HP34401a reading low in 4W ohm - Page 1
Forum section: electronics / repair

author: gmac34
author_rank: Regular Contributor
timestamp: May 13, 2025, 11:17:12 am
content: I have recently aquired an old hp 34401a, 1994 vintage.It is mostly functional, as was confirmed by a questionable calibration certificate. The unit was tested by the seller with a 5502a, which is insufficient to calibrate a 61/2 digit meter, but gave me the (probably wrong) confidence that the thing was not broken.And in fact it seems to agree quite well in voltage and current measurements, but, comparing it with my other meters (a Keythly 2000 and a HP 3456a), I see there might be an issue with the 4w measurement in particularly the 1Mohm and 10Mohm (cannot properly test the 100M range).The symptoms are as follows:- Apart from the 100-ohm range, 2w mode gives significantly closer results between the 34401a and the other two meters.- in 4W mode, 100 ohm to 100kohm ranges appear to be consistently 0,015% low. This might not be too bad considering that I'm comparing 6 1/2 meters with each other.- in 4w mode 1Mohm and 10Mohm measure 0,07/0,08%, low this seems quite too much.(I'm comparing my meters using an assortment of Vishay foil resistors, 5ppm/c plus a couple of ceramic Caddock Metal Film /ceramic resistors for 1M and 10M. They are all in the same metal box with 4W terminals)So far, I have only tried to clean the area around the front and rear switch, as it seemed there were some stains, but with no success. I don't see any other sign of rework or dirt on the boardIs this "normal" for a 30-year-old DMM and should just be adjusted, or should I start looking for some leaky JFETS?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2025, 12:43:29 pm
content: The verification certificate is indeed odd. They use tolerance intervals that are quite a bit larger than the 1 year accuracy of the 34401.It at least shows the ranges somewhat working and no major fault - could still be off.It is also odd to see such a large difference between the 2 W and 4 W ohms for the high resistors. This suggests some leakage issue there - with the 34401 most of the switching is however done a custom (hybrid ?) chip, not individual JFETs. For 1 M and 10 M and especially one may live with using 2 Wire mode.Leakage can hardly explain the difference for the low resistance (100 Ohm) case. Here the 2 Wire mode is hardly relevant - if may read high due to contact resistance. So the issue is the 4 wire 100 ohm range being off quite a bit off.So 2, likely different problems.For the higher restance, one could check the input currents for the sense H input and the normal voltage input. The sense L input should not be critical at all.

author: gmac34
author_rank: Regular Contributor
timestamp: May 13, 2025, 01:35:47 pm
content: The discrepancy at 100ohm is difficult to explain. I'm quite sure that the 4w measurement is more accurate. But I'm not sure why nulling the leads is not enough to get a good measurement in 2W.If I put 10M across the sense high and high input connectors, I measure circa 2mv across, a little bit more across Low and sense Low and across High sense to low sense, I measure less than 1mv.I'm not sure if this is the right way to measure it, as I'm probably mixing in the result with the input bias of the 3456a.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 13, 2025, 01:38:57 pm
content: I have a 34401A with FW 03-01-01, same vintage from around 1994, long time ago that I calibrated it, so I just compared 2W and 4W readings for all ranges from 100 Ohm to 10M with my FLUKE 5450A. I use a shielded 4W PTFE cable assembly.First off, forget about the comparison to your other meters , that's confusing in first place. In the beginning, just let's focus on the comparison of 2W vs. 4W readings of your 34401A alone.Usually, 2W and 4W Ohm fully agree, but you have to Null 2W readings first, for 100, 1k, 10k ranges, otherwise 2W reads about 0.2Ohm higher than 4W, due to cable resistance. This effect vanishes for 100k and upwards. In my case, all these higher ranges agree in 2W and 4W to a few ppm difference.Please indicate, or make a photo, which kind of cables you're using, especially which insulation, PTFE or PVC, or else, and if you have twisted the cables, and which one you've twisted (S+/In+ and S-/In-. or S+/S- and In+/In-).The difference between for these higher ranges is too far off, but this can have different root causes:1) If you have low insulation type cables, and you twist eg S+/S-, you might get big leakage currents which lead to big errors, in the 100k range it might be 1.. 10 ppm, the higher ones 10 and 100 times bigger.This failure is not so probable, as the seller already detected these different readings.2) Usually, both Ohm modes are calibrated only in the 4W mode, see Service Manual, p. 79:When performing a 4-wire ohms gain adjustment, a new gaincorrection constant is also stored for the corresponding 2-wire ohmsmeasurement range. If desired, the 2-wire gain can be adjustedseparately after the 4-wire ohms gain calibration is completed.That means, it might be possible, that the seller adjusted your 34401A in both modes, i.e. 4w first, then a separate 2W adjustment for the same range.If something went wrong, then both modes disagree.If you re-adjust all ranges against your KEI2000, but only in 4W mode, then the 2W mode readings must fully conform to the previous 4W adjustment.3) Only after the status of the calibration / adjustment is claified, then a real hardware defect should be taken into consideration.I don't want to speculate about possible failures at this point in time: That makes no sense as well, to blindly try a failure search.4) What can be said in general, that the overall adjustments of the resistance ranges are too far off, compared to your KEI2000, it should be less than the 1 year specification (<100ppm), as the 34401A is very long term stable. This might as well be due to the drift of the KEI2000, so you'd need a really good resistance standard, or a freshly calibrated other instrument.Please provide some information, how far off the DCV ranges are, in comparison.That indicates to me, that either there occurred a problem during Ohm calibration (wrong method, wrong cables, ..), or there is REALLY a hardware problem, like the infamous U201 (AD706) leakage defect, but latter should not make a difference between 2W and 4W.Conclusion:Please provide the requested information, and maybe you know, whether the seller tried an adjustment of all ranges (not only this verification).Could you pls. supply the complete verification data for all ranges/modes?Then we'll find out, what's going on here.I again suggest, NOT to make any "repair" attempt at this status.Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 13, 2025, 01:51:43 pm
content: Quote from: gmac34 on May 13, 2025, 01:35:47 pmAs said, pls. just compare the readings of the 34401A 2W vs. the 34401A 4W reading in first place.Then, for 100, the 2W reading is 0.13 Ohm higher, the 1k 0.24Ohm, and the 10k 1.43 Ohm.Otherwise, you still might observe differences up to 100mOhm, depending on your cables.If you did not use NULL, then this looks nearly ok. for 100 and 1k ranges.10k and higher is too far off, but I gave 3 possible things to check first.your differential measurement of S+ and In+ makes no sense, and probably creates other errors.You need to check differently.Frank

author: gmac34
author_rank: Regular Contributor
timestamp: May 13, 2025, 03:08:35 pm
content: Hi, thanks for the detailed response.If I check the cal number, I see "87" This seems odd, maybe someone tried many time to adjust itThe cal date is still 1994, but maybe no one bothered to update it. The seller did not mention it being recently adjusted, only compared to a standard.Attached is the first page of the "calibration", and I have written down a few voltage comparisons. The voltage is provided by a hp3245A up to 10v, 100v comes from a much less stable 6634BFor the 4W measurement, I use 2 shielded cables with two inner twisted pairs. I think the insulation is polyethene, but the twisted pairs are (S+/In+ and S-/In-), so that should not create such a large error. I will make a PTFE cable soon. I ordered the materials for that a week ago. For the 2-wire measurement, I also used regular silicon test leads.The two wire measurements were made nulling the wire first.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 13, 2025, 04:16:37 pm
content: For a 30 y/o instrument, it has been adjusted maybe three times only.One complete adjustment increases the counter by 37, afair, i.e. +1 for each adjustment point.You don't need to adjust each mode/range every time, you can also do single point adjustments.These silicon leads should not be twisted for high Ohm ranges, just leave them separate.The other ranges look fine.This discrepancy of the 2w/4w ranges i can still not assign to already known errors.The calibration is anyway compromised, therefore you can't make it worse, I guess.So, you need to re-adjust the 6 Ohm ranges in 4W mode only, by your 6 relatively stable resistors, and taking their nominal values from either instrument you trust most. Then pls. verify, if 2w and 4W readings agree afterwards.If not, we'll have to look for hardware failures.Frank

author: gmac34
author_rank: Regular Contributor
timestamp: May 13, 2025, 04:19:16 pm
content: Silicon leads for the 2w measurement were not twisted. I try to keep them dangling in the air as much as possible. I have also tried to use 4 silicon leads, again, not touching each other for the 4W, the result is very similar

author: gmac34
author_rank: Regular Contributor
timestamp: May 13, 2025, 04:44:35 pm
content: Do you then suggest that I adjust the 34401a resistance range to match the Keithley?I don't have all the references I would need at the moment, I would need to get a 100ohm (maybe my 95 is ok), a 10K (my 10.2K is possibly out of range) and a 100Mohm. As well as the PTFE cables that are coming. It's a reasonable expense, but I can start to put together what I need.I'm just worried (apart from ruining the calibration even further) that this would hide possible hardware problems.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 13, 2025, 09:44:12 pm
content: Quote from: gmac34 on May 13, 2025, 04:44:35 pmYes, I really suggest this, in case your KEI 2000 has the lowest uncertainty compared to the 3456A.Please read the Service Manual for the procedure, i.e. you only need 100, 1k, 10k, 100k, 1M and 10M, all can be 0.9 .. 1.1 times from nominal.The zero calibration is as well recommended.And I was under the impression, that you already have all these 6 resistors, as you're doing comparisons between your 3 DMMs: 94.9756 Ohm, 1000.04 Ohm, 10.20063 k, 99.8046 k, 1.000033M and 10.001380M ?!They don't need to have low uncertainty, only a short term stability is required, to first measure the nominal value with your KEI 2000, then apply the resistor to your 34401A, type in the measured value and calibrate.Nothing can go wrong, either there really a mis-calibration happened, so your DMM is off anyway, or it's really defect (we'll see..) and then a new calibration is required after the repair, anyway.Frank

author: gmac34
author_rank: Regular Contributor
timestamp: May 14, 2025, 07:08:28 am
content: Yes, I do have them, up to the 99.8K, they are all from this serieshttps://sm-elektronik.pl/pdf/RCK-VISHAY.pdf0.1 or 0.01% 5PPM not the best Vishay can offer, but they should be stable.1M and 10M are from Caddock, also 0.1%-5PPM. And I do have a short.I will be away for a few days, but I will get back to this, perhaps with the PTFE cables then.I was looking to see if there was a way to save calibration constants before I do anything, but it seems I would need to take the EEprom off and read it.Also, the resistors are in a metal box. I have a guard terminal connected to the box with the 3456a. I would connect the shield of the wires to the box and the guard terminal, not sure how I should use it on the 34401a. Should the shield go to Earth?

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 14, 2025, 07:50:32 am
content: Quote from: gmac34 on May 14, 2025, 07:08:28 amPerfect! Even the Caddocks are good quality and sufficiently stable.There's no known way to save the cal constants, unlike with the HP3458A.. forget it, it's not necessary at all.You might write down all readings on the 34401A of your 6 resistors, and if you'd like to keep the calibration "As is", afterwards re-enter exactly these values in another calibration run, reproducing the old cal constants.No, the 34401A has no GUARD, so better leave the other side open.Does your box and the plastic of the jacks have high insulation?Frank

author: gmac34
author_rank: Regular Contributor
timestamp: May 14, 2025, 08:00:49 am
content: No, the box is metal and the jacks are just ABS, but if that was a problem affecting the 1M and 10M measurement, wouldn't I also see a problem with the other 2 meters?I cannot send a picture of my box at the moment but I was "inspired" by this onesame type of box, exact same type of binding posts, cheaper resistors....

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 14, 2025, 08:24:28 am
content: Fine, should do the job!

author: gmac34
author_rank: Regular Contributor
timestamp: May 14, 2025, 11:08:20 pm
content: Well, I did not wait, and took some time this evening - night.First thing, I noticed I had quite a bit more resistance shorting the front terminals vs the rear in 2W mode (about 0.3 ohm on the fornt 0.02/3 on the back). So I took the switch off and cleaned it as described in other posts.The switch now measured okay, >0.02 ohm on all gangs outside of the meter, but I still had the same difference between front and rear.I checked there was something wrong between the front terminal and the switch, but the resistance was very low between the binding posts and the switch. I imagine something was funky because someone tried to adjust the zero on the front with a switch that was not going to zero correctly.I then proceeded to adjust referencing the K2000 and 3456A.I used values from the 3456A for 100 and 1000 ohm while the rest comes from the K2000. I have more confidence in the 3456A for the low ranges as the Keithley uses a quite "drifty" resistor to set the current for the low ranges, and I'm quite sure mine is a bit off.I'm not too happy with the 10M "transfer" i get a lot of noise on the K2000 I have to try again with better cablesAttached the current situation. I remeasured everything with all three meters, and for the 34401A, both 2 W and 4 W.Now the meter seems to behave normally, shoudl I now check the voltages around U201to check if this adjustment is hiding an issue with that?

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 15, 2025, 09:28:34 am
content: Congrats, I guess you already solved the problem.Root cause, as you described, and which is consistent for me, was that bad front/rear switch with following erroneous zero and 2W Ohm calibration.Sorry, that I forgot this time, to hint you upfront to this very well known switch problem, for such old instruments.I also had that problem a few years ago, and you might have found my cleaning description and failure mode analysis.If I remember correctly, the switch contact resistance for such an old, contaminated switch could reach several kOhm.But you are right, such a re adjustment could as well hide possible problems in the constant current circuit, especially when the deterioration of U201 is not yet dominant.It makes sense, to check the voltages in the schematic, and compare to proven working 34401As.Alternatively, you could simply use the 34401A as-is, until you detect real problems.Frank

author: gmac34
author_rank: Regular Contributor
timestamp: May 15, 2025, 09:38:26 am
content: Well, since I anyway probaby overpayed for this meter i might as well excract some extra entarteiment and check the voltages... I will do it wen I get back form a trip as well a redo the 1M an 10M calibration with better cables.I would like to get one of my meters calibrated, but it has been a struggle to find someone that is willing to engage with private individual here in Italy, now I have 3 meters and I fiddled inside all of them...In the meantime, thanks for the help! I wouldnt have dared adjsuting the meter otherwise

author: gmac34
author_rank: Regular Contributor
timestamp: May 23, 2025, 06:20:01 pm
content: As promised, I took the time to check the voltages (red ones are my measurements). I see only two significant deviations, in yellow. Overall, the values are quite close compared to the other units, which had a completely bad U201. However, could the low output of U201a still be an indicator?Voltages are measured in 2w mode with a short on the output (hopefully I understood correctly the conditions from the other post)

author: gmac34
author_rank: Regular Contributor
timestamp: May 23, 2025, 08:16:40 pm
content: It seems I lost the picture on the way

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 23, 2025, 08:42:33 pm
content: Quote from: gmac34 on May 23, 2025, 06:20:01 pmThank you, always very interesting to follow.This circuit and U201 A/B work perfectly fine.The deviations of the Gate voltages arise from sample variation only; maybe they assembled a different lot of FETs in your instrument, as mine is about the same vintage, but probably not the same manufacturing date.Frank

author: wraper
author_rank: Unknown
timestamp: May 23, 2025, 08:53:21 pm
content: Quote from: Kleinstein on May 13, 2025, 12:43:29 pmWouldn't that give nonlinearity within the range? From what OP said it looks like readings are off by constant percentage.

author: Psychonaut
author_rank: Newbie
timestamp: June 12, 2025, 02:13:07 pm
content: Hello!I don't know if you actually solved the problem or not, but this problem is mentioned for a certain range of multimeter serial numbers.For more information, go to the website:https://support.keysight.com/s/service-notes?language=en_USand enter the device model and serial number, this way you will learn about possible potential problems with your device.

author: gmac34
author_rank: Regular Contributor
timestamp: June 12, 2025, 08:10:52 pm
content: Well, I see two main problems addressed, one is about the stability of R201(10 M and 100 M ranges) and R202 (all other ranges)I noticed that R202 in later revision is replaced by a foil resistor, while older ones (like mine) use Metal Film.But given that I saw excessive difference on the 10 M and 1M ranges this should not be the issue.The other (34401A-11) directly involves 1 and 10M , and talks about capacitors that suck up moisture and start to leak, but does not mention exactly which one, it suggests a board swap... I will try do the "10M-ohms linearity test" to see if anything odd comes up there