Author Topic: Agilent 34401A repair log  (Read 7417 times)

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Offline golf32Topic starter

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Re: Agilent 34401A repair log
« Reply #25 on: September 05, 2022, 08:35:40 pm »
(Attachment Link)

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?

Yes, 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.

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.

Frank

I 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.
 

Online Dr. Frank

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Re: Agilent 34401A repair log
« Reply #26 on: September 05, 2022, 09:16:46 pm »
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
« Last Edit: September 05, 2022, 09:27:06 pm by Dr. Frank »
 

Offline Kleinstein

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Re: Agilent 34401A repair log
« Reply #27 on: September 05, 2022, 09:18:55 pm »
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.
 

Offline golf32Topic starter

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Re: Agilent 34401A repair log
« Reply #28 on: September 05, 2022, 09:45:11 pm »
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.
« Last Edit: September 05, 2022, 09:55:49 pm by golf32 »
 

Offline floobydust

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Re: Agilent 34401A repair log
« Reply #29 on: September 05, 2022, 10:56:35 pm »
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.
 

Offline golf32Topic starter

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Re: Agilent 34401A repair log
« Reply #30 on: September 06, 2022, 12:34:37 am »
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.

I 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.
 

Online Dr. Frank

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Re: Agilent 34401A repair log
« Reply #31 on: September 06, 2022, 07:18:51 am »
it seems there is a constant ~10-20 uA offset in the output current across all ranges. No dependence in output current vs resistance.

...

How did you discover this?
That could indicate a slight Ibias error of U201, as it pushes this amount of current into the switch, pin 6, I think.
Frank
 

Offline golf32Topic starter

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Re: Agilent 34401A repair log
« Reply #32 on: September 07, 2022, 01:21:02 am »
it seems there is a constant ~10-20 uA offset in the output current across all ranges. No dependence in output current vs resistance.

...

How did you discover this?
That could indicate a slight Ibias error of U201, as it pushes this amount of current into the switch, pin 6, I think.
Frank

I put my multimeter in series with the resistance standard.
 

Offline floobydust

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Re: Agilent 34401A repair log
« Reply #33 on: September 07, 2022, 01:30:08 am »
Give it a curveball, put your finger on U201 heat it up.
 

Offline golf32Topic starter

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Re: Agilent 34401A repair log
« Reply #34 on: September 11, 2022, 04:39:45 am »
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.
 

Offline Kleinstein

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Re: Agilent 34401A repair log
« Reply #35 on: September 11, 2022, 07:03:18 am »
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.
 

Online Dr. Frank

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Re: Agilent 34401A repair log
« Reply #36 on: September 11, 2022, 10:31:15 am »
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 values

All 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 106 times 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.

Frank

PS: 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.



« Last Edit: September 11, 2022, 07:43:48 pm by Dr. Frank »
 
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Offline artag

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Re: Agilent 34401A repair log
« Reply #37 on: September 11, 2022, 12:42:56 pm »
Yes I know that I'm sometimes too direct.. that's my way anyhow, and nothing personal, of course.

Some 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.
 

Offline golf32Topic starter

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Re: Agilent 34401A repair log
« Reply #38 on: September 13, 2022, 05:25:27 am »
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!
 

Online iMo

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Re: Agilent 34401A repair log
« Reply #39 on: September 13, 2022, 07:17:12 am »
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"..
« Last Edit: September 13, 2022, 07:34:59 am by imo »
Readers discretion is advised..
 

Offline Kleinstein

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Re: Agilent 34401A repair log
« Reply #40 on: September 13, 2022, 07:51:23 am »
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.
 

Online Dr. Frank

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Re: Agilent 34401A repair log
« Reply #41 on: September 13, 2022, 10:03:13 am »
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!

The 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/#msg823545

There 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
« Last Edit: September 13, 2022, 10:06:31 am by Dr. Frank »
 

Offline golf32Topic starter

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Re: Agilent 34401A repair log
« Reply #42 on: September 16, 2022, 01:27:59 am »
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.
 

Offline strawberry

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Re: Agilent 34401A repair log
« Reply #43 on: September 16, 2022, 07:11:48 am »
PTFE might be but not shielding
4core shielded cable?
reading may change up to ~5ppm depending on cable placement (external factors) (depends)
« Last Edit: September 16, 2022, 07:59:36 am by strawberry »
 

Offline Kleinstein

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Re: Agilent 34401A repair log
« Reply #44 on: September 16, 2022, 07:19:51 am »
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.
 

Online Dr. Frank

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Re: Agilent 34401A repair log
« Reply #45 on: September 16, 2022, 09:32:04 am »
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 
« Last Edit: September 16, 2022, 01:16:05 pm by Dr. Frank »
 


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