Agilent 34401A repair log - Page 1
Forum section: electronics / repair

author: golf32
author_rank: Regular Contributor
timestamp: August 28, 2022, 11:11:49 pm
content: My work was getting rid of a beautiful Agilent 34401a. It was in a pile with craploads of other test equipment (most not worth taking, but if I get around to it I will post the repair log of other stuff I got). It had a tag saying that the ohms ranges were way off. I didn't write them down, but it was throwing basically every ohms range error code on self test. I started my troubleshooting by measuring the current coming out of the front panel when measuring resistance. Every range was basically zero.Some quick theory on the ohms range current source from the manual:U201-A, Q201, R201, and R202 work together to turn the reference voltage into a current which produces a precise voltage drop across the 28.57k resistor in U102D. U201-B and Q202 work together to replicate the voltage drop across the other resistors in U102-D to create a constant current. Q203-Q210, CR202, and Q211 are a protection circuit to prevent overvoltage at the input.My first port of call was the reference voltage drop from the 28.57k resistor. When I measured it, it was way out of spec. I suspected the JFET Q201, so I removed it. My transistor tester measured it as two resistors, meaning my suspicion was correct and the part was bad.Next, I bypassed the protection circuit by lifting one leg of Q202 and CR202 and jumpered them. I was still not getting the output current required and suspected Q202. I measured it out of circuit and again, it was dead. Finally after that, I was getting the correct current readings at the front panel. When I reconnected Q202 and CR202 however, the current was off. This made me suspect the protection circuit. I didn't want to troubleshoot each individual transistor, so I replaced them all. I tested the jfet Q211, and lo and behold, it was dead. I did not test any of the BJTs.Finally, after all the replaced parts, it did not error out! upon testing with my fluke resistance standard, the readings were nearly spot on. Pretty great considering the non-exact replacements and poor job at cleaning flux.I hope this helps someone in the future with this same issue. Don't trust the jfets!

author: floobydust
author_rank: Super Contributor
timestamp: August 29, 2022, 04:57:36 am
content: I believe the 34401a Ohms source does not have protection for -ve going transients, it only has protection from +ve overloads by CR202. So the JFET Q211 gets hit and does fail, or worse.Also note there is an ECN for early S/N's below 3146A30500 that made changes to the Ohms source, was R207 5k62 and CR203 4.7V zener.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 29, 2022, 06:27:31 am
content: The block of BJTs provides the protection for negative going voltage. This should reasonably work up to some -1000 V, though maybe only for a limited time with the 1 mA test current. The voltage rating of the risistors is also a bit on the low side.Very fast transients (e.g. ESD) could be a problem, as the transistors and CR203 have a limited speed.It is still odd to also have a bad Q201, as this one is quite a bit away from the input.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 29, 2022, 08:15:28 am
content: Quote from: golf32 on August 28, 2022, 11:11:49 pmHello, I completely disagree with your conclusion, and I think that you did not repair the DMM properly at all.The readings are not spot on, but completely out of specification.100Ω range is -227ppm, 10kΩ is +400ppm, 10MΩ is -2800ppm.These extreme deviations I call "still defective".Btw, you should set the 34401A to 6 digits, and always use full scale reference values, if you want to make a statement about its accuracy, which might be down in single ppm figures.I "dislike" your analysis and repair method:You use too much trial and error, instead of extensively testing all voltages in the circuit and from that find the root cause, and in turn ALL defective components.I'm saying that in a harsh manner, for several reasons.First, do not try to adjust the DMM in this condition, that would hide the faults into the calibration.Second, you still do not know, which kind of event caused the damage, and vv. you can't derive which components as well had been damaged by this event. I'm wondering, if you always have used the equivalent/correct replacement types, especially for the FETs.Third, your trial-and-error method should not be an example for others.If even Q201 REALLY was defective, how should this have happened?All the components towards the CC output must have been damaged as well, especially U201.I don't remember precisely at the moment, but the value of VGSof Q202 was crucial for correct operation, I think. Maybe it was even a selected part? See BOM.Yep, here you go:Q202 1855-0863 1 Transistor J-FET P-Chan D-Mode 27014MMBF5461SELPlease search for similar 34401A repair threads, where we had documented the correct voltages of the circuit, and retrace that for your DMM.In most cases, only U201 was defective, causing the same failures which you described initially.Frank

author: strawberry
author_rank: Super Contributor
timestamp: August 29, 2022, 04:19:26 pm
content: ohms range protection work up to +/-300VQ211 high leakage ~500Mohm or test methodMy 34401A got on repair bench for ~20ppm but 2800ppm is crime to universe

author: wje
author_rank: Contributor
timestamp: August 29, 2022, 04:35:11 pm
content: "My 34401A got on repair bench for ~20ppm but 2800ppm is crime to universe". Love it, hilarious.I really like my 34401A as a regular bench meter, but I'm currently having a crisis because my 2 Datron 4910's disagree by 2ppm, and my Solartron 7081 doesn't agree with either of them, although it's between the two. Life for us electronics guys is so hard.PS - I sent one 4910 to Fluke for calibration against their JJ array some years ago, that was way too expensive to repeat. Unless I can't stand the uncertainty anymore. And yes, I'm also a volt-nuts member.At least modern GPS disciplined frequency refs meant I sold my cesium-beam std.

author: golf32
author_rank: Regular Contributor
timestamp: August 29, 2022, 04:57:47 pm
content: I don't know how much I agree with your idea that too much trial and error is bad, but different strokes for different folks I guess. If I had the time I'd measure the voltage of every node in the circuit and do the math to figure out what's wrong, but I don't.I never did the math to check if it was in spec, I was just happy to see it close. I'm going to calibrate my resistance standard before I do anything else. It probably hasn't been calibrated in decades.I never expected the high resistance range to be entirely accurate. I did a crappy job of cleaning flux, so leakage is my first thought.I thought Q201 being dead was strange too, but it's kind of a moot point if the custom ICs are dead. Maybe I killed it with my fiddling. Haven't looked into replacements but I'd imagine it's hard.Thanks for the feedback all.

author: strawberry
author_rank: Super Contributor
timestamp: August 29, 2022, 06:49:06 pm
content: https://www.eevblog.com/forum/testgear/34401a-ohms-drift/My 34401A was calibrated being faulty and drifted again. possible reason it was dumped on ebayleft std Cynel 60/40 solder hard flux on PCB and couldnt register difference (dont have reference at some ppm range to compare with)cleaned with IPA ~99

author: Kleinstein
author_rank: Super Contributor
timestamp: August 29, 2022, 07:03:44 pm
content: There is a chane that Q201 was not bad: with Q202 not working there can be quite some current between the inputs of the U201B OP-amp and this may look like a broken Q201.Transistor testers are often not that great recognizing JFETs, expecially those with a higher threshold.Usually the 34401 does not drift very much and often is in spec even after decades. Some of the difference seen could also be from the reference resistors used. Flux residue and related leakage could explain the higher resistance ranges beiing bad. It is not a good idea to just get a new calibration, because the leakage current from flux residue can be quite variable, and may change with time and humidity. The correct way is to get it cleaned well. Some flux types may be OK, but it is hard to know upfront and can depend on the temperature profile.If one has a suitable meter for the small current, one could check if the current of the ohms current source in the 1 M and 10 M range depends on the DUT voltage. Ideally there should be not much change as this would be something like extra resistance in parallel to the DUT causing a linearity problem.Another possible problem is U201 being damaged in a way to have higher than normal input bias or offset. Changing U201 could effect the calibration noticible.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 29, 2022, 08:53:58 pm
content: Hello golf32,obviously you still didn't get the point, that your DMM is still defective.the readings are not even close, they are extremely way off. For the 10MΩ range, instead of speculating that it wouldn't be that precise, better read both specifications of the 34401 and the 5450A, and you'll see, that 2000ppm is an order of magnitude off, at least, I'd guess.if you you'd caught all the failures, and not implemented new ones by wrong Q202, your DMM would deviate not more than maybe 20ppm. That's the only math you have to do.Even the 5450A is very stable, and probably wouldn't deviate more than 20ppm in ranges up to 10MΩ. Especially 10k is ultra stable.. so definitely there's still a big fault in your 34401A.You also don't have to measure every node, and you don't have to calculate anything inside the circuit.please search for these diagrams, where me and others documented the reasonable voltages. From that you can easily derive, where your errors are residing. Especially Q202 is crucial.I expect, that the OpAmp is defective as well. That can easily be seen, if + and - are not exactly on the same potential. That's really no big effort, and requires no math at all.Another volt-nuts had a defect Q202, but the circuit dud not work properly before a correctly selected VGSwas assembled.About the method:You suspected all three JFETs to be defect, plus the HV transistor chain.as others said already, transistor tester sometimes/often, I don't know, don't handle these correctly. So I really doubt, that all are defective.A better way would have been, to properly identify really defect transistors, so you could trace the assumed over voltage event, how it digged backwards into the circuit. Just blindly replacing parts does not allow to trace that any more.Frank

author: golf32
author_rank: Regular Contributor
timestamp: August 29, 2022, 10:05:22 pm
content: Well I'll do some more poking. I believe I already measured the +- differential on the opamp, but I will see if any of my "fixes" have changed that.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 30, 2022, 09:21:54 am
content: https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/In this thread, there are many failure modes described, and you'll find this diagram with the faulty or correct voltages.The case with wrong Q202 VGSis not contained, so please search for yourself, using a similar headlineFrank

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 30, 2022, 09:36:57 am
content: i do agree with Frank, sorry..you meter is still too much off,but come on, be happy :-) and be friendly frank :-)i know you, and i know this hp meter, it is WOW nice and good and stable,and deserves respect and to be handled with white gloves on..But :it is alive, clearly found the spot and the problem, keep digging, replace with the correct parts,be sure to clean the area very good, perform all the calibrations, and then see if it can perform to spec again,a super score, i take it any time of the day

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 30, 2022, 04:14:39 pm
content: Quote from: oz2cpu on August 30, 2022, 09:36:57 amYes I know that I'm sometimes too direct.. that's my way anyhow, and nothing personal, of course.42 years ago, @ Airforce Lab, when each instrument had to be field repairable, we had to order expensive HP spare parts, 10times the regular price.Therefore, I've learnt to analyse first, and then only to replace parts on root cause find.I've been doing failur analysis this way for the last 40 years, so I will always un-like trial-and-error methods.In this case, I'm really interested, which parts really have been blown (check the replaced parts by proper methods), which further components are damaged and what was the initial event, probably.

author: strawberry
author_rank: Super Contributor
timestamp: August 30, 2022, 04:32:27 pm
content: now we know why repairs are dumpedparts cost 10x and must pay someone to do repairsnew instrument would be cheaper they sayonly f..er are those who cant afford new instrumentsless engineering "better" for industrymore cr..p brakes more money is mademore pollution on earth

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 30, 2022, 06:02:44 pm
content: >Yes I know that I'm sometimes too direct.. that's my way anyhowno problem, we love you anyways :-)>more cr..p brakes more money is made>more pollution on earthExactly, I love to score old defective equipment, play with it, repair it, use it, or sell it or trade it, when i am done playing..win win for all parts

author: Kleinstein
author_rank: Super Contributor
timestamp: August 30, 2022, 06:54:43 pm
content: The repair was not that nonsystematic: The initial guess that Q201 was bad was not out of thin air, but based on measurements - though possibly with the error in assuming that the OP-amp input current is small. That is a bit tricky. Q201 is also a relatively easy to replace part.The next step of suspecting Q202 is also not that bad - it is known to be a part that can fail, and testing outside may be easier than analysing the circuit. Even after neasuring all the votlage it is not so easy to come up with which part is broken - it may work with 1 broken part but 2 broken parts could already have many odd combinations. Especially if one happens to have a somewhat suitable P JFET at hand, swapping Q202 is not such a bad idea.replacing all the transistors of the protection is a bit odd - most broken BJTs show up as a dead short and are thus easy to find also in circuit. A test to narrow down the fault would be relatively easy. I would prefer it over unsodering all the parts.The result after the repair is not so clear. The deviations are at least suspect and deserve more tests. Even of the 3 values would be spot on, a little more testing would be a good idea.The observed difference can have severa reasons, e.g. flux, leaky relays, a leaky Q202 or Q201, a partially bad U201 (chances are it got stressed beyond specs), a damages switch chip, drift of the 34401 internal resistors or just some drift in the reference resistors. Diviations in both directions are a bit strange.

author: TaylorD93
author_rank: Regular Contributor
timestamp: August 30, 2022, 07:17:18 pm
content: well done, i scored a 34401A which was being scrapped at work with AC and Ohms failures at Calibration, and during self test.Iirc U201 was the fault about £12 for the IC and it was back up and running, for memory it buffers the 7VDC reference. I maybe mistaken. Such a great feeling of achievement when you can repair such a capable DMM for so little, when the Calibration house said it was not financially viable to repair it, so pushed us to replace it with a 34461A (imo a less reliable DMM)

author: floobydust
author_rank: Super Contributor
timestamp: August 30, 2022, 10:29:00 pm
content: To finish the repair, OP would have to dig in a bit. This is supposed to be fun, enjoy the puzzle lol. I'd be so happy to pull a 34401A out of the scrap heap.I repaired a (different) dumpster dive multimeter that ended up literally 1" away from my own garbage can, my patience running out. It ended up being one FET op-amp input bias current that measured 70nA instead of spec 1pA typ./60pA max., which caused loading and offset. When a semiconductor gets damaged and not totally wiped out due to an overload, it can be very difficult to troubleshoot. One clue I found was the leakage current varied greatly with small temperature changes of a few degrees.I would say either the new Q202 JFET is far off from the selection criteria, or U201AD706is not well. Warm it up a little (non-ESD finger touch) and see if readings move too much.JFET's will naturally read as ohmic resistance D-S and some transistor testers cannot supply enough -ve voltage to turn off the JFET, and then wrongly report the part as something else.There must be a sweet spot for the AD706 output voltage with the zener CR201A and Q202MMBF5461spec is VGS(off)+1 to +7.5V, IDSS-2.0 to -9.0mA pretty wide range off the reel.

author: golf32
author_rank: Regular Contributor
timestamp: September 01, 2022, 01:53:24 am
content: Just commenting to say that I am not abandoning this repair. I do agree that I didn't do the best job, but hey, that's how you learn. I haven't had much time to work on it, but I will over the long weekend.Rationale for replacing all the transistors in the protection circuit was that when bypassed it "worked" (which I now know is not actually in spec), so I thought the problem was somewhere in there. I also saw where I could still buy the parts so I thought, hey, I'll just replace them all instead of tracking down an individual transistor.I didn't know about transistor testers not being able to test certain JFETS. Definitely good to know.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2022, 07:09:56 am
content: If the threshold of Q202 is wrong one would expect the circuit to be not possible to turn the fet all the way off or on. So the problem would be with the lowest or higherst current settings to be likely way off. It would rather very unlikely to be just at the edge to be only a little off.As far as I see the circuit the main point is to have a threshold of less than some 5.5 V to be able to turn off and an IDss of more than some 1-2 mA. So there is quite some tolerance window.It still looks like they need some selection for the MMBF5461.The current problem with some 200 ppm off is more like a thing of leakage or an offset. A test with an increased temperature of some critical parts is a good idea. Leaking semicinductors usually get way worse when warmer. This is not guaranteed, but the normal case.Another short at cleaning is another idea - when clean an additional cleaning step should not change much. So it would help to check the reading in the 10 M range before and after to get the difference with good resolution.If possible it would also help to measure all the ohms ranges: so far it looks a bit odd with some values high and some low. Leakage current would mainly effect the 10 M range and an offset should efect all ranges in the same direction.Another point to check is the input current in the voltage mode and also for the 4 wire ohms sense Hi. Higher than nomal input bias is possible after things like ESD damage or with contamination / dirt.

author: golf32
author_rank: Regular Contributor
timestamp: September 04, 2022, 08:01:58 pm
content: I've taken some measurements and now I suspect U201. I'm seeing a differential of ~150uV on U201A and ~500uv on U201B. The first is technically within spec if this was the worst AD706 to leave the factory plus long term drift, but the second is obviously not. Neither of the outputs are pinned to a rail so this leads my to believe that the chip has excessive offset.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 05, 2022, 02:19:13 pm
content: Quote from: golf32 on September 04, 2022, 08:01:58 pmI still can only recommend a systematic approach, i.e. measuring these about 10 test points in the schematic, and writing that into the picture which I have linked, or see the unit that I have repaired:https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg3571485/#msg3571485This way only we together could identify the still existing (U201?) or new (Q202?) failures.Btw: This AD706 always fails with excessive input currents (it's a low Ibiastype), so that afterwards causes high offsets on its inputs.Your description does not necessarily imply a defective U201.Frank

author: golf32
author_rank: Regular Contributor
timestamp: September 05, 2022, 06:57:51 pm
content: It seems the Q201 transistor I used is an adequate replacement. I get a ~5.05 and ~0.505 drop across the 28.57k resistor. I don't think there's an actual specification for that (datasheet says "About").I assume for that post you linked the first value is pre-repair, and the value after the error is post repair?

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 05, 2022, 08:10:29 pm
content: Quote from: golf32 on September 05, 2022, 06:57:51 pmYes, I think I described that in my text, that the first value is faulty, the second behind the => is correct/repaired.The voltages are about ok.Which instrument did you use to measure these? One with 10M or 1GOhm input impedance?The difference between +/-, pins 5, 6 is 20mV, that seems to be a bit too high, although I measured the same, or a 10MOhm DMM could create an error.The Gate voltage of Q201 is too high, 6.4V, all others measured around 4.6V or lower.That might be an indicator, that there is a slightly excessive bias current out of pin5 of U201, which drives an additional current into node pin 1 of Q201 / 28k57.Is there a big difference of this Gate voltage when you switch through the different, upper ranges?There's a table in the description of Theory of Operation, p. 98, which of both current sources R201 or R202 is used.If you apply a short in 2W mode, and switch all ranges from 100Ohm through 1MOhm, there should be no difference in Gate voltage, as R202 only is used.In 10M and 100M range, this could change, but these both ranges should have the same Gate voltage.That could indicate an excessive current out of pin 6.Q202 seems to operate correctly.How old is your instrument? The AD706 often is going to leak after about 20years.I would replace U201, if this Gate voltage varies strongly, as described.Such an effect in this place could explain the varying big errors, including the sign, from 100Ohm over 10k to 10MOhm.You could measure and calculate the errors between your 34401A and the 5450A for each of the resistance ranges, maybe a systematic trend can be seen, like growing error from 100Ohm to 1M, and negative error for both 10M and 100M ranges.Frank

author: golf32
author_rank: Regular Contributor
timestamp: September 05, 2022, 08:35:40 pm
content: Quote from: Dr. Frank on September 05, 2022, 08:10:29 pmI used a 3478A, which has a 1M input impedance.There is no difference in Q201 gate voltage when I change ranges except for the two highest ranges (no variation between those two)The case is stamped with 26, April 2011, so not as old as I thought.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 05, 2022, 09:16:46 pm
content: 1M really? I didn't know this.. could explain these 20mV difference.This does not give a direct hint for a faulty U201.Anyhow, if you discover a systematic error trend between the 5450A and the 34401A over the ranges, I would replace AD706.I assume that you have meanwhile cleaned your soldering flux.And maybe you should also check how The Gate Voltage of Q202 changes over the ranges. If it's not the correct VGS, this could fail for different output currents. Just to make sure...Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: September 05, 2022, 09:18:55 pm
content: The higher gate voltage at Q201 should not be a problem (unless more than 7 V) - it would be just a different threshold for the FET.The slightly different gate voltage for the 10 M and 100 M range is due to used the lower current ( 17 µA vs 170 µA) case. So some difference is normal and the amount can be larger for a smaller JFET.If the calibrator can provide more good resistor values one should also check other ranges / values to get more data to work with. A problem with U201 should shift all the ranges to the same direction, though possibly more of an error for the 10 M range.An increased input bias, like an offset voltage for U201 would effect the calcibration. In circuit it would be quite difficult to detect a moderate error.

author: golf32
author_rank: Regular Contributor
timestamp: September 05, 2022, 09:45:11 pm
content: it seems there is a constant ~10-20 uA offset in the output current across all ranges. No dependence in output current vs resistance.I don't think Q201 or Q202 are the issue now. I believe that if they were, I would see a change in output current vs resistance. I will still look for the Q202 that I replaced. I built a new workbench between taking it out and now so I don't know where I put it.Seems like best course of action is to replace U201?Edit: I'll have to wait on taking vgs measurements for a while, we're having power issues due to the hot weather.

author: floobydust
author_rank: Super Contributor
timestamp: September 05, 2022, 10:56:35 pm
content: Something I might try is put a potentiometer and ammeter in series with the 34401a in Ohms mode on a manual range. You should read the Ohms source current, and hopefully the same despite the compliance voltage changing due to the resistance changing.Said another way, on 10k Ohm range expect 100uA to the max. 4V compliance (9V available) according to the service manual. That is about a 100k ohm limit. If you went down to say a 1k ohm test resistance (100mV compliance voltage) is it still tracking at 100uA?If there is a problem area, you can measure the usual op-amp /JFET voltages which should indicate what is struggling. It might give some clues.

author: golf32
author_rank: Regular Contributor
timestamp: September 06, 2022, 12:34:37 am
content: Quote from: floobydust on September 05, 2022, 10:56:35 pmI didn't use a potentiometer but I changed between decades and 1.9x decades with no change. My fluke has the option to multiply any range by 1.9. didn't see any variation in output current between decades and 1.9x decades.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 06, 2022, 07:18:51 am
content: Quote from: golf32 on September 05, 2022, 09:45:11 pmHow did you discover this?That could indicate a slight Ibiaserror of U201, as it pushes this amount of current into the switch, pin 6, I think.Frank

author: golf32
author_rank: Regular Contributor
timestamp: September 07, 2022, 01:21:02 am
content: Quote from: Dr. Frank on September 06, 2022, 07:18:51 amI put my multimeter in series with the resistance standard.

author: floobydust
author_rank: Super Contributor
timestamp: September 07, 2022, 01:30:08 am
content: Give it a curveball, put your finger on U201 heat it up.

author: golf32
author_rank: Regular Contributor
timestamp: September 11, 2022, 04:39:45 am
content: Update: I replaced U201, and have taken some measurements. I buttoned everything up and let it warm up for two hours. Measurements in the range of 10-100k ohms seem to be in spec, the 1 ohm and 1M ohm ranges are out of spec, and the 10M and 100M ranges are wildly out of spec. Kind of a relief and kind of not. It's slightly concerning that 1 ohm is out of spec when 10 and 100 are not, because they are all on the same range.secondary metrology question: how do I deal with different resolution between my multimeter and resistance standard? My resistance standard reports more sigfigs than my multimeter, so the accuracy spec changes by about 25% depending on if I match the sigfigs or not.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 11, 2022, 07:03:18 am
content: The 1 M and higher ranges are sensitve to leakage currents and the inpput bias of U201. To meet the specs they may have to include some of this in the calibration factors. So an exchange of U201 can effect the calibration for these ranges. The 100 M range is especially sensitive, because it measures with the 10 M divider in parallel to the DUT.The very high resistance from the calibrator is also a bit tricky - the cabling and possible leakage currents inside could also reduce the accuracy there. So there can also be some contribution from the calibrator. High humidity can increase the leakage currents.A higher error at 1 Ohms could be caused by thermal EMF and similar effects. Here again the use of a calibrator is not ideal, as internal relays can add some extra termal EMF. Testing very low resistors also needs a careful check of the zero. Checking an intentional 0 ohms ref also with cables in between can give an idea on how good the cables are.There is no problem with the different resolution, one can always consider an additional 0 digit. 1.0 is the same as 1.00. The absolute error part, sometimes given in digits of cause always relates to the given resolution with the specs. Usually the specs are valid for the full resolution setting. The math used to calculate the differences does not care about the number of digits. With the higher resolution DMMs the resolution limit on the display is usually not significant for the linearity and accuracy. If at all it is a small contribution to the noise. The specs of the meter include the resolution limitation.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 11, 2022, 10:31:15 am
content: Hello golf32,at first, please ignore these confusing and partly misleading explanations by Kleinstein, although he already gives some hints to your measurement and metrology problems.The most relevant topics of your measurement issues are:- resolution vs. uncertainty (accuracy)- thermal voltages / e.m.f.- leakage currents in cables- 2W vs. 4W measurement for higher valuesAll resistors of the 5450A were measured to 1ppm resolution, or 6 digits, that is what is shown on the display.That means the 1 Ohm is resolved to 1µOhm!The 34401A can also resolve 1ppm, when set to 6 digits. To achieve stable results, you should use 6 Dig Slow, and statistics.As its lowest range is 100 Ohm only, it is simply not capable to measure 1 Ohm (i.e. to 1µOhm resolution) and 10 Ohm (i.e. to 10µOhm) with sufficiently high resolution.Therefore, you only can make a useful judgement about 'accuracy' and comparing 'resolution' for 100 Ohm to 10MOhm ranges only.@ Kleinstein: For 1 Ohm and 10 Ohm, there exists DEFINITELY a resolution problem!I hope, this answers your 2nd question.For the low value resistors, 1 Ohm to 100 Ohm, you have an additional problem, which the 34401A cannot deal with directly, these are thermal voltages, on the order of µV to several tens of µV, depending especially on your test cables. These e.m.f. have to be removed by a method called 'Offset Compensation'.Btw.: That has absolutely nothing to do with the 5450A calibrator, this effect occurs for every low Ohm resistor measurement.The 34401A uses 1mA test current for the 100 Ohm and 1kOhm ranges. Its last 6th digit therefore resolves 100nV (equivalent to 100µOhm) for 100Ohm and 1µV (equivalent to 1mOhm) for 1kOhm.If you measure 1 Ohm in its 100 Ohm range, a 1µV offset voltage would create a 1mOhm error, or 1000ppm of 1 Ohm.. and that order of magnitude is exactly what you see in your table!There is a trick for the 34401A to manually apply this Offset Compensation method:With 4W , 6dig slow, manual 100 Ohm range, first connect the Input+ cable to the Input- jack of the 34401A, instead to the 5450A, and take a NULL measurement.This measures and eliminates the e.m.f. voltage at the resistor w/o the test current.Then apply the cable back to the 5450A, and you will measure the resistance w/o e.m.f.Use this method for 100, 1k, maybe also for 10k and 100k ranges. The measurements of 1 Ohm and 10Ohm will be much more precise as well, but anyhow limited by the resolution of the 34401A.The replacement of U201 evidently changed the measurement of all three resistors, (10 Ohm, 10k and 10M) so it was very probably defective.A working U201 has practically no impact on the calibration of its ranges up to 100k, because the test currents are about 106times higher than its bias current, which is on the the order of tens of pA .For 1M and 10M ranges, this might be different, as these are using 5µA and 500nA. Anyhow, when I replaced the U201 in my 34970A, also these ranges were accurate again to about 30ppm afterwards. I assume that the 1M and 10M ranges only might have to be re-calibrated in your case; the 100M range has no special calibration constant, as it's relying on the 10M range only.The 1k range is now accurate to +8ppm (what was it before?), 10k to -75ppm (+400ppm before), 100k to +29ppm, which is a good sign.If you offset compensate the 100 Ohm range , I 'd guess it will be accurate to a few tens of ppm, instead of 100ppm.For the higher ranges, 1M to 100MOhm, you have to take care about leakage currents, but regarding your cables.Again, that effect has nothing to do with the calibrator, because its own leakage currents are reduce to near zero by design, and otherwise are part of its calibration.These cheapo cables you are using, might have PVC as an insulator, which might create errors on the order of 100 to 10000ppm, when plus and minus leads are drilled. I recommend to use shielded PTFE cables instead.For 1M to 100M, you only need 2W method, to simplify your setup, and reduce leakage currents. Use short cables, and don't let them touch each other.Please repeat these measurements with the proposed methods, and lets see what happens.FrankPS: I briefly tested the 100 Ohm range on my 34401A, vs. 1, 10 and 100 Ohm from my 5450A.The complete setup revealed about 80 digits offset, which varies a bit from resistor to resistor.That is about 8µV of e.m.f. present, yielding 80ppm error for the 100 Ohm resistor, 800ppm for the 10 Ohm plus 10ppm resolution error for the 34401A, and 8000ppm plus 100ppm for the 1 Ohm.When using this manual Offset Compensation method, the readings agree to the 5450A within a few tens of ppm at most, although the 34401A was not warmed up at all.Hint: My specific 5450A might have higher or additional e.m.f. compared to other units for some reason.The basic idea for this problem stands as described, though, as well regarding the order of magnitude for this e.m.f. error.

author: artag
author_rank: Super Contributor
timestamp: September 11, 2022, 12:42:56 pm
content: Quote from: Dr. Frank on August 30, 2022, 04:14:39 pmSome people have such a way and assume their advice is so valuable that criticism won't be valid.But you took the trouble to apologise for yourself in advance. Such honesty does make a difference.

author: golf32
author_rank: Regular Contributor
timestamp: September 13, 2022, 05:25:27 am
content: Here's the measurements with nulling.You are correct about the cables, they are the standard Pomona banana leads. I can't yet justify buying a nice set of test leads (though with this I am close) so I will call this good enough.At this point I'd start to wonder about the accuracy of my resistance standard. I bought it from a garage sale, and have not have it calibrated yet. I have access to a 3458a at work, so I might bring it in one day to check on it. Definitely wouldn't overwrite the current calibration until I do have some nice leads.Thanks everyone for all the help. I'm glad to have a new addition to my bench!

author: iMo
author_rank: Super Contributor
timestamp: September 13, 2022, 07:17:12 am
content: Connect your meter via rs232 or gpib and you will get 1 more digit..PS: you should multiply the number in your "diff in ppm" by 1E6 in order to get the "diff in ppm"..

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2022, 07:51:23 am
content: For the high ohms part the thermal EMF is usually not a problem, but the insulation could, if the cable touch the table. So cheap short cables may be good enough if they are not touching anything.For the low ohms part the leakage from not so good isolation is usually not critical, but thermal EMF (e.g. from the plugs) is. One can get an idea on how bad the cables are from the error one sees just after handling the plugs. Not all cheap cables are bad in this respect.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 13, 2022, 10:03:13 am
content: Quote from: golf32 on September 13, 2022, 05:25:27 amThe 34401A also is not belonging to the highest precision class DMMs, therefore PTFE would be a sort of overkill.Assuming, that you now caught all defective components, the OHM calibration of the 34401A now seems to be messed up.10k is off in the negative direction, compared to 100, 1k and 100k ranges. 1M, 10M and 100M had a systematically increasing deviation recently, always 10 times, i.e. 275ppm, 2600, 26000ppm, as if a systematical current error would be present.But now, these 3 ranges are reading completely different, maybe due to 2W method.It could have happened on your 34401A, that it was calibrated once or several times with an increasingly deteriorated U201, i.e. the increasing current source errors would have been hidden away into the calibration.You really should check your 5450A by a calibrated 3458A, that would be great to rule out issues with the calibrator, and to perform a calibration on 34401A Ohm ranges.I published a repair thread about my 5450A. As it was (also?) stored in humid environment, the 10M, 19m and 100M resistors were initally off by 1000 to 5000ppm:https://www.eevblog.com/forum/blog/eevblog-544-fluke-5450a-resistance-calibrator-teardown/msg823545/#msg823545There you can also get an idea, how stable over time and temperature a 5450A really is (when working OK)So it would be very interesting if you would report about the outcome of your check with the 3458A, please.Frank

author: golf32
author_rank: Regular Contributor
timestamp: September 16, 2022, 01:27:59 am
content: I brought my 5450a into work and calibrated it against a 3458a. 2 hour warmup, 4w, autocal, 100NPLC, autozero, offset comp. I don't know if I set it up as well as I could have because that meter is too smart for me.The first attachment is of the 5450a. Very significant deviation in resistance, much higher than what you had recorded. I have no idea when mine was last calibrated, so comparing my numbers to yours is kind of apples to oranges. I suspect that my measurement technique was wrong. I'm not going to overwrite the current calibration.Second attachment is updated 34401a chart with new 5450a cal.

author: strawberry
author_rank: Super Contributor
timestamp: September 16, 2022, 07:11:48 am
content: PTFE might be but not shielding4core shielded cable?reading may change up to ~5ppm depending on cable placement (external factors) (depends)

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2022, 07:19:51 am
content: The readings of the calibrator don't look good, especially the 10 K case. This somewhat points to a possible problem with the calibrator. A few of the other differences are also a bit higher than the calibrators specs, but there are also contribution from the 3458 and in parts a not ideal setup (e.g. cables , shielding, guard connections). So these cases may still be OK.AFAIK the offset comp. part is working well for small resistors, but it can have problems with higher resistance (e.g. 100 K and up, maybe also 10 K ?) and may need a longer than normal delay in this case. The higher resistance tests usually don't need the offset compensation and some meters alread turn it off in those cases. Some Fluke calibrators use simulated resistance to get more available test points and this can also cause some delays / extra capacitance. So the calibrator may need more delay than just a reference resistor. So to be sure one should have also looked at the case without offset compensation for comparison.The higher resisance ranges (1 M und up) work usually better with 2 wires, as leakage is the larger problem than cable resistance.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 16, 2022, 09:32:04 am
content: Hello golf32,it seems, that you have successfully repaired your 34401A.These residual deviations may be caused by erroneous calibration of the 34401A, as U201 deteriorated more and more.Please, check its CAL COUNT.The initial is 32 (from factory), each consecutive full calibration should increase that number by about 32.To make precise measurements with the 3458A in 10k and 100k ranges, please use:OHMF 10000;APER 1;OCOMP ON;DELAY 1 for 10k and OHMF 100000;APER 1;OCOMP ON;DELAY 5 for 100k.Please also use statistics, 16 measurements, and check, that the achieved standard deviation is around 0.2ppm, i.e. 2E-3 for 10k and 20E-3 for 100k.Probably you need shielded PTFE for these ranges. 1k and below are uncritical; DELAY 0.1 is sufficient there.Explanation for these settings can be found elsewhere.The 10k range resistors of your 5450A seem to have a defect, as these increased by 4 Ohm unexpectedly. This range is usually the most stable one.These +4 Ohm can also be seen in its 100k range. Check the 1.9k and 19k resistors as well.1M to 100M ranges do not use OCOMP, and should be used with 2W only, as the cable resistance is usually below 1ppm.10M to 100M ranges might suffer from leaking (deteriorated) relays inside the 5450A, see my repair thread.As your 3458A readings are higher than the calibrated values, this is not that likely; moreover this could just simply be the drift over the years, see my table showing the 2003 vs. the 2015 values. The last calibration date may be displayed, whenever you power your 5450A on.You could use all the 1.9 times ranges, to make further checks, and to identify, which ones of the stacked resistors inside the 5450A might be defective.Anyhow, thanks for your repair and test reports about both the 34401A and the 5450A!Frank