Author Topic: HP 34401A reads a little low on AC  (Read 600 times)

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Online bdunham7

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HP 34401A reads a little low on AC
« on: September 20, 2020, 05:20:05 pm »
I have a recently acquired HP 34401A that appears to be accurate as far as I can determine on DCV and R, but reads a little low on AC.  I've read about issues with the power supply to the TRMS converter circuit, but this is so slight I'm wondering how I can test for it.

The unit looks minty fresh and clean with a bright, even display and very few marks or dings.  I disassembled it and removed the inner shields and found no burned components, no corrosion and not even any dust.  I couldn't tell it apart from brand new.  It has a CAL count of 35 (one calibration) with a CAL date of 24 AUG 1999.  The firmware is 10-05-02.

It reads about 0.2 to 0.25% low on all AC ranges, tested on three different sources at various combinations from 1VAC to 120VAC and 50Hz to 1kHz.  I don't have a source or reference accurate enough to legitimately calibrate the unit, but I do have several that are in the same range of 0.06-0.08% basic accuracy and states of calibration ranging from just back from Father Fluke to factory calibrated 10+ years ago.  The important thing is that all of those references uniformly agree to well within their specs--0.01 to 0.02%, while the HP is consistently below that by 3X the spec or so. 

I'm not terribly concerned about the actual calibration right now--I could tweak the gain constants to match up with my 8846A or actually have it calibrated--but I'm thinking there is some other single issue that is affecting the entire TRMS conversion process.  It's not yet broken enough to make it easy to troubleshoot, but I don't want to wait until something lets go. 

Any ideas?

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Offline mawyatt

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Re: HP 34401A reads a little low on AC
« Reply #1 on: September 20, 2020, 06:07:23 pm »
I have a couple old 34401A's, one is an HP and the other an Agilent. I recently purchased a Keysight 34465A, and will later do a home-brew cal and use the 34465A as a "reference".

I developed a few simple calibration PCBs to help, one includes a reasonably accurate low frequency Square-Wave reference which is generated from an accurate reference voltage (5.000VDC). The duty cycle of the SW is very closed to 50%, and guaranteed by design from a CMOS FF. The FF output is buffered with a discrete CMOS inverter (low Ron PFET and NFETs) for a lower output impedance (you could just parallel a bunch of CMOS inverters for the same effect).

So with a 5.000VDC reference, the 50% duty cycle should produce 2.500VAC rms, and the 34465A confirms this, and the 34401A's agree as well. Also have some resistor references, and testing a reference with 10, 1 and 0.1VDC outputs that's based upon an LM399 (also a REF01) and another that includes a LTZ1000 & LM399. All these are just for my home use as I've retired so don't need NIST traceability.

Edit: Just did a pair of quick readings on the 34465A and 34401A, 5.00020VDC/2.499983VAC and 5.00023VDC/2.50016VAC respectively.

Best,
« Last Edit: September 20, 2020, 07:16:36 pm by mawyatt »
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Offline mawyatt

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Re: HP 34401A reads a little low on AC
« Reply #2 on: September 20, 2020, 06:10:08 pm »
Here's the reference PCBs using the LM399 and the LTZ1000 & LM399.

Best,

Research is like a treasure hunt, you don't know where to look or what you'll find!
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Offline guenthert

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Re: HP 34401A reads a little low on AC
« Reply #3 on: September 20, 2020, 06:10:48 pm »
     Sorry, when I read 34401A and low AC readings, I thought of the obnoxious "feature" of that unit displaying a "satisfying" 0, when the voltage becomes less than 100uV as Joseph Geller published (I knew Geller Labs sadly closed shop, but I just learned that their web site went off-line too.  A copy of his findings is preserved at https://pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/20_HP_Agilent/01_HP_3400B_20MHZ_True_RMS_Voltmeter/Geller%20Labs/34401A%20AC%20zero.pdf). 

    Alas, that has nothing to do with the issue you're reporting.  To me it sounds like that unit simply drifted out of spec (3 times max. spec'ed error in 20 years wouldn't concern me too much).

 

Offline nfmax

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Re: HP 34401A reads a little low on AC
« Reply #4 on: September 20, 2020, 09:12:25 pm »
Send it out to a competent cal lab for calibration & adjustment, then you will know. If the DC ranges are as good as they look to be, its probably a good meter, and worth spending the money on calibrating. It's all closed-case, software based calibration, but the AC ranges need some high-voltage, high-frequency signals which require specialised calibration equipment.
 

Online bdunham7

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Re: HP 34401A reads a little low on AC
« Reply #5 on: September 20, 2020, 09:26:35 pm »
Send it out to a competent cal lab for calibration & adjustment, then you will know. If the DC ranges are as good as they look to be, its probably a good meter, and worth spending the money on calibrating. It's all closed-case, software based calibration, but the AC ranges need some high-voltage, high-frequency signals which require specialised calibration equipment.

The DC and ohms ranges appear well within specs.  One of the sources I have available to use is a Fluke calibrator, so I can feed it anything it needs except that 750VAC 50kHz input--that would require a newer, better calibrator.  I'm currently not needing to calibrate it--I want to know how it works, or doesn't work, as the case may be.  If it is in the beginning stages of a breakdown, then calibrating it wouldn't be all that helpful in the long run.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Online bdunham7

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Re: HP 34401A reads a little low on AC
« Reply #6 on: September 20, 2020, 09:32:54 pm »
To me it sounds like that unit simply drifted out of spec (3 times max. spec'ed error in 20 years wouldn't concern me too much).

That sounds reasonable at first, but my experience with top-tier modern (relative term here) closed-case calibration meters is that they maintain their calibration pretty well over the years unless they are damaged or something breaks.  I have a number of similar class meters that are well within their 90-day specs, or even 24-hour specs, more than a decade after their last calibration adjustment. If there is something in the TRMS circuit that is prone to drift, I'd like to find it. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Offline mawyatt

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Re: HP 34401A reads a little low on AC
« Reply #7 on: September 20, 2020, 11:35:18 pm »
To me it sounds like that unit simply drifted out of spec (3 times max. spec'ed error in 20 years wouldn't concern me too much).

That sounds reasonable at first, but my experience with top-tier modern (relative term here) closed-case calibration meters is that they maintain their calibration pretty well over the years unless they are damaged or something breaks.  I have a number of similar class meters that are well within their 90-day specs, or even 24-hour specs, more than a decade after their last calibration adjustment. If there is something in the TRMS circuit that is prone to drift, I'd like to find it.

One reason I got the Keysight 34465A was because how well these old 34401As have held up, one (Agilent) after 15 years since cal, the other (HP) even longer.

The KS34465A computes the AC RMS rather than with specialized circuits like the 34401A uses I believe, maybe someone that actually knows how these AC RMS values are created in both DMMs will comment, I certainly would like to know.

Think that if these DMMs are to be used for work rather than a hobby, then a full certified calibration is in order.

Best,
Research is like a treasure hunt, you don't know where to look or what you'll find!
~Mike
 

Offline RoadRunner

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Re: HP 34401A reads a little low on AC
« Reply #8 on: September 21, 2020, 08:46:28 am »
Hello,

Nothing to add to this post but i have question

Hello mawyatt , In your images there is a nice looking tweezers, Is this possible to know make and model of that?

Regards
 

Offline Dr. Frank

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Re: HP 34401A reads a little low on AC
« Reply #9 on: September 21, 2020, 11:43:22 am »
Hello,
I also see a problem that AC accuracy is off, but the other modes are fine.
It should be < 0.1% accurate in the ranges you described, even after many years.
Basically, the conversion is done by a TRMS chip, which has a fixed conversion factor, and the AC-attenuator resistors probably do not drift that much over time.
The DC-output from the TRMS chip (AD636 or so) is measured by the DCV part of the 34401A, and that does not drift that much either, especially as you found out, that DCV is still quite accurate.

You mentioned that you have no appropriate tools to calibrate ACV properly, so how would you know that you measure (compare) ACV properly?

As Guenthert already mentioned, there's the 'Geller-problem' with ACV on the 34401A, but also on many other DMMs with TRMS chip.
This simply means that their AC specification is valid only for signals between 5% and 120% of full scale, see footnote [4] in the specification.
If you measure consistently on 1% to 5% F.S. level, i.e. 0.5Vac on the 10V range, then you might already encounter systematically low readings (additional -0.1%) , worsening at even lower levels, and exact Null at I think < 0.1% F.S. signals. That your reading is 0.2% LOW, but not 0.2% HIGH, already indicates that this very probably explains the effect.
Make sure that you make your measurements at 100% F.S.

Distorted AC signals and similar might also be the root cause for worse accuracy, e.g. additional 0.15% for Crest Factor between 2-3.

If you can rule these effects out, then there might have happened an overload condition on the AC input attenuator network, i.e. especially on this 1MOhm divider resistors, R301, R302, see schematic, which led to an excessive drift. Maybe an overload with > 1kV occurred, which might also have damaged input circuit, C301, C302, U301. I would try to check these components.

Anyhow, if no damage can be found, but only a drift occurred, usually a new calibration will solve the problem.

Frank
« Last Edit: September 21, 2020, 02:04:28 pm by Dr. Frank »
 

Offline mawyatt

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Re: HP 34401A reads a little low on AC
« Reply #10 on: September 21, 2020, 12:43:40 pm »
Hello,

Nothing to add to this post but i have question

Hello mawyatt , In your images there is a nice looking tweezers, Is this possible to know make and model of that?

Regards

These are from Techni-Tool, they are very old with no model number but have Anti-Acid and Non-Magnetic indicated.

Best,
Research is like a treasure hunt, you don't know where to look or what you'll find!
~Mike
 

Online bdunham7

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Re: HP 34401A reads a little low on AC
« Reply #11 on: September 21, 2020, 02:14:32 pm »
You mentioned that you have no appropriate tools to calibrate ACV properly, so how would you know that you measure (compare) ACV properly?

My definition of proper tools would be currently certified NIST-traceable standards with uncertainties 5 to 10X less than the tolerances of the meter being calibrated, or whatever is called for in the HP calibration manual.  That would typically be a Fluke high-performance calibrator and the corresponding power amplifier.  I don't have anything at that level.  I also don't have anything to produce the 750V 50kHz test signal for the last step of the AC calibration sequence, so even if I had an appropriate reference meter, I wouldn't be able to do a 'proper' calibration.

I do have stable sources and several DMMs that I know are at least as accurate as the HP 34401 specs, all of which agree to well within their published tolerances.  The sources at the moment include a Fluke 5100B (repaired, not current cal), a Fluke 5200A (repaired, not current cal), a Siglent SDG2042X and for DC, a Fluke 731B and a Power Designs 5020.    Meters include an in-cal Fluke 8846A, an expired but fairly recent cal HP3455A (which also is surprisingly good) and a trio of old 8842A models that were last calibrated from 2 years to 20 years ago.  I can get 5 meters to agree on an AC signal to better than 0.02% and on DC to better than 20ppm.  The HP 34401 is also well within 20ppm of the others on DC, but consistently sits at -0.2% or so from the others on AC.

Quote
As Guenthert already mentioned, there's the 'Geller-problem' with ACV on the 34401A, but also on many other DMMs with TRMS chip.
This simply means that their AC specification is valid only for signals between 5% and 120% of full scale, see footnote [4] in the specification.

Distorted AC signals and similar might also the the root cause for worse accuracy, e.g. additional 0.15% for Crest Factor between 2-3.

I've tried this with 1V, 1.414V, 10V, 50V, 100V, sine waves from three sources, square waves, etc.  All the other meters agree and the HP34401A is very consistent, but low.  The 8842A model uses the same or similar AD converter chip, so if there were a distortion, noise or crest factor issue, I would think they would be similarly affected.  So it seems to me that the circuit is basically working well, but something is causing a scaling error somewhere. 

Quote
If you can rule these effects out, then there might have happened an overload condition on the AC input attenuator network, i.e. especially on this 1MOhm divider resistors, R301, R302, see schematic, which led to an excessive drift. Maybe an overload with > 1kV occurred, which might also have damaged input circuit, C301, C302, U301. I would try to check these components.

Anyhow, if no damage can be found, but only a drift occurred, usually a new calibration will solve the problem.

Frank

I'll check those if I can figure out exactly how that first section works or how it could fail.  It is labelled '50 kHz Flatness' and my error is the same at 50Hz and 50kHz, so I sort of ignored it.  I've attached a schematic in case anyone wants to follow along.  I also ruled out the middle gain sections because the same error occurs at all different levels.  Perhaps I should reconsider those areas.  However, I'm pretty sure the problem lies somewhere in this AC schematic.  People with more severe AC measuring problems on this model have reported leakage issues with tantalum caps and power supply issues of various sorts.  I'm reluctant to calibrate a drifting meter until I know what it is doing.  The real challenge will be tracing a signal through the circuits looking for an 0.2% error without causing that error by circuit loading. 

[attachimg=1]
[/quote]
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Online bdunham7

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Re: HP 34401A reads a little low on AC
« Reply #12 on: October 03, 2020, 03:44:37 am »
If you can rule these effects out, then there might have happened an overload condition on the AC input attenuator network, i.e. especially on this 1MOhm divider resistors, R301, R302, see schematic, which led to an excessive drift. Maybe an overload with > 1kV occurred, which might also have damaged input circuit, C301, C302, U301. I would try to check these components.

After looking at the circuit and how the ranges work (see attached) I took the following measurements:

Compared 5VAC, 500mVAC, 50mVAC 1kHz nominal output from SDG2042X, both set to 20Hz filter

Fluke 8846A: 4.9974, 0.49782, .049594
HP34401A: 4.9880, 0.49687, .049507

Error from 8846A -- 5V: -0.188%, 500mV: -0.191%, 50mV -.175%


So I have a pretty consistent -0.19% error.  The maximum error at these test levels should be about +/- 0.08%, and of course we usually see better than that. I tested the 100V range separately and got a similar result.  This seems to rule out anything in the 1st stage other than R301 and R302, as well as the second stages since the error is there regardless if one, both or none of the amplifiers is switched in.  I thought carefully about U307-A and the RMS converter section, but I just don't see anything there that would behave with such a constant error over both voltage and frequency, other than perhaps the AD637 itself.  So, I went back to R301 and R301 and measured them:

Measured with Fluke 8846A:      

R301: 500954R
R302: 501307R
R301+R302 1002263R


Now I have a conundrum.  If these resistors together were exactly 1M, the meter would have an apparent error against the 8846A of less than the allowable tolerances.  It would be off by perhaps +0.03%.  I actually calculated that at 1000359R, the two should match, excepting noise.  The trouble is, I don't know what the original resistances of these were, so I've no way of knowing if they've drifted somehow.  The resistors are Vishay PTF65500K00CT16, which is an epoxy-coated 500K 0.25W 0.25% 5ppm/C 500V mil-spec resistor that is still in production.

https://www.mouser.com/datasheet/2/427/ptf-1763051.pdf

So by the datasheet, they are still within their listed tolerances.  If they were those values originally, the closed-case calibration method would have just stored the appropriate gain constants and the meter would be accurate--the nominal value doesn't matter much within limits.  Just to make sure my numbers were right, I bodged on 2 parallel 1G resistors.  Sure enough, the readings now agree with the others well within listed uncertainties--although the frequency response became poor with a +10% error at 50kHz due to the parasitic capacitance of the 1G resistors at 0.375pF each.  I can get replacements of the same type with a 0.05% tolerance, which should bring it into spec.  I'm just wondering if anyone can think of anything else to look at in this circuit.

One other thing--the series input capacitor C301 is rated for 400VDC.  That doesn't seem appropriate.  Any thoughts?[attach=1]
« Last Edit: October 03, 2020, 04:03:43 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 


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