
content: Quote from: tszaboo on August 16, 2023, 03:37:04 pmHey guys,I do not understand this whole speculation about any random root causes any more, tszaboo, not yours in specific, but more the others, who wildly speculate about e.g. other components which might cause this issue, w/o any proper rationale.We have solved this problem about 9 years ago already, by proper measurement on this front / rear switch, on the correct switch contacts, macro pictures, and so on.The front contacts are very unlikely the problem, on 34401A and on on 34465A, those are pure copper contacts, and very probably will not be the culprit, but much more probable this contaminated switch, which will be very probable deteriorate after 20..30 years, like mine from 1993, or so , in the very same manner in each instrument of the same age.That's a simple causality consideration.I also do not understand, why the thread opener does not simply follow exactly our best practice from way back when.Sorry, that I became emotional. Therefore, this will be my very last post in this thread.Frank

author: martinr33
author_rank: Frequent Contributor
timestamp: August 17, 2023, 02:09:21 am
content: I am 100% with Dr. Frank here.The 2W short measurement isvery sensitiveto the condition of the switch and terminal wiring, and is the ONLY measurement that exposes a problem in the switch (and wiring, for a 3458A).Cleaning the contacts sounds like a great next step. Don't leave too much Deoxit around, it will cause green corrosion to appear. PPE might ba a better choice, or maybe Deoxit gold (I have not tried it).If it is like the switch on the 3458A, you may not have to take the springs out if you are careful.

author: bdunham7
author_rank: Super Contributor
timestamp: August 17, 2023, 01:53:53 pm
content: Quote from: Dr. Frank on August 16, 2023, 06:29:42 amI haven't had a F/R switch issue specifically on a 34401A, but on other DMMs where I have I've never had one that was off but perfectly stable when repeatedly switched. Perhaps there is some difference in the design of this one that would allow that. There are three possibilities here--the switch has 0.5R of resistance in it's contacts, there is actually a stable 0.5R resistance in the front jack wiring or the zero calibration is off.1mA test current and 0.5R indicated correlates to 500µV being dropped somewhere. It shouldn't be that hard to set up and find where that voltage drop is happening, if it actually is.

author: alligatorblues
author_rank: Regular Contributor
timestamp: September 14, 2023, 04:49:31 pm
content: Quote from: cskilbeck on August 11, 2023, 02:51:18 pmTry working the froont/rear switch, Clean out terminals. Get a 4-wire short and try that on the DCV setting. Calibrate zero volts. Try again. If necessary, Reflow the solder joints on front terminals.

author: martinr33
author_rank: Frequent Contributor
timestamp: September 15, 2023, 01:00:39 am
content: K101 can also be a problem. It is a reed relay that sits in the high input line. If it has a problem, it only shows up on the 2W resistance range.You can try tapping it. You might also try CAL 0 front and rear, becasue a bad 2W 0 cal can also cause problems.Use a good short.

author: JohanH
author_rank: Frequent Contributor
timestamp: September 15, 2023, 06:56:49 am
content: 0.5 ohm is way too high. Even cheap leads shouldn't have higher resistance than 0.1 ohm. Typical leads that I have are around 0.02 to 0.05 ohm.With a proper short, as in the picture below, the values are much lower. The values aren't stable, they toggle a few milliohm up and down. The front and rear terminals are equal, though.There was an issue with the 4W measurement with this meter when I bought it. The front panel was cracked and the plastic clip that supports the front panel and front terminal assembly was missing. Due to this the terminal assembly easily bent inwards when you inserted leads into the jacks. This had led to pressure on the PCB, repeatedly bending it and cracking the solder connections of the 4W resistors at the front, near the terminal. Resoldering the resistors and securing the front panel fixed the issues.

author: cskilbeck
author_rank: Contributor
timestamp: October 02, 2023, 09:39:30 am
content: Just to close this out - I cleaned the front/rear contactor as best I could and then did the zero ohm calibration using this jerry rigged short. Everything looks good now, the readings look very reasonable and agree with my other DMMs.