Repair and experiments thread: HP 3458A U2

[TiN]

Intro

I hear you screaming, this is thread for the repair section, not metrology! Yes, yes, but hear me out on evil plan to capture the world first.
To my big surprise, my EEVBlog HP3458A repair log thread is the most popular thread in Repair section by far with 75K views after a year. Not expected result covering DMM which cost 10 grand. 

Meter I had covered in there I've got little over a year ago, September 15, 2015. Since the successful repair after January 2016 unit was in 24/7 daily operation and use, logging my voltage/resistance standards, other instruments, doing current measurements, and very little bit of AC measurements with GPIB + Raspberry Pi. I'm happy with it and it simplified volt-nuttery by a lot, thanks to it's great ACAL feature and low-noise and super linear ADC. If someone serious about metrology and volt-nuttery like we call it here, I'd strongly suggest to skip all that middle-way solution meters like 3446x/70,K7510 and similar and just go straight to 3458A. Nothing can beat calibration from single 10V and 10K reference Given that, it's not everyday generic use meter either, so it should be tied and used over GPIB, really.

Alright, cut the talk. While I have now good working 3458 with great help and support from volt-nuts community, I wanted even more and to tinker with it's circuits and design to understand analog better. However to me it was not worth the risk to solder stuff inside the working unit. Because if you try to fix something which is working - it will eventually break. No exceptions from this rule in engineering world. So there was the only solution from this point.
YES, get the SECOND unit for all the wonderful and magical experiments I wanted to try. 

But buying one of those eBay 3000$ boxes is still over my homelab budget, and I wouldn't like the idea of destroying good unit, even for sake of science, like those tarded youtube videos... We need better plan.
Which is - get one of those zombie-dead-rusty 3458's which do popup sometimes for less money. Few weeks ago, one of such happened to be listed, and it was a love from the first sight. No, I actually lied, I saw the unit from seller and considered 999$ too high for a box without digital A5 CPU board and busted case. But seller sold for less, so here we are.

Reference information and documentation

• EEVBlog HP3458A repair log thread
• xDevs.com HP 3458A Unit 1 restoration project log
• xDevs.com GPIB guide for HP 3458A
• HPAK 3458A Multimeter User’s Guide, Edition 7
• Youtube video on HP 3458A Unit 1 post-repair
• Youtube timelapse video on HP 3458A Unit 1 assembly
• 10V Comparison test of HP 3458A, 2xKeithley 2002, 4xKeithley 2001

Experimental unit, initial condition and status checks



Ouch. How these expensive instruments end up in state like this? 



Front panel is literally torn apart... And if you think those damaged binding posts are no big deal to replace, Keysight does not agree with you, with 270$ USD price tag on that 6-post therminal block.



I can see VFD in there though I was sure it's 100% shattered, as it was looking that meter had a drop on it's face. Multiple times.

Attempted to remove front panel plastic..



Every single latch and notch is busted..



And the photo which saved me 1000$...



VFD glass seem to be good, no vacuum leak. Will see if FP board still functional. Pleasant surprise, given the condition of front face.

Rear is in much better shape. Even terminal block is fine. MY-serial means unit was assembled in Malaysia, and more recent than most of 2823Axxxx boxes. No 001,002 options, but who cares anyway.



Missing GPIB connector means that there is no A5 outguard processor board.
I bought one Rev.B board from another seller, and it's currently on it's way to me for initial checks and NVRAM replacements. After which it will be installed in this unit and we could test how much life left in this project.

Transformer and initial power supply measurements revealed no problems there too.  As some might remember, my unit N1 had blown transformer from using on 230VAC mains in 110VAC switch, which cost me 337$ for replacement. No need go this route for this unit  .

And a teaser for upcoming work:



Per datecodes on IC's, it was made in 1999-2000 year, so it's fairly bit younger than my rusty friend.

Planned experiments and ideas

Here are few ideas I was tinkering about to try inside 3458A. Now since I'll have two units together, it's possible to perform direct comparisons between the "stock" unit and modified to see if there are any measurable gains, or it's all just waste of time.

* Replace current shunt resistors and current source range resistors for better TCR/stability for ohms/currents
* Replace A9 LTZ reference with multi-LTZ reference for lower noise, to see if ADC can be lowered in noise too
* Swap terminal block with triaxial ports, route OHM guarding to guard to test if that can help lower noise in OHMs/low current ranges
* Replace LDOs in various parts for lower noise modern ones
* Test various calibration things, like -10V cal, cal from 1K resistor, etc.
* Try battery power for the unit, instead of mains.
* many more

So the new box in case of successful repair would be used as guinea pig to try all the crazy stuff, which normal audience of these instruments would not risk to do.
Before doing these modifications, I'll spend few months on running unit in stock condition, to determine correlation and performance versus my reference 3458A meter.

[Nuno_pt]

Who does that to that kind of equipment?!?!?!?

TiN with a second unit this will be interesting to follow all the experimentation and mods on this DMM.

Bookmark

[Macbeth]

Ooh this is gonna be good 

[pelule]

Sound like a good plan and I am exited to have anoher interesting thread to follow.
One additional test I would have done myself, if I had a 2nd 3458A:
how does the drift differ between a 24/7 powered against a only powered time by time.
Have read many threads and available online information.
Conclusion both seems the best for lowest drift 
My personal theory: on/off has much lower drift (physics) but may have an ref hysteresis effect.

[TiN]

It's a crime to own 3458A and not have it measuring something. 

[ManateeMafia]

Looks like Dr. Frankenstein is just in time for Halloween! 

[Vgkid]

Looking forward to another one of your 3458a threads.

[pelule]

It's a crime to own 3458A and not have it measuring something.

Yes, absolutely correct - if it wouldn't be your reference and you wouldn't have another 8.5 digit precision gear like the 1281 for your daily work.

[lukier]

Let's hope the ADC chip is OK (no drift, no convergence error). 

Seems you want to beat even the HFL version

Side question to fellow Volt-Nuts. Where did you buy your 3458A? I'm not counting TiN here, because he is brave and his purchases are on the extreme side.
These meters show up on eBay once in a while, for rather high prices (4-6 k$) and often in a very poor condition (not as bad as TiN's, but still missing buttons, broken binding posts, sometimes with self test failures etc). I would be dead scared buying one of these, spending so much to find it has U180 problems and then either return (if possible) or spend another thousands of dollars on the missing parts and Keysight repair agreement (requires no missing parts AFAIR). In the worst case the total expense might reach levels close to a brand new unit.

[pelule]

I bought mine from a trustable source - another volt-nuts.

[TiN]

In the worst case the total expense might reach levels close to a brand new unit.

That can happen even after 1 month of fully functional 1-year old used unit.

That's why I consider for myself getting dead beaten-up unit for 1000$ is more fun than one of those nice looking ones for 3-5K$, as risk is about the same. And for me it's actually better, as it's easier to understand if whole thing is BER from the start. It's better to loose 1K than 5K. In the end chances are equal. Busted front panels, missing binding posts, keys, rusty chassis does not really matter and fixable with few hundred $$, but something like flaky U180 or missing A1 board is another game, where prices go high quick without much choices.

Also logistics does matter on larger items like this one, as even just to ship unit for whatever reason to/from Taiwan costs 150-200$USD. So you want to watch out and talk with seller for everything you can, to have understanding of what you are going to pay for. My rule to treat every ebay purchased item as "Dead, full repair/diagnostics required" disregarding what item condition from seller showing saved me unthinkable amounts of money (by preventing on bidding/buying sketchy "too-good-to-be-true" stuff).

[Macbeth]

Also logistics does matter on larger items like this one, as even just to ship unit for whatever reason to/from Taiwan costs 150-200$USD. So you want to watch out and talk with seller for everything you can, to have understanding of what you are going to pay for. My rule to treat every ebay purchased item as "Dead, full repair/diagnostics required" disregarding what item condition from seller showing saved me unthinkable amounts of money (by preventing on bidding/buying sketchy "too-good-to-be-true" stuff).

I agree, and there is another danger which is one thing I can't wrap my head around - the lottery of customs getting involved and charging some arbitrary tax based on what their valuation of it may be  This is despite the stuff being scrap that not only ended up being exported as e-waste and ended up smashed and bashed as landfill in a third world dump, but was eventually recovered too! Just what is going on with these "green" politicians? 

[plesa]

These meters show up on eBay once in a while, for rather high prices (4-6 k$) and often in a very poor condition (not as bad as TiN's, but still missing buttons, broken binding posts, sometimes with self test failures etc). I would be dead scared buying one of these, spending so much to find it has U180 problems and then either return (if possible) or spend another thousands of dollars on the missing parts and Keysight repair agreement (requires no missing parts AFAIR). In the worst case the total expense might reach levels close to a brand new unit.

I'll never buy meter on eBay. The error can be intermittent, if unit pass self test on picture does not mean it will pass every-time.
$2k is resonable price, but everything above is lottery.
BTW if you needs 3458 service, send it to Loveland only.

Looks like Dr. Frankenstein is just in time for Halloween! 

Mr. FrankensTiN 

[lukier]

I'll never buy meter on eBay. The error can be intermittent, if unit pass self test on picture does not mean it will pass every-time.
$2k is resonable price, but everything above is lottery.
BTW if you needs 3458 service, send it to Loveland only.

If not eBay then there are not so many options left. Liquidation auctions may be even worse, as you bid (and bids can go quite high + bidder premium + taxes) and cannot check anything. There might be a label attached/sticker and that's it, no returns. And the bidder is responsible for collection etc.

So what's left (well except a fellow volt-nut like mentioned above) are used-line companies. I don't know how trustworthy/reliable they are.

So maybe TiN's strategy is not so bad after all 

[TiN]

Yey, plesa not buying stuff on ebay, more goodies for us 
Just joking. It's boil down to how much one would like to gamble. Some people have extra 100$ to play tinkering around 20-year old ham radio, others may have 1K for rusty 3458A with missing PCBA. There are also some calibration labs and refub shops which buy these units for parts, as some of parts are not available from manufacturers anymore. These guys often the ones to fork 10-20-30K$ on stuff like faulty calibrators and high-end RF gear.

Another point of reasoning these "extreme end" items for me is to produce articles on my site. I'm not great on theory or teaching to write 20-page article about the physics and math behind precision analog designs, like Shahriar could do (I always enjoy SignalPath videos, even when they are far outside of my comfortable engineering fields), so this is another way. Get something, try to fix it and hopefully learn something.

This is the way I hope to get more tinkerers and lurkers to start contributing too, by providing example, that trying to do something is not that hard actually. But it does require investment of time and funds to make it all worthwhile. Of course ,there is no need to start with stuff like 3458A, I'd suggest trying waters first in something easier and cheaper, like those old Fluke/Keithley DMMs for 50-100$ flooded on ebay. Those are much simpler designs, yet still with lot of good (and not so good) design ideas to learn from.   

[pelule]

No risk no fun. If you request a garantee, you need to buy new products, but even than...
As allways one should be able to judge the risk - or stay away, based on the personal know-how and like TiN all your personal needs.
An example of my own:
my Datron 1281 is reborn out of two "for parts only" - I asked many detailed question before did the bidding.
Finally:
total investment $1300
~200 hrs fun on repair and cleaning.
A very very good friend, having access to a cal lab (it's not that easy to cal like the 3458A  ).
And I am "in like Flinn" a fully equipped 8.5 digit DMM - "Winner, Winner, Chicken Diner".
Would I do it again? Yes, at any time.
Is it serious? Asolutely not, I am a totally crazy volt-nut. Just ask my wife
Would I recommend other to do the same? If TiN would ask, probably yes. Never to any unknow to me.

[lukier]

I guess so, it's a matter of risk and how much one can gamble.

Just 3458A is quite peculiar (compared to most T&E gear I think) that the risk curve doesn't fall off with the price. TiN-like $1000 deals are rare and extreme, other meters selling for 3-5 k$ are only slightly less risky.

Compare that to K2001 that I recently repaired. Got it for parts/repair for just over $200, doubled that in parts and with some help here and hours spent the meter is working. "Good" ones go on eBay for $1000-$1500 and probably for twice as much from used equipment dealers. Brand new price is $4900 (Newark for example).

Maybe it is related to the fact that 3458A wasn't produced in such volumes as the other gear, so the "rarity" factor increases the price.

[quarks]

Bookmark!
I have had similar thougts when I caught the voltnut virus.
For me it is a lot of fun but besides the money, you need a lot of time and also space which I unfortunately can not expand.

[HighVoltage]

This will be a nice thread to follow again.

I am just a fresh owner of a 3458A, bought it on ebay and it was like new condition, even the screen protector foil was still on the display, warranty expired in mid of 2015. So, we can get lucky once in a while on ebay as well with such a high end instrument.

TiN, it seems you are on a path to build the ultimate 3458A, surpassing the HFL, may be.
May be one day you will start the same with a Fluke 8508A ?

[TiN]

8508A at 14920$? That is very unlikely, even if not considering worse ADC performance of Fluke meter. I mean Fluke is even too shy to list linearity in their specifications.  There are good ideas in 8508A though, like electrically controlled rear/front terminal switching to allow ratio measurements, lower 20 and 2 Ohm ranges, reduced (?) reference temperature. But nothing we can't do with bit of help using 3458.

I mean if someone has it targeted for dumpster, I'd love to take one apart, as very little known about 8508A. People who have one seem to be too busy/shy to show any results, such as LTZ log or anything like it. So I still think 30-year old 3458 has the floor. 

[pelule]

even if not considering worse ADC performance of Fluke meter. I mean Fluke is even too shy to list linearity in their specifications.

Even I am a proud owner of a Datron 1281 I have to agree to you.
My personal judgment on the 8508A (it may fully wrong): Fluke isn't too shy, they simple don't know.
The 8508A in principle is a re-designed Datron 1281. Wavetek aquired the complete IP (as far as it may have been documented that time), but the brain's, having designed it, are gone long time ago. Then Fluke later aquired it from Wavetek - two times a break in know-how.
As Wavetek stated in the 1281 manual: The instrument’s A-D converter takes the form of a highly linear, low noise, fast and flexible multislope integrator (the typical no-spec of marketing). At the specs: Excellent [meter] linearity of 0.1 ppm of full scale.
But nowhere (even not in the original Datron service manual) is stated, what the ADC linearity should be (by measurment principles it must at least 1 class better to be able to guarantee. I assume its ~10 times better, thus ~0.01 ppm - this is the usual security margin in high performance measurements).
If I would be a Fluke designer, having the mission to redesign the 1281 with modern components, how should I give a more crisp guarantee for that re-design, I just have schematics, some other paper, but no "sense behoyned". They only proofed method would be a "worst case error calculation by parts", I wouln't be able to do the "worst case error calculation by design" - this small but important difference makes my guaranteed specs worse. Only alternative: a complete new design based on my knowledge. I guess that's the reason, why Fluke has no own 8.5 digit DMM.
The "philosophy" behind the HP 3458A is the US typical definition of useability (you don't need high educated/expensive know-how for the most common tasks == customer saves costs during use and is willing to pay higher price).
Thus that "easy calibration", the most important advantage of the 3458A.
I still have the impressen (I own both DMMs), the 1281 is at same level of performance, even better in drift, AC and Ohms. But nobody is able to proof that nowadays with an acceptable/realistic amount of efforts/measurements.

[HighVoltage]

People who have one seem to be too busy/shy to show any results, such as LTZ log or anything like it. So I still think 30-year old 3458 has the floor. 

My LTZ1000A reference will go to a CAL lab next week. I will ask them to run it for 24h on a 8508A and log the data. This will also be an interesting comparison to my own 3458A.

Attached is the 8508A spec sheet with extended specifications.
Although the differences to the 3458A are not so big, why does the 8508A cost twice as much?

 

[lukier]

There are good ideas in 8508A though, like electrically controlled rear/front terminal switching to allow ratio measurements, lower 20 and 2 Ohm ranges, reduced (?) reference temperature. But nothing we can't do with bit of help using 3458.

Fun fact, 3457A has relay switched front/rear (and 30 Ohm range, but this is due to the overall ranges arrangement I suppose). I've read somewhere that HP took some inspirations from 3457A in 3458A (binding post arrangement, a lot of keys in the front for quick function access), but somehow they didn't use relays for front/rear. I guess it's because 3457A can accept a scanner card and maybe relays were not good enough for 3458A specs (but mechanical switch is? strange).

On the other hand Keithley meters accept scanner cards, but they too have mechanical front/rear switch and one has to route leads from the card to the rear banana jacks. 

BTW If you are planning to mod the frontend, esp. for improved resistance, here's an interesting paper attached.

[TiN]

pelule
I don't think Fluke metrology DMM engineers don't know what it's ADC linearity. I just think it's not told to public directly, as it would not look good versus old proven 3458A. Fluke Primary standards lab have JJA unit, and I'm sure they are more than capable to perform measurements and validation of 0.1ppm-linear ADC. Publication of such data on website though is another question.

There are often two ways to design things, one is just to use better and more expensive components with minor circuit modification, and other way to innovate new designs to improve performance in principle. But the later is way more involving and difficult, not just swapping obsolete BOM parts with modern replacements.  Of course, these are just my own wild guesses, so take with grain of salt.

Why not prove the performance of 1281? It's doable to good extent even by homelabs, if you have verified LTZ-based reference and stable resistors. I don't own 1281, but I see no reason why not to try at least run both meters together in same conditions measuring voltage reference output. Surely many readers here would enjoy seeing such a thread here in Metrology section

HighVoltage
Would be great to see data if they do sample run like that. At least we could estimate meter's noise and stability. If they could variate ambient temperature (e.g. 24C during working hours, 28C at night after shift), we can get even ballpark on tempco. But that's probably impossible, as I'd expect any serious calibration facility keep ambient temperature stable 24/7/365. One of the marketing benefits of 8508A is that it's modern design does not need ACAL feature to still meet the spec. Surely, the whole thing is filled with VPG foil networks and hybrids.

That could also explain pricing (>16K$ new, I heard). Much less popular instrument, less volume in production -> higher pricing to keep stock and maintenance. Calibration of 8508A is another level of fun, I bet. 3458A on opposite is known proven metrology standard, with strong community of many techs and engineers knowing it's caveats and issues. So it's a go to instrument when someone needs such stability and performance. I read many research papers, DAC/ADC test reports, production test setups, and almost every single one of them uses HPAK 3458A as precision measurement box.

Jeez, I sound like HPAK pays me for marketing and promotion  . Maybe one A3 PCBA would be good to settle with?  I have a feeling that will be needed..

[dr.diesel]

Much less popular instrument

I too find it surprising that with all the members here nobody has access to one laying around at work.  Must be very rare instrument indeed.

[saturnin]

I don't think Fluke metrology DMM engineers don't know what it's ADC linearity. I just think it's not told to public directly, as it would not look good versus old proven 3458A. Fluke Primary standards lab have JJA unit, and I'm sure they are more than capable to perform measurements and validation of 0.1ppm-linear ADC. Publication of such data on website though is another question.

FYI. A typical example of type testing linearity on the Fluke 8508A is shown in the following application note from Fluke (page 7):

http://support.fluke.com/Calibration-Sales/download/asset/2114953_a_w.pdf

[quarks]

I do not know why this is causing discussion here, but as an owner of several 8.5 DMMs I have made comparisions between them and the 8508A is by far the most stable instrument and does indeed do not need ACAL. Also it is within 2ppm accurate within only about 2min. For me, this alone makes it my favorite DMM, as I do only run my 10V Ref. 24/7. There are a lot more things which makes the 8508A remarkable and most likely better than any other metrology DMM available.

But of course there are downsides to a 8508A. First of all is the price, which is more in the range of 24k$ (with option1) in the US and above 20k€ in Europe and there is no artifact cal (which makes it even more pricý, because you cannot calibrate it in a homelab, even if you have the very best calibrator i.e. like my Wavetek 4808 or Fluke 5720/5730).

Therefore for hobbiest like us, there is nothing better available then a 3458A, because of its exceptional linearity, artifact cal and ACAL function (which compensates very good for temp changes). So all you need besides a working 3458A is a good enough 10VDC reference and a as stable as you can afford 10K resistor to recalibrate it any time (exception of course is AC calibration with SCAL).

[TiN]

quarks
I think this topic brought up by lack of on-hand experience on 8508A. We see many of us run number of tests with 3458A (memory brings excellent Dr.Frank test data experiments), but I hardly recall any datalogs from Fluke box. Best part of 8508A on paper for me is it's ohms features and ratio abilities if one has Opt1 with rear ports.

[ManateeMafia]

Who has the $$$ to pony up so this can be rented? 

http://www.transcat.com/fluke-8508a-reference-multimeter-for-rent

[TiN]

Alright, back on topic.

Yey, it's alive...Well, it powers on with donor A5. However, it would be too easy to repair, don't get happy too fast. See that ERR?
I guess we lucky again, dreaded ERR 114 haunting us in this box too. Last time that eaten over 200 hours... Not expecting this to be any easier, while still there is a hope that issue somewhere not in U180. But chances are slim. 





It's a balanced rundown convergence, not the multislope rundown convergence one, which I had most issues with.
Any measurement with slow slope speeds fail faster than a second. NPLC 0.001 works, so we know what to do.
ST fails ofcourse, instantly after 10V test. Can't read 10VDC gain/timeouts.





For further tests another donor A3 was used, so we can check other boards first. With that board we can get some readings on input signals, such as 5VDC and 5VAC.





But on 100VAC and 1kVAC ranges it reads junk:



Resistance checks:





Current checks:





Somebody still believed in miracles and was thinking it would be easy 2-evenings fix? 
Can't be so naive. 

[Kleinstein]

The 100 V and 1000 V range not working has at least a common part with the divider. So these faults are likely related. This fault is likely relatively easy to fix, and even if not - still a useful unit without those ranges. So this error can wait to get fixed.

I am afraid with those badly broken units from ebay or similar source, chances are high to find a bad U180, as there seem to be quite some of these boards around. At least now we know where to look for trouble. As far as I remember a more obvious point pointing to trouble with U180 was a drift in the +- reference generated with resistors inside U180. These voltage are a good test-point anyway.

Balanced rundown could also be a more general problem with the convergence - so could still be something else than U180, like the comparators.

[TiN]

Exactly what I'm thinking too, Kleinstein.

Had trigger BRC errors from bad reference +12/-12V voltages on ADC before too when I was messing up with drifty A3 on my first unit, which had no MSRC error. So that's what my hope is, that BRC is due to something else.

I don't worry about high-voltage ranges much right now either. But I don't agree on approach leaving unit half-repaired with faulty ranges. Full function repair or nothing, otherwise why bother?

So the cliff-hanger here is if and when we get ADC working properly. Rest stuff is doable relatively easily.

[TiN]

3458A A5 outguard controller PCBA (P/N 03458-66505 Rev.B, made in 1995) from eBay seller vloutlet13 arrived.
Time to swap NVRAMs on it, upgrade FW to 9.2 and have it installed into meter 
Was it a cheapie? Not quite, I was desperate, so 400$.

[TiN]

You didn't think I forgot, right?



A5 PCBA luckily is the one which I had missing photos, Rev.B. ROM is now single chip on it, rest is almost same as Rev.A. This particular one made in 1995 or so. Good enough.



No criminal on the bottom except minor delamination around RAM pins due to desoldering sick NVRAMs.

ROM is 27C4002 from ST. UV EPROM, yey! I think I'll desolder it, put socket instead and erase with my bodge-UVC lamp setup and write Rev.9 FW over. According to label, it's Rev.7 now.



Can you see firmware bits in that die photo? No? But they are there 



And of course, refresh NVRAMs   
I'll plonk the board into my rusty good 3458A to test, so we can make sure it's good and close A5 issues once and for all.
This will be first time I open 3458A since 24/7 operation from spring 2016. I'll use a chance to blow some dust out.

[TiN]

No problems encountered  .



I think we can use my old A5 in 3458U2 instead for now. Calibration not lost on digital board swap. External LTZ1000 reads 0.01ppm within pre-swap value.

[Nuno_pt]

It's starting to look promising the fixes.

Maybe few parts then first thought.

[TiN]

Em, I think you confused a bit.

This is my good 3458A, not the U2. Just check to make sure that A5 from eBay is good and has no problems. So of course it's self-test passed.
Doesn't mean that we will see this message on 3458-U2 any time soon though.

[TiN]

Alright, seems no major issues with A1 yet, with use of good A3 board as temporary swap.



This project will soon turn into long-waiting one though, to get A3 replacement... I still can use drifter-A3 to do some tinkering though. 

[TiN]

Btw, in case someone wondered what A3 boards are installed in new meters or serviced in.

Here's photo of one from KS site:



Obviously, Keysight ran out of the control gate arrays chips for A3, so they bodged in patch board with ALTERA CPLD with bunch of 3.3V-5V level translators and pair of LDOs on top of the old board footprint. 
Mark my word, in few years from now this A3 board will be even more patched up. 

[ManateeMafia]

There's almost enough room for a Pi Zero. 

Looks like they are battling to get a replacement meter to market before it's too late.

[plesa]

Mark my word, in few years from now this A3 board will be even more patched up. 

For 27 years old instrument it is quite good, I must say. A5 board is already redesigned.
It is good start to design new instrument   With production between 700-1000 units per year the amount of components and investment is still in acceptable level until U180 became obsolete and not available....
I'm still curios how long they are going to support 3458A ( in EU are min 5-7 years from production stop).
The info on website is also good and give opportunity to order 3458A in Europe after RoHS deadline
Notice for European Union Customers: This product is not updated to be EU RoHS compliant. Please contact Keysight Authorized Distributors for quotation and ordering. Keysight will continue service and support to the end of worldwide support life.

[VintageNut]

There's almost enough room for a Pi Zero. 

Looks like they are battling to get a replacement meter to market before it's too late.

If you read the volt-nut email group, one of the participants states that all of the HP brains that designed the 3458A are no longer at KS. 

[saturnin]

Keysight has been working on a successor of 3458A for quite a while and it is a real challenge for them as they mentioned in this webcast (listen from 46:00 to 47:40):

http://www.keysight.com/main/eventDetail.jspx?cc=GB&lc=eng&ckey=2689915&nid=-11143.0.00&id=2689915

[VintageNut]

Keysight has been working on a successor of 3458A for quite a while and it is a real challenge for them as they mentioned in this webcast (listen from 46:00 to 47:40):

http://www.keysight.com/main/eventDetail.jspx?cc=GB&lc=eng&ckey=2689915&nid=-11143.0.00&id=2689915

Thank you for the link. I listened to that segment. I heard them say that they were trying to make a better one and have not been successful yet. I did not hear that they were designing a replacement for the 3458A.

In fact, they never said that they have designed a new up-to-date style instrument that is as good as the 3458A.

[Dr. Frank]

Keysight has been working on a successor of 3458A for quite a while and it is a real challenge for them as they mentioned in this webcast (listen from 46:00 to 47:40):

http://www.keysight.com/main/eventDetail.jspx?cc=GB&lc=eng&ckey=2689915&nid=-11143.0.00&id=2689915

I know that passage very well, guess who this "Frank" was, who posed that question..
My question was about an updated/improved specifications for the existing instrument, as it's known, that many improvements had been done (40k reference for example).
They answered in that manner, I think they did not really catch my intention.

I also was under the impression, that they were NOT working on a successor, and their statements even emphasized my idea, that they in fact do not have budget, brain and knowledge to even make an updated copy of that design.
Very probably, there  is no market at all for such an instrument, not to talk about the R&D cost, that will never be rewarded, even over many years.

I asked Scott Stever directly (former PM for the 34465/470A and engineer on the 3458A), and he simply refused to make any word about plans of re-design... at that instance, I had the very same feeling.

For me as a PTN manager in a big electronics company , it is really a miracle how they are still able to produce that unit with all these ancient component technologies and so many obsolete parts.

Frank

[VintageNut]

Keysight has been working on a successor of 3458A for quite a while and it is a real challenge for them as they mentioned in this webcast (listen from 46:00 to 47:40):

http://www.keysight.com/main/eventDetail.jspx?cc=GB&lc=eng&ckey=2689915&nid=-11143.0.00&id=2689915

I know that passage very well, guess who this "Frank" was, who posed that question..
My question was about an updated/improved specifications for the existing instrument, as it's known, that many improvements had been done (40k reference for example).
They answered in that manner, I think they did not really catch my intention.

I also was under the impression, that they were NOT working on a successor, and their statements even emphasized my idea, that they in fact do not have budget, brain and knowledge to even make an updated copy of that design.
Very probably, there  is no market at all for such an instrument, not to talk about the R&D cost, that will never be rewarded, even over many years.

I asked Scott Stever directly (former PM for the 34465/470A and engineer on the 3458A), and he simply refused to make any word about plans of re-design... at that instance, I had the very same feeling.

For me as a PTN manager in a big electronics company , it is really a miracle how they are still able to produce that unit with all these ancient component technologies and so many obsolete parts.

Frank

I knew that was you. I agree with everything you state except the size of the market. It is a very large market for just that one instrument.

[plesa]

It is a very large market for just that one instrument.

With 700-1000 units per year...?? We will see... But it looks that market for 8.5 DMM is small.

[Dr. Frank]

I knew that was you. I agree with everything you state except the size of the market. It is a very large market for just that one instrument.

That's to be discussed .. let's go:

They usually sold / still sell about 2000 units per year, that's about 20M$ turnover/year.
The BOM cost is quite high, as would be a re-design, with probably improved features, also HW ones.
I guess that it needs a decade to make the return-on-invest.
KS nowadays is completely cost driven, whereas in the 1980ties, the Cold War financed each of such exceptional projects.

So I really doubt that..
Other estimates and opinions, please!

Frank

[dr.diesel]

We'll have to rely on Keithley, <4PPM DMM8510, 

[plesa]

I knew that was you. I agree with everything you state except the size of the market. It is a very large market for just that one instrument.

That's to be discussed .. let's go:

They usually sold / still sell about 2000 units per year, that's about 20M$ turnover/year.
The BOM cost is quite high, as would be a re-design, with probably improved features, also HW ones.
I guess that it needs a decade to make the retrun-on-invest.
KS nowadays is completely cost driven, whereas in the 1980ties, the Cold War financed each of such exceptional projects.

So I really doubt that..
Other estimates and opinions, please!

Frank

I will reduce the amount of units sold per year, based on serial numbers it is less.
If I will be in marketing department I will try to support current 3458A as long as possible. Thats what we can see (A3 piggy back boards, A5 redesign).
That require relatively small amount of money than investing into the completely new product from scratch.
I suppose that everything depends on availability and manufacturing process for U180 ASIC.
In Europe it can be tricky next year, because unit is not RoHS compliant, but we do not know what is not RoHS compliant.
It it is components like VFD, it is possible to make replacement  and make 3458A available in Europe again. Maybe in EU they will sell used units only
But if non RoHS components is for example U180, it can be difficult.

Sorry for off topic.

[saturnin]

I know that passage very well, guess who this "Frank" was, who posed that question.

I thought so 

Very probably, there  is no market at all for such an instrument, not to talk about the R&D cost, that will never be rewarded, even over many years.

Well, 3458A is an industry standard. It covers a wide variety of applications (exactly as it was designed, i.e. to support applications in metrology (slow, but precise measurements) and at the same time to serve as fast system multimeter in production - all in one box), so imho market is not so small. 3458A is at the end of its life (see how many its internal components become obsolete), so it really makes sense to develop a new product. With a new product, they can ask more money. Although they didn't say it explicitly, they clearly stated they have been trying to do better than that (i.e. 3458A design). If you are already investing time&money in such research/development, you really want to come with a new product otherwise you become less and less competitive...

Keysight has one big advantage. Once 3458A successor is developed, only they can state it is 1:1 replacement of the older design (as they did it with 34401A->34461A upgrade). Only Keysight can guarantee full backward compatibility. And this is really important in industry applications - nobody wants to risk a loss of production due to an error in rewritten scripts for ATE...

I asked Scott Stever directly (former PM for the 34465/470A and engineer on the 3458A), and he simply refused to make any word about plans of re-design... at that instance, I had the very same feeling.

I don't know what relationships you have with him , but I would guess he simply couldn't say anything. Actually, it is probably a business secret...

For me as a PTN manager in a big electronics company , it is really a miracle how they are still able to produce that unit with all these ancient component technologies and so many obsolete parts.

I fully agree. As I wrote, it is one of reasons to move forward...

[VintageNut]

I knew that was you. I agree with everything you state except the size of the market. It is a very large market for just that one instrument.

That's to be discussed .. let's go:

They usually sold / still sell about 2000 units per year, that's about 20M$ turnover/year.
The BOM cost is quite high, as would be a re-design, with probably improved features, also HW ones.
I guess that it needs a decade to make the retrun-on-invest.
KS nowadays is completely cost driven, whereas in the 1980ties, the Cold War financed each of such exceptional projects.

So I really doubt that..
Other estimates and opinions, please!

Frank

$20M/yr is a lot of money for one instrument no matter any of your other points. There are many instruments in production that reap less then $20M/yr

[krivx]

I knew that was you. I agree with everything you state except the size of the market. It is a very large market for just that one instrument.

That's to be discussed .. let's go:

They usually sold / still sell about 2000 units per year, that's about 20M$ turnover/year.
The BOM cost is quite high, as would be a re-design, with probably improved features, also HW ones.
I guess that it needs a decade to make the retrun-on-invest.
KS nowadays is completely cost driven, whereas in the 1980ties, the Cold War financed each of such exceptional projects.

So I really doubt that..
Other estimates and opinions, please!

Frank

$20M/yr is a lot of money for one instrument no matter any of your other points. There are many instruments in production that reap less then $20M/yr

That's turnover, not profit. We're in the dark as to how much they make on each sale, and how that compares to other instruments.

[Dr. Frank]

In Europe it can be tricky next year, because unit is not RoHS compliant, but we do not know what is not RoHS compliant.

Very probably, it's simply the solder.
In automotive industry, we had to switch over to lead-free a few years ago, and that was the main issue with RoHS; other substances like Cadmium or Cr(VI) were also banned, but I think that happened already earlier. I don't think, that they used Cd alloy solder for lower thermocouple voltages.

And it's clear, that changing to lead free solder and components requires a complete redesign and re-validation.
Some special components were not available in lead free, I guess.

Frank

[TiN]

Also recently it was discovered that online shop on Linear site does not allow you to add non-Pbfree parts into cart, including LTZ1000CH/LTZ1000ACH and others hermetic metal packages I tried. #PBF part - no problem, non-PBF - page refreshed and nothing get's added

[VintageNut]

If there was no profit, they would not sell it.

I knew that was you. I agree with everything you state except the size of the market. It is a very large market for just that one instrument.

That's to be discussed .. let's go:

They usually sold / still sell about 2000 units per year, that's about 20M$ turnover/year.
The BOM cost is quite high, as would be a re-design, with probably improved features, also HW ones.
I guess that it needs a decade to make the retrun-on-invest.
KS nowadays is completely cost driven, whereas in the 1980ties, the Cold War financed each of such exceptional projects.

So I really doubt that..
Other estimates and opinions, please!

Frank

$20M/yr is a lot of money for one instrument no matter any of your other points. There are many instruments in production that reap less then $20M/yr

That's turnover, not profit. We're in the dark as to how much they make on each sale, and how that compares to other instruments.

[mimmus78]

Lock what I found on ebay!!! It's incredible!!! Never seen one of this before, it should be an experimental/prototype unit.

[TiN]

Happy holidays to every -nut  .

I'll celebrate with good DCV results on my 3458B so far. After ADC replacement, ran SN18 test for few days, to confirm board normal function.
Then 5 days meter calibrated to 10.0000V using DC standard. CAL passed w/o issues, so time to rerun SN18 with correct gain and get some initial 10V data to look at.

CAL? 72 monitoring:

Date/time	Meter temperature	CAL? 72	CAL? 1,1	CAL? 2,1	Deviation, ppm
21.DEC.2016 09:06	35.5	983.331695E-03	39.9991607E+03	7.07037491	Reference, 0.00
21.DEC.2016 20:48	36.2	983.331710E-03	+0.0153 (+0.0153)
22.DEC.2016 08:26	36.2	983.331676E-03	-0.0346 (-0.0193)
23.DEC.2016 00:52	35.6	983.331701E-03	+0.0254 (+0.0061)
23.DEC.2016 08:30	36.0	983.331641E-03	-0.0610 (-0.0549)
24.DEC.2016 17:42	36.2	983.331664E-03	+0.0234 (-0.0315)
24.DEC.2016 18:16	36.1	983.331626E-03	-0.0386 (-0.0702)
24.DEC.2016 23:21	35.5	983.331697E-03	+0.0722 (+0.0020)
25.DEC.2016 16:45	36.6	983.331594E-03	-0.1047 (-0.1027)

So far looking good, stable and behaving. 10V reading stable within 0.1ppm over 10 minutes (transfer spec) and indeed stable better than 0.3 ppm window over >24 hours (24h is 0.55ppm spec).
Room is airconditioned, so to make test more challenging, there are temperature oscillations below 1C. Still meter shows good enough data. 

10V datalog:



Live data (may take a while to draw plot, many points

RAW 10V from 732B Data for those who feel bored during holiday...

And you see this only on EEVBlog (well, and xDevs.com - how the 16 week 2016 year 3458A A3 board looks like today..
Perhaps all new 3458A from Keysight have this version of board now.



Old gate array logic chip is now replaced with a patch PCB, with ALTERA MAX CPLD in TQFP144, some 245 bus transievers and few LDOs. Transievers are due to conversion of 3.3V CMOS levels into 5V TTL for rest of the A3. More parts get obsolete - more patch boards we see..



I cooked little comparison table of common 3458A A3 PCBA versions:

A3 PCBA Version SKU	03458-66503 Rev.A	03458-66503 Rev.C	03458-66503 Rev.D	03458-66513 Rev.A	03458-66513 Rev.B
Board photograph
First released	1988	1989	~1995	2000	2015
U180 Hybrid	Same package and footprint	Same package and footprint	Same package and footprint	Same package and footprint	Same package and footprint
Interface MCU 8051	Mask 03458-85501	Mask 03458-85501	Mask 03458-85501	Rev.2 Mask , Intel 8051	Rev.2 Mask , ATMEL 8051
U210 Gate array ASIC	Fujutsu MB651314 PGA	Fujutsu MB651314 PGA	Fujutsu MB651314 PGA	HP 1820-5770 Orbit 6968A PGA	Patch PCB 03458-26550 with ALTERA CPLD
Optical interface	HP 1005-0097/HP-1005-0096	HP 1005-0097/HP-1005-0096	HP 1005-0097/HP-1005-0096	Avago HFBR-2521Z/Avago HFBR-1521Z	Avago HFBR-2521Z/Avago HFBR-1521Z
Integrating cap	MLCC 50V	50V black plastic NP0	50V black plastic NP0	yellow coated MLCC	yellow coated MLCC
U213 position	74F112PC	HP 1820-2924 IC	Jumper pin 8 - pin 10	Jumper pin 8 - pin 10	Jumper pin 8 - pin 10
ADC U142/U405 comparator	EL2018CN DIP8 (HP 1826-1817)	EL2018CN DIP8 (HP 1826-1817)	EL2018CN DIP8 (HP 1826-1817)	Patch PCB 03458-66530 with EL2251CM	Patch PCB 03458-66530 with EL2251CM
U404 ramp opamp	Hermetic LF400C	Plastic LT1122	Plastic LT1122	Plastic LT1122	Plastic LT1122
C401,Q402	Hermetic metal can	Plastic	Plastic	Plastic	Plastic
C146,C147 capacitors	None	Present	Present	Present	Present

[mimmus78]

Nice. When I finish my honeymoon with my 3458a I will open it and check how looks like. Also my unit should have a new A3 board too.

Inviato dal mio Nexus 6P utilizzando Tapatalk

[guenthert]

If there was no profit, they would not sell it.

While I have no doubts that they make some profit on the unit, this isn't necessarily the case with large companies.  It might be of 'strategic' value and funded by their marketing department.  They will make much more profit on the many lower spec'ed meters, but want to be able to say 'yes, we can' to large customers which rather deal just with one vendor.

[MadTux]

Perhaps a slightly stupid question, but that MLCC Integrating capacitor mentioned above isn't the one that does the voltage integration in the multislope ADC process?

Just wondering, don't have a 3458A (much too expensive), but know the 3456a quite well and there they always used foil capactors if they needed accurate and stable ones (e.g. the C403  Integrating cap is a foil cap). If that is indeed the integrating cap, what are the reason for using an IMO rather poor (leakage, piezoelectric microphonics, thermal drift, dissipation factor ...) ceramic cap instead of e.g PTFE foil cap?

[TiN]

Nothing stupid in your question. You correct about purpose of integrating cap. 3456A using much slower cycle and ramp times than 3458A does, so it's integrating cap much bigger in value, and need to be stable during integration time period. 3458A's ADC however is much faster, so integrating capacitor is only 330 pF. So C0G/NP0 Type I ceramic was used for it, as it's stable and much less problematic than regular Type II ceramic dielectrics. This is all to achieve max speed 100 KSPS which 3458A's ADC capable of.

I suggest great read about 3458A design in HP Journal 1989-04, where you can learn more about this instrument.

[CalMachine]

I have an idea on how to improve upon the 3458A ADC.  This idea is in it's infancy still, and I've been doing a lot of research into the subject to perfect my understanding on how to implement my idea!  If it works out, we could see DMMs with a few orders of magnitude better specs.

[Pipelie]

I have an idea on how to improve upon the 3458A ADC.  This idea is in it's infancy still, and I've been doing a lot of research into the subject to perfect my understanding on how to implement my idea!  If it works out, we could see DMMs with a few orders of magnitude better specs.

Looking forward. 

[guenthert]

I have an idea on how to improve upon the 3458A ADC.  This idea is in it's infancy still, and I've been doing a lot of research into the subject to perfect my understanding on how to implement my idea!  If it works out, we could see DMMs with a few orders of magnitude better specs.

An improved ADC can yield higher (finer) resolution and I would be surprised, if there aren't better ADCs around than the one in the HP3458a designed about 30 years ago (surely there are ADCs with higher resolution, not sure about better linearity).  Improving precision and accuracy of the overall DMM will be quite tricky though.  Hint: ask yourself what voltage the input voltage is compared to in order to yield the reported result ...

[Echo88]

The Linearity of the 3458A is, as far as i know, still unrivaled by any other DMM. Resolution means nothing, if theres not the Linearity to back it up. Apart from the fact that it needs a JJA and really good Nullvoltmeter to even test a selfbuilt 8.5-DMM-ADC which states that its as good or better than the Multislope-ADC in the 3458A.

Massive analog knowledge is what it takes to design this stuff, not new parts with better specs than their counterparts 30 years ago.

[Kleinstein]

The ADC of the 3458 is a compromise in getting high speed readings (e.g. 100 kHz) and super high resolution / linearity from the same circuit. So it is a little strange to have it as still one of the best. Today there is little need to use the same ADC for high speed readings too, as there are superior fast ADCs available (LTC2368 and similar). So it is mainly the slow high linearity part that might need to be solved better. In my view the ADC type used in NI's DMM cards with continuous integration and a separate ADC for the residual charge would be a good bet.

Still I don't think there will be so much improvement. There is noise of the reference voltage as an important source - one would need to improve there too, to have lower noise not only for a zero reading. Also many signal sources have at least comparable noise - so the only thing to do there would be to reduce the noise bandwidth by having 2 amplifiers + ADCs and do a kind of commutating auto zero - so not missing any of the signal. So this is an obvious possible factor of 2 in speed - but at a price.

Linearity at the ppb level is rather difficult - hard to model and hard to measure. Though there is a relatively easy way to check for the two most important parts of nonlinearity. DNL via the histogram and the large range INL sampling via adding voltage sources. There just is no way one can fully test a slow 40 Bit ADC - only sample testing is possible.

The more practical path would be more a smaller, cheaper solution to comes close to the 3458 performance (at longer integration times) at lower power.

There are a few modern parts that are now available that can help to make a better ADC: one are good integrated ADC to get something like 12-16 Bit readings in the MSPS range and the other is more processing power to get a more intelligent control / adjustment. So no more limit to 14 steps up and 2 steps down. Also analog simulations are much better today.

[CalMachine]

I wasn't trying to spill my guts here... but I am not sure if my idea will even increase performance of the ADC, it's just a hunch.   What I noticed while reading the specs of a lot of long scale DMMs which specifically used integrating ADC was how poor the oscillator accuracy was.  My idea is to use a GPS disciplined double ovenized oscillator doing my timing measurements of the ramp up and ramp down phases to get a more accurate time measurement with greater resolution, as well as clock references for everything.  I am also wanting to try to implement these new 2DW232 zener references  as voltage reference.

[Echo88]

I cannot comment on the clock reference, i assume if a more stable clock reference would better the 3458A specs they would have used a OCXO (since those are available since ages). As far as i read nobody has yet determined how much those 2DW232 drift over time; im more for the concept of clever filtering the LTZ1000 if needed: http://electronicdesign.com/energy/filter-trims-ultra-precision-voltage-reference

[Andreas]

I cannot comment on the clock reference, i assume if a more stable clock reference would better the 3458A specs they would have used a OCXO (since those are available since ages). As far as i read nobody has yet determined how much those 2DW232 drift over time; im more for the concept of clever filtering the LTZ1000 if needed: http://electronicdesign.com/energy/filter-trims-ultra-precision-voltage-reference

Hello,

why should the clock stability be more critical than short term stability of the integration capacitor?
Why do you think that you can filter a reference with 1.2uVpp by a chopper OP with 1.5uVpp.

The best filter for noise will be a long integration time >=100NPLC.
Of course you have to sort out the references with popcorn noise.

And in my opinion the (symmetry of) charge injection of the integrator switches is more a issue than clock stability.

with best regards

Andreas

[Echo88]

Who said that one cant use better Chopper OPs than MAX4238, for example LTC2057? For course it all has to be simulated and built in the end to test it.

[Theboel]

Integrating an GPSDO in 3458A can cause a more trouble than benefit
I do try use GPSDO to lock a stereo generator for FM transmitter and I found there is a lot of worse result in phase noise.
if I have 3458A and I need to keep it accuracy I will do 3 thing :
1. use 4 LTZ1000 in paralel to bring the reference noise down
2. Use temperature control system to keep everything inside has a stable and low humidity especially reference and ADC (yes I know about ACAL but if I keep everything 23 degree with 1/10 degree variation there is nothing wrong ?)
3. Better PSU and filtering.
 

[CalMachine]

Integrating an GPSDO in 3458A can cause a more trouble than benefit
I do try use GPSDO to lock a stereo generator for FM transmitter and I found there is a lot of worse result in phase noise.
if I have 3458A and I need to keep it accuracy I will do 3 thing :
1. use 4 LTZ1000 in paralel to bring the reference noise down
2. Use temperature control system to keep everything inside has a stable and low humidity especially reference and ADC (yes I know about ACAL but if I keep everything 23 degree with 1/10 degree variation there is nothing wrong ?)
3. Better PSU and filtering.
 

How so?  With my 53132A I can resolve 12.5 digits with the last digit fluctuating maybe 3 or 4 with my GPS reference.  That's a 12 digit stable measurement with a 1 second gate time.  This multimeter would be designed for metrology purposes, so the super fast integrating capabilities of the 3458A ADC I could care less about.  I'm aiming for like a 1 second-ish, user selectable, integration time in my design.  If you can more precisely control and time the ramp up phase as well as measuring the ramp down phase with much much greater precision and resolution... can't you then, with better certainty, determine the input signal with simple Tu/Td ratio?  Most component discrepancies are canceled out with a dual/multi slope method.

[Theboel]

How so?  With my 53132A I can resolve 12.5 digits with the last digit fluctuating maybe 3 or 4 with my GPS reference.  That's a 12 digit stable measurement with a 1 second gate time.  This multimeter would be designed for metrology purposes, so the super fast integrating capabilities of the 3458A ADC I could care less about.  I'm aiming for like a 1 second-ish, user selectable, integration time in my design.  If you can more precisely control and time the ramp up phase as well as measuring the ramp down phase with much much greater precision and resolution... can't you then, with better certainty, determine the input signal with simple Tu/Td ratio?  Most component discrepancies are canceled out with a dual/multi slope method.
[/quote]

I think its related to phase noise caused by PLL use in my GPSDO.
if you like You can try (I assume You already has 3458A) find a suitable GPSDO with a programmable output like this
http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=234
and then see how the noise level.
in my kindergarten knowledge how DMM works I still not understand what is more important low phase noise or stability of the frequency ?
if the low phase noise better to use a low phase noise OCXO like WENZEL ULN (perhaps a custom made for 3458A frequency need)   

[CalMachine]

How so?  With my 53132A I can resolve 12.5 digits with the last digit fluctuating maybe 3 or 4 with my GPS reference.  That's a 12 digit stable measurement with a 1 second gate time.  This multimeter would be designed for metrology purposes, so the super fast integrating capabilities of the 3458A ADC I could care less about.  I'm aiming for like a 1 second-ish, user selectable, integration time in my design.  If you can more precisely control and time the ramp up phase as well as measuring the ramp down phase with much much greater precision and resolution... can't you then, with better certainty, determine the input signal with simple Tu/Td ratio?  Most component discrepancies are canceled out with a dual/multi slope method.

I think its related to phase noise caused by PLL use in my GPSDO.
if you like You can try (I assume You already has 3458A) find a suitable GPSDO with a programmable output like this
http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=234
and then see how the noise level.
in my kindergarten knowledge how DMM works I still not understand what is more important low phase noise or stability of the frequency ?
if the low phase noise better to use a low phase noise OCXO like WENZEL ULN (perhaps a custom made for 3458A frequency need)   

Ahh I gotcha.  I'll definitely look into phase noise effects.

I've got 2 3458As and a 1281, as well as 2 Trimble ThunderBolt E GPS references I can use for testing purposes.

[Kleinstein]

Filtering of the reference noise is possible for the higher frequency range (e.g kHz), where many DMMs are also somewhat sensitive, but this not such an important issue and many DMMs don't even use filtering here.
For the below 1 Hz range filtering is not practical, just because resistors / caps get to large. However it is possible to use separate references for the short time (e.g. up to hours / days) and long time ( > days) time scale. Many DMMs have that naturally (though they usually still use the same ref.): one for the ADC and one to measure directly in the 10V range to adjust the gain. So a low noise DW232 could be combined with a log time stable LTZ1000. The Keithley 2001 goes the way of two references - though I am not sure it is implemented very well.

Comparing ref and AZ OPs noise is difficult, as AZ noise is more or less white, while the reference noise usually has quite some 1/f component. So in the 0.1 to 1 Hz range things are much more in favor of the AZ-OPs.

Clock Jitter is the least problem - so a normal crystal clock should be good enough even for 9 digits. On the critical short time scale a GPSDO is not better than a crystal anyway, as they internally use a crystal as well. The kind of phase noise such an integrating ADC is sensitive is more like phase modulation in the 10s of kHz - not frequency drift in the 10s of ms range. The problem is not so much measuring the time, it is more about parasitic currents and nonlinearity due to higher order effects like output voltage dependent bandwidth, ringing after fast switching or nonlinear resistors and just simple current noise of BJT based OPs. Also nonlinearity and dielectric absorption in the integrating capacitance can become an issue - this is why such a small capacitance is preferred, despite of more frequent reference current switching and thus higher sensitivity to clock jitter.

[Theboel]

according to "High resolution digitizing techniques with an integrating digital multimeter by David a czenkusch" You can read it with "HP 3458A Component-Level Information Packet.pdf"
3458A use 10Mhz as reference and the clock has less than 150PS at 10KHz jitter if somebody can measure this and compare to better clock we can now find out what the result with a better clock.
the other thing in the paper are talking about are capacitor accuracy 0.001pF 
so my list now get longer :
1. use 4 LTZ1000 (or more) in paralel to bring the reference noise down
2. Use temperature control system to keep everything inside has a stable and low humidity especially reference and ADC (yes I know about ACAL but if I keep everything 23 degree with 1/10 degree variation there is nothing wrong ?)
3. Better PSU and filtering.
4. Better clock
5. Better capacitor divider network.
sorry if I made mistake take my opinion as dummies thinking about the "king of the hill" like TiN said

[TiN]

CalMachine

I'm skeptical about "magnitudes" of improvements possible in any key performance aspects. Even for testing alone, hope you get that programmable JJA system in place, as that's pretty much only way to really split beans during testing whatever ideas we try. As was already mentioned, even if we design and get 9.5 digits, they mean nothing (other words - just noise) without linearity and stability to backup. And testing sub-ppm linearity is already hard enough, even with PJVS.

Having that said, replacement of onboard oscillator with more stable clock module is one of the things we can try in few months, once I confirm 3458B is fully stable and functional in stock condition. I'd expect rubidium-based source might be a better option with less phase-noise compared to GPSDO+OXCO combo. I have both anyway.

Btw, so far new ADC is good and stable, -0.01ppm drift over 7 days per SN18 procedure.

I also did a side test for 10V, 100V, 1000V ranges using same 10V reference.
To confirm the numbers, I ran same test with my reference 3458A using 10V out of 3245A (+/-0.5 ppm stability).

3458B 10V range Deviation	3458A test	3458A 24h spec
+0.25ppm, Reference, 10V range	0.0 ppm 9.999978 VDC reference from 3245A	+/-0.275 ppm
-1.6ppm 100V range	-1.5 ppm Datalog 100V range	+/-14 ppm (+/-3 ppm for 10 min )
+6.0ppm 1000V range	+3.4 ppm Datalog 1000V range	+/-13 ppm (+/-7.75 ppm for 10 min )

[Theboel]

Hi TiN.
I really doubt about GPSDO or Rubidium but I think the best chance is OCXO with clean PSU.

[CalMachine]

CalMachine

I'm skeptical about "magnitudes" of improvements possible in any key performance aspects. Even for testing alone, hope you get that programmable JJA system in place, as that's pretty much only way to really split beans during testing whatever ideas we try. As was already mentioned, even if we design and get 9.5 digits, they mean nothing (other words - just noise) without linearity and stability to backup. And testing sub-ppm linearity is already hard enough, even with PJVS.

Having that said, replacement of onboard oscillator with more stable clock module is one of the things we can try in few months, once I confirm 3458B is fully stable and functional in stock condition. I'd expect rubidium-based source might be a better option with less phase-noise compared to GPSDO+OXCO combo. I have both anyway.

Btw, so far new ADC is good and stable, -0.01ppm drift over 7 days per SN18 procedure.

I also did a side test for 10V, 100V, 1000V ranges using same 10V reference.
To confirm the numbers, I ran same test with my reference 3458A using 10V out of 3245A (+/-0.5 ppm stability).

3458B 10V range Deviation	3458A test	3458A 24h spec
+0.25ppm, Reference, 10V range	0.0 ppm 9.999978 VDC reference from 3245A	+/-0.275 ppm
-1.6ppm 100V range	-1.5 ppm Datalog 100V range	+/-14 ppm (+/-3 ppm for 10 min )
+6.0ppm 1000V range	+3.4 ppm Datalog 1000V range	+/-13 ppm (+/-7.75 ppm for 10 min )

Yeah I doubt 'Magnitudes' as well, but it's still a hope!  The only reason why I popped out improvements of that order was the fact that DMM oscillators are like 0.01% at worst to 1ppm at best.  With a GPS referenced DOXCO or Cesium you can get stability of 1x10^-11 to x10^-12.  That is several orders of magnitude increase in accuracy of the oscillator for timing measurements.  I have high hopes that it could perhaps yield 1 order of magnitude increased measurement accuracy.  But yes, it is super high hopes, and most likely implausible.

Your numbers are looking really good!   

As Kleinstein said, there are still non-linearities and parasitic effects that would wash out any improvements made.

[ap]

Again, what is relevant here is the phase noise/jitter, not the absolute accuracy. The linearity of the A/D converter is already that good that it was hard to even determine it. In reality, it is probably much better. How would anybody identify an improvement after a mod? It will be very hard even with a JJA.
Linearity, btw, is not only driven by the A/D converter, but also by the amp stages in the input board.
Not sure overall it is a good idea to invest time trying to improve the world's most linear A/D converter installed in any test instrument.

[TiN]

Clock stability is not required to be much more stable longer than integration cycle, so all that effort would likely be just a snipe hunt. It's easy to confirm these claims however, much easier than anything else.

I'd agree that improvement on primary reference is a main area, also current source and current shunt resistors. Used resistors are not stellar by any means, leaving room for "easy" improvement with modern BMF's. If we find anyone with 3458HFL, we could verify this as well, since HFL's A1 seem to have better resistors as well, to support their improved resistance ranges accuracy. Other option to add active guard drive port for low-current measurement functions. Binding posts not very great for this application.

And get rid of that fan blowing over A1 PCBA with some thermal managment care. However there could be correction coefficients factored in 3458A's firmware for this effects compensation, so lots of hours testing is a must anyway.

[TiN]

3458B is getting final calibration love. Still has busted front terminals, so all calibration will be done using rear terminals.
But for sake of completeness, zero cal also ran on front. Innovation was required to attach a short, lol.



Don't try this at home, folks.. 

[Dr. Frank]

3458B is getting final calibration love. Still has busted front terminals, so all calibration will be done using rear terminals.
But for sake of completeness, zero cal also ran on front. Innovation was required to attach a short, lol.

Don't try this at home, folks.. 

Why didn't you exchange front and rear jacks?
I thought, that they are identical?

Frank

[Dr. Frank]

Yeah I doubt 'Magnitudes' as well, but it's still a hope!  The only reason why I popped out improvements of that order was the fact that DMM oscillators are like 0.01% at worst to 1ppm at best.  With a GPS referenced DOXCO or Cesium you can get stability of 1x10^-11 to x10^-12.  That is several orders of magnitude increase in accuracy of the oscillator for timing measurements.  I have high hopes that it could perhaps yield 1 order of magnitude increased measurement accuracy.  But yes, it is super high hopes, and most likely implausible.

Your numbers are looking really good!   

As Kleinstein said, there are still non-linearities and parasitic effects that would wash out any improvements made.

Hello,

A GPSDO is not a good idea, as the short time noise is bad, due to the satellites signal jitter.
A compact and decent OCXO is a much better idea, but look for good phase noise performance.
One of the metrological institutes, either NIST or PTB, they also modified the clock for some purpose, but I don't remember the background.

The LTZ1000A reference is quite noisy, about 1..2uV or 0.2ppm, but has good mid to long term stability.

Have a look on the different Allan diagrams, where best performance, about 0.02ppm or 8 digits, is achieved at about 10..20sec averaging.

At NPLC 1..100 you definitely see the short term noise, which might be improved by a less noisy reference, but for the price of bad long term stability.

If you would really achieve 9 digit performance at short to mid term time, I have my doubts, but an experiment would be great.

Frank

[TiN]

Yes, terminal blocks are same, but there is no need for that swap. This unit destined for lot of adventures, including (but not limited to) use of custom terminals block with active ohm guard and remotely programmable reversing.

Calibration today is to serve two purposes:
1. Use meter as one more transfer venicle between homelabs, so I can again test my accuracy on DCV/Ohm. This time also for AC, as SCAL to be done as well.
2. Get idea of aging rate and stability of used F5700 artefact cal using 3458 and guardband. Three artifact calibrations were done in sept/16, november/16 and today. I'll post details in related thread.

I found two last PCBs for my LTZ modules. If I get worty resistors I'd be able to have bank of 5 modules to try as A9 sub for lower noise. .

[CalMachine]

Yes, terminal blocks are same, but there is no need for that swap. This unit destined for lot of adventures, including (but not limited to) use of custom terminals block with active ohm guard and remotely programmable reversing.

Calibration today is to serve two purposes:
1. Use meter as one more transfer venicle between homelabs, so I can again test my accuracy on DCV/Ohm. This time also for AC, as SCAL to be done as well.
2. Get idea of aging rate and stability of used F5700 artefact cal using 3458 and guardband. Three artifact calibrations were done in sept/16, november/16 and today. I'll post details in related thread.

I found two last PCBs for my LTZ modules. If I get worty resistors I'd be able to have bank of 5 modules to try as A9 sub for lower noise. .

I'm interested to see the results of your mods to your 3458A.  Having a bank of 5 LTZ modules would be sexy!

[TiN]

I heard some 30kg of boxes have finally arrived. Wonder what that could be... Pile of shattered electronics or winner-winner-chicken-dinner? 

[Theboel]

I heard some 30kg of boxes have finally arrived. Wonder what that could be... Pile of shattered electronics or winner-winner-chicken-dinner? 

what 
2 pcs of broken 3458A ? 

[TiN]

No, but I do have extra broken A3 board. There were more parts in boxes than just 3458A for some other fun happy projects too.
And since I have meter in lab for these couple of days, let's hook it to 2000VA programmable AC source and test with difference mains, eh?

DUT is LTZ-reference, which is HP A9 STD in this case in a box, powered by +15V linear Agilent E3649 PSU.
It's calibrated value is +7.1846680 VDC +/-0.2ppm with unmeasureable tempco, obtained August 2016. Reference was sitting cold on the shelve since then.



Testing results are indeed interesting.



There seem to be visible relation to input mains voltage in sub-ppm level. I will also test with 200 and 240VAC, as well with 50 and 60, and perhaps 400 Hz settings.
Does this mean we need to monitor not just temperature/humidity but also mains power input for most accurate sub-ppm transfers? 

Btw, does anyone know how to remove binding post terminal without breaking stuff? I mean the rotating part from the threaded stud.

[Kleinstein]

Changing the frequency from 60 Hz to 50 Hz might need a call to ACAL, as this changes the timing and might also cause small gain changes.

With the AC source there is also the question about EMI - this could be an issue to the reference and the meter itself.

For the changes in the voltage, this can change the temperature distribution in the case, as the power dissipation at the voltage regulators will change and so will temperature gradients. So there is more than just one internal temperature.

[TiN]

There are ACALs after each voltage change and also on each 0.3c TEMP? change.

Only meter is powered from AC source, DUT reference PSU is constantly powered from regular mains.

As of temperature variance, that was exactly my thinking how different mains could make such result. Higher voltage on same transformer winding cause bit higher output voltages on secondary side, which in turn make LDOs run bit hotter. Since meter's own DCV path is about 0.1-0.3ppm/K it doesn't take much of the raise to bump similar change in measurement value. This just highlights another source of error for such accurate measurements to consider. If meter used in place where mains stability vary a lot, this could generate extra errors into otherwise stable readings. Perhaps double-transformer arrangement, such as used in Datron 1271/1281/4920/4950 and Fluke 8508A also helps to reduce this, as analog power section is powered there from intermediate regulator, so there is more decoupling from mains variations.

Another point for me is that meter's power well within 30W window, which is well manageable amount of power conversion need if we decide to try battery powered opeation. If we drop front panel and it's VFD, this number likely go down 5-10W. But I'd expect battery DC power may cause more bad than good, since 50/60/400Hz mains frequency used for syncronize/clock some parts of meter/ADC and so on. Yet still interesting to attempt.

P.S. Playing with different ideas.... no, these are not the ones which will go on meter

[Kleinstein]

With the meter running from a different supply frequency than the grid (and thus externally coupled "noise"), there could be a beat frequency signal superimposed to the signal. Worst case this could upset the ACAL adjustment and thus add some more or less random noise at that step. If could be interesting too look at some of the data where the beat frequency is low enough to be visible - might need an extra measurement.

The separate (higher frequency) transformer would make a battery powered operation easier. Still some of the meters still synchronize that frequency with the grid, to improve suppression by the integration time. It should at least keep the loss rather constant for the analog part.
I don't think battery operation for the 3458 is such a good idea - rather difficult at least, as there are several voltages in the signal referenced part.

A modified thermal layout might be possible, e.g. more separation of the voltage regulators, more thermal coupling. Are there thermal images of the 3458 internals ? Could be a nice reference for fault finding anyway.

[TiN]

Not as many voltages on both inguard and outguard actually, just +/-15 and +5s and 20V. Remove VFD and it's high-voltage DC-DC removes need for AC as well. Perhaps few battery packs from notebook could provide enough margin to downscale voltages for both negative and positive rails and have enough juice to run meter for a day or two. Custom regulator board would have a good spot in original A4 location.

Mains frequency impact can be tested by adding an clock input, locked to mains frequency, so both cases can be tested. So far it's running 50Hz and reading noise is in 0.15ppm range, within normal for LTZ ref.

I have thermal images somewhere, but never posted them before for some reason I don't remember now. Perhaps I'll take few more images on this unit this Friday after mains test done and before hauling it back to my homelab. It also missing one of inguard shields (ebay seller had it missing) so that could affect things a little too.

Another reason for custom A4 is to have more efficient regulator, so possibility of removing fan could be evaluated. The way airflow is directed with covers and placement suggest main purpose of the fan cooling A4 regulators and directing airflow onto A6/A5. So lot of things to play with 

Anyway, first I need to get meter back home and compare it's readings with my 2002s/3458A and all standards, so I can confirm their accuracy and not worry about calibration. As the moment I change fan, replace caps and do other things, calibration we done two weeks ago would be gonsky. I'll post report in next few days with magic numbers  .

[ap]

The 3458A sets the sampling synchronous to the mains frequency. A RESET e.g. does that (except for firmware 6 and below, a bug always sets it to 60Hz). It can be manually set, by LFREQ, in any case it is important that it is set correctly.

[TiN]

Here are the money shots. Few hot suckers detected



More to come...

[Kleinstein]

I don't see anything really bad on the thermal images. The power supply board is still not that hot - just a little. Also no surprise to see higher temperatures one the AC board: lots of fast amplifiers here an thus the high power. Still only a few parts with more than 60 C.

Some of the "diodes" near the reference seem to be rather hot and provide a hot spot in a possibly sensitive area. One has to keep an eye on these when using a different, higher power ref module.

The fast comparator at teh ADC board is rather hot - but also not on the main board and thus not a big issue. I don't think drift of this comparator would be important.

[TiN]

3458B in the house...finally, after 3 months of travel, remote calibration, ebaying and replacing boards, unit from this thread joined the pack.

Stashed it on top of poor 2002's, powered on, connected LTZ reference have currently online (I keep them all stored "cold", only power on for a week one at a time to do measurement), and here's what we see after 20 minutes runtime...



For those who don't play with ppm-calculator every day, here's the distilled data from photo:

LTZ module recorded value : +7.13665729 VDC, reference value obtained from comparison to F732B calibrated within 10 days to +/-0.55ppm in August 2016.
Bottom 3458A, this is my original rusty meter, never left homelab, DIY calibration transferred from K2002s from 7VDC cal. Reading is +7.1366591 V, +0.254 ppm
K2002 (GPIB 4) - handcarried and calibrated in August 2016. Reading is +7.1366605 V, +0.450 ppm
K2002 (GPIB 6) - handcarried and calibrated in August 2016. Reading is +7.1366588 V, +0.212 ppm
Top 3458A - is this unit from thread. Reading is +7.1366579 V, +0.086 ppm

Can I say now that I've joined sub-ppm 10DCV accuracy club? 

[ManateeMafia]

Looks great. I am glad it all came together for you in time for the Chinese New Year.

Btw,  I see no problem in having too many clocks.

[VintageNut]

You are the honorary president of the club. From a repair and restoration standpoint, as well as volume of work, you are supreme.

I very much enjoy reading your methods and results.

Can I say now that I've joined sub-ppm 10DCV accuracy club? 

[TiN]

Some overnight data came thru. Looks like K2002-4 and HP3458B are very good friends now, running together down in the noise Also shows that 2002 is no much worse than 3458A, if we forget about ACAL and linearity of the latter for a second.



Green line is rusty 3458A.
Now having four "clocks" we can with good confidence say that 0.3ppm bump in first few hours did not come from meters, but from reference, as all four of them shown same data. You would be never be able to say this if use just one (even very good and just calibrated) meter.

Also there are often many concerns over shipping precision gear internationally, due to temperature, mechanical, pressure stresses, etc. In this case it all seem not to affect our homebrew DCV transfer (both 2002 and 3458B were calibrated in USA, while my lab is in half-globe away in Taiwan, for those who not frequent here) in significant way. Of course there is little chance that I just dont see the effect (e.g. all meters drifted same way due to shipping), but I found that very unprobable due to sample size (6 x LTZ1000A references, one of them being cross-shipped twice and retested 4 times, 3 x DMMs).

We also reaching the level on which calibration accuracy level (0.5-1ppm on 10VDC to be on safe side) cannot be improved much, no matter amount of money spent. Short of having own JJA system, surely. 

Having all this, resistance transfers likely be much more sensitive for shipping/stresses, as it's easier to damage sensitive elements than little semiconductor zener. 
We will see more once I get to resistance comparisons.

[Kleinstein]

It is no a surprise that the K2002 and 3458 are at about the same level for this test: it is expected that the main source for low frequency noise are the references and here both use the same type of LTZ1000, just like the DUT. So nearly half of the noise is expected to come from the DUT anyway.

The only possible problem for the K2002 could be the possibly not so good AZ implementation of the Keithley, as shown in the DMM7510 thread, but this might already be swamped by the references.

[TiN]

Cooked more data over weekend.
Three logs from LTZ modules and one from HP 3245A(modified with LTZ1000) set to 10.00000V.
Legend: HP3458A, HP3458B, K2002-4, K2002-6

[TiN]

Time has come...

I was thinking a while which meter to use for the wild experimentation, and after all things considered, decision was made to use my rusty 3458A as a patient. 3458U2 will have replaced front terminal block and missing inguard A1 shield, but remain unchanged for now.

Let the game begin, starting with easy part - DCI function. Why easy? Because it's simple component swap.

Current shunt modifications



I have broken A1 board, from which I removed original R213, R212, R211, R207, R209, R206 resistors and tested their TCRs to establish our baseline expectations.
Test results:
* R212 1 ohm - +8.3 ppm/K Datalog
* R211 9 ohm - +4.7 ppm/K Datalog
* R207 40K "reference" VHP101 - +1.4 ppm/K Datalog



I don't have much better replacement for 40K VHP101, so decided to leave that one alone for now, as we as 90 ohm (1mA range) and 4.53K shunts (10uA range), for but replaced rest with better resistors:

* R213 replaced with VPG foil shunt 319790 0.1 ohm 9W, measured TCR +0.55 ppm/K
* R212 replaced with VPG VPR221Z 1 ohm, measured TCR -0.86...-1.47ppm/K
* R211 replaced with AE foil resistor 9 ohm, measured TCR +1.5…+0.94 ppm/K
* R209 replaced with S102 foil resistor 634 ohm, measured TCR -1.2…1.25 ppm/K
* R206 replaced with CADDOCK USF340 resistor 500Kohm, measured TCR +0.78 ppm/K



After installing A1 board back, no SELF-TEST errors detected, ACAL ALL passed clean too.
Running GPIB app now collecting same 100mA running thru both meters from HP 3245A (LTZ-powered).

[ManateeMafia]

Looks like you are adding the undocumented pimp option 003. 

Will the alternating + and - TCR's have any impact on the readings or are the values too far apart?

[TiN]

There is still a long way before any celebrations can commit. 3458A holds many secrets within. One of which likely to be internal software corrections (unrelated to calibration constants) of components/resistors tempco. Why I think so. Well, here are some results. Test procedure was simple. DC current sourced thru both 3458X (modified) and stock 3458B and logged as usual. After getting somewhat stable current levels, data was corrected by computing offset and meters were covered with a blanket to raise internal temperature. Both meters stacked together, so temperature raise is expected similar.

Change of temperature (internal TEMP? sensor from A1's) was used as temperature delta and resulting tempco was calculated.

10mADC range:
3458B : +3.36 ppm/K measured, 10ppm/K spec. Datalog
3458X : +10.92 ppm/K measured, 10ppm/K spec 



Next range 100mA

3458B : -10.6 ppm/K measured, 25ppm/K spec. Datalog
3458X : +61.6 ppm/K measured, 10ppm/K spec   



1A range

3458B : -4.65 ppm/K measured, 10ppm/K spec. Datalog
3458X : -1.88 ppm/K measured, 25ppm/K spec   



At first glance, 100mA range data does not make sense, right? We changed +8.3ppm/K original resistor to much better one +1ppm/K. That's 8 times more stable resistor. But if firmware actually uses +8ppm/K correction factor for DCI measurement calculation it apply +8ppm correction per each degree of temperature increase, right? TEMP? reading of 3458X increased +7.5C, which gives about +60ppm total correction. 

And that's about what we see in measurement data. However this brings more questions, how does meter reads it's temperature for compensation, as TEMP? sensor require switching, and only read by GPIB command in my datalog program each 50th sample, not every sample. Sounds like a conspiracy theory...

DCI 1A range however improved significantly, so perhaps correction factor not used for this range? Again more questions, than answers. Setting up my TCR setup again, to retest TCR of original shunts removed from A1 PCBA to see what was their original tempco, while 100uA datalog runs now.

[Kleinstein]

Right after soldering, there is a chance that the resistors can show drift. This can be due to a high temperature, but also due to relaxation of mechanical stress. So it may need a kind of burn in for the new resistors. Still the curves really look like correlated to temperature and not like aging / relaxation.

With the temperature there is also a change in the normal ADC calibration, ideally adjusted with ACAL if the temperature changes significant. Is this done in these curve too ? It might be worth a try to do the comparison for a voltage (e.g. 100 mV or 1 V) reading. Depending on how much is adjusted during such a normal ACAL run, they could also adjust for the TC of the shunts. And it is only the super stable 40 K resistor that might mater. So the TC and stability of the shunts might not be so important, except for self heating - so it would be mainly the 1 A range that could profit from a better shunt. 

For the resistors there could also be an effect of thermal EMF. Some resistors can show quite some thermal EMF if they get warm due to self heating, or external temperature gradients. So the TCR is not the only important property of the shunts.

[Tony_G]

This is a great thread but I've clearly missed something somewhere - Is it being called a 3458B because of the repair/modification work?

Don't mean to derail the thread, just wondering - I get the 3458X as the eXperimental one, just can't find the reasoning behind the 3458B.

TonyG

[TiN]

I just reference B as second unit, as I have two meters, and calling them both 3458A likely to confuse.

Kleinstein
ACAL does indeed correct all DCI shunts/ohm source errors by comparing to 40K reference resistor, but it takes 830 seconds to do so and not very convinient for transfers. So if tempco can be improved without ACAL, it can be useful for multiple comparisons/logs. I measured some temperatures before with nominal currents flowing thru shunts, and temperature raise was very small, less than 8K for 1A current.
ACAL was not used during data collection charts other but at the start on both meters. 

I might tap voltage output directly after shunt network, to see if it does actually change that much, or introduced tempco increase caused by meter's firmware math/processing.

[Dr. Frank]

TiN,
you measured the T.C.s on the resistors of this burnt board..
Are you sure it also had this VHP101, but not the older type with unknown (at least 1.3ppm/K) T.C. characteristics?

Your measurements were not very distinctive, but the T.C. you've measured were clearly above the original HP T.C. specification.
I'd assume T.C. ~ 0.3ppm for the real VHP 101.

To answer the question of Tony_G.. This improvement on the T.C.s of the shunt resistors might as well improve the DCI specification.
If you look closely at these, you'll come to the conclusion, that most of the uncertainty might arouse from the T.C. specification.

Also, the Ohm calibration stability might be improved, as several of these DCI shunt resistors were used in the Ohm ACAL routine.

Frank

[Kleinstein]

For the current measurements there is more than the TC of the shunts. There is also the drift of the 100 mV or 1 V DC range (I don't know if there might be an intermediate amplification used). So the drift of the shunts is only one part.
So it might be really worth comparing one of the DC ranges as well - maybe the 3458X was is not that good in this respect too.

The full ACAL takes a long time and is thus not that practical. Is there such a thing as a partial ACAL, e.g. only for the DC gain and ADC ?

[TiN]

Yes, one can run ACAL DCV (takes 130 seconds), ACAL OHM, ACAL ACV separately. I often use ACAL DCV when doing LTZ comparisons, to achieve transfer spec over longer times (hours-days).

Previous tests of 1V DCV show max 0.6ppm change with 0.5C TEMP? variation over multiple hours. Perhaps worth to note, that 3458X is my older meter, which I use 24/7 since it's repairs finished February 2016. It's <1ppm stable for all DCV ranges. I can't tell same about newer unit, so that's why older one went for modifications.

Idea was to start with shunts and go down the signal path as it goes, just didn't expect 60ppm TC increase from only shunt change .

Dr.Frank
Could be. But they all look exactly same. I'm not going to replace 40K off my A1 just yet.

I can repeat 40K TCR measurement (of resistor from dead A1) once again with different meter, to confirm the value.

100uA range seem to be about same for both meters, below 1ppm/K.

[Dr. Frank]

TiN,
The earlier 40k resistors were different, and looked different.
They did not have that VHP101 marking, which was used from about 1995 / 96 onwards.
So you simply have to verify that, on your DUT, please.
Frank

[Kleinstein]

If on provoking the higher internal temperature there also is a significant change in air flow, this could cause additional drift / shifts. This is because this would change temperature gradients inside too. So changing the temperature due to reduced air flow is different from higher air temperature, even if the same internal temperature reading is reached. Some of the really hot parts (e.g. AC board) might even overheat.

It might be worth measuring the 1 Ohms resistor removed from 3458A - is this really so bad to show a TC in the 50 ppm range, so that old compensation would cause the now higher TC.

The resistance of the copper line between the 0.1 and 1 Ohms shunt could also contribute. So may the resistance of the E1 terminal of the 0.1 Ohms shunt, as thus is added to the 1 Ohms value. Also the 1 Ohms resistor looks like it is 4 wire, but used as 2 wire - so it's wire resistance also adds, while it is included in the simpler 2 wire resistor. Still this should not be enough to cause a 60 ppm/K effect.

[mimmus78]

TiN you must also check what happens with EMF.

Let the meter rest with zero current to "stabilise temperature", and remove leads from binding posts.
Do an ACAL. This should zero the current range to read zero with zero current (yes I know you know).

Than run your test and make the reading/temperature stabilise. After this happens remove stuff from binding posts, what is reading at ZERO now?
This should be EMF contribute to error.

On my unit at 1A range, after 10 minutes of running at 1A I have +4uA due to this "heating" effect.
Most of the heating/effect happens on fist minutes.
After removing current the meter returned to ZERO in a couple of minutes.

This effect seems not to influence much the 1A range as I swapped cables and got the "same" current reading, estimated to less than 1uA difference but maybe this EMF will produces more error on the lower current ranges.
Can you repeat this experiment with your BAD ranges?

EDIT: I tested also the other ranges and this thermal effect seems to be present only on the 1A range.

[mimmus78]

And anyway this remember me of someone ...

https://youtu.be/_CvNe-JhjD4

Inviato dal mio Nexus 6P utilizzando Tapatalk

[ManateeMafia]

Are you trying to convince TiN to repair some of these bad U180's?   

[mimmus78]

No I do not need to convince him, he already tried ... I never thought possible of repair stuff this way ...

[TiN]

Bit more data captured.

Retest 10mADC range:
3458B : +3.47 ppm/K measured, 10ppm/K spec. Datalog
3458X : +12.35 ppm/K measured, 10ppm/K spec 



1mA range

3458B : +0.55 ppm/K measured, 10ppm/K spec. Datalog
3458X : +5.48 ppm/K measured, 10ppm/K spec



100uA range

3458B : -1.42 ppm/K measured, 10ppm/K spec. Datalog
3458X : -0.45 ppm/K measured, 10ppm/K spec



No problem with 100uA range at all.

100nA range

3458B : <100 ppm/K measured, 210ppm/K spec. Datalog
3458X : <100 ppm/K measured, 210ppm/K spec 



Error/tempco of 100nA range mostly delivered from amplification stage, so this measurement have no meaning. It matches unmodified 3458, so replacement of 500K resistor was for nothing really.

DCV comparison, 100mV range

3458B : +1.72 ppm/K measured, 2.2ppm/K spec. Datalog
3458X : +0.63 ppm/K measured, 2.2ppm/K spec   



DCV comparison, 1000mV range

3458B : -0.2 ppm/K measured, 1.3ppm/K spec. Datalog
3458X : -0.128 ppm/K measured, 1.3ppm/K spec   



As we can see, DCV 100mV and 1V ranges are good and cause zero problems, and not cause of DCI excessive tempco. We can close A3 effect theory for now.

Original A1's resistors TCR tests:

Test results:
* R211 9R ohm from A1 - -1.38 ppm/K Datalog - measured directly by K2002
* R209 634R ohm from A1 - +4.65 ppm/K Datalog - measured directly by K2002
* R206 500 Kohm from A1 - +1.86 ppm/K Datalog - measured directly by K2002
* R212 1 ohm shunt from A1 - +1.7 ppm/K Datalog - measured directly by K2002
* R213 0.1 ohm HP 03458-82501-1 shunt from A1 - -6.2...-13 ppm/K Datalog - measured directly by K2002
* R213 0.1 ohm HP 03458-82501-1 shunt from A1 - 7.5 ppm/K from 25C to 35C, 1000mA test current Datalog - measured by Keithley 182-M

Summary table:

Resistor	Original A1	New resistor	Range	Test result
R213 0.1R	+7.50 ppm/K	+0.55 ppm/K	1 A	Better
R212 1R	+1.70 ppm/K	-0.86...-1.47 ppm/K	100 mA	Out of spec
R211 9R	-1.38 ppm/K	+1.5...+0.94 ppm/K	10 mA	Out of spec
R210 90R	Unchanged	unchanged	1 mA	Out of spec
R209 634R	+4.65 ppm/K	+1.2...+1.25 ppm/K	100 uA	Better
R208 4.53K	Unchanged	unchanged	10 uA	Not tested
R207 40K	Unchanged	unchanged	1 uA	Not tested
R206 500KR	+1.86 ppm/K	+0.78 ppm/K	100 nA	Same

Perhaps I'll swap 9R for 10mA range back to see if that "fixes" the 10mA TC.

Dr. Frank
40K resistor from dead A1 is Vishay 301031 hermetical



40K on my 3458A's A1 is Vishay VHP101 (photos posted above). I think same is in my second unit, as it's 5 years younger.

[Kleinstein]

The 1 Ohms resistor is also used in the higher ranges. So the 60 ppm / K drift of that resistor would also cause something like 6 ppm/K for the next higher range.
How was the PC of the new 1 Ohms shunt measured: As a 4 wire resistor, or as a 2 Wire resistor like use in the circuit and 4 wires only from than on ?

If the contacts to the resistor have a rather high resistance, this could cause the problems, not only for the 100mA range, but also the 10 mA range. Wire resistance at the other side would add another 6 ppm/K and thus would end up at 12 ppm/K as observed. So chances are the 1 Ohms resistor is 4 wire mode only and may thus not be suitable here.

[TiN]

That thought crossed my mind today. TCR was measured as 4-wire resistor, with each pin connected individually to K2002.
I connected both current and sense pins together in meter, so it's definitely different that TCR measurement was done.
Also test current is different, K2002 source 7.2mA during measurement, instead of 100mA used in actual meter.

Here's graph during TCR test, 1 ohm resistor is rosewood color. Simple box calculation gives -1.9ppm/K (ramp from +20°C to +40°C).


As bonus for thread, I got cover off 3458B for some checks, took few pics. A1 R207 confirmed to be indeed VHP101.
A3's logic is now in ALTERA MAX V CPLD (1270 ALE).



Those unpopulated test points on the corner are JTAG. Curious to see if PLD is actually secured  . 



CPLD Reads just fine Backed up just in case. CRC matches the sticker label.



Love these Tek grabbers, always handy 



Reference A9 module. 15K VAR installed (prior to calibration in January) to reduce LTZ temperature  .

[mimmus78]

"Curious to see if PLD is actually secured ..." and few post later someone got fired 

Anyway TiN why you used this "not even encapsulated" audio resistor?
OK it's 100K over 15K so any change in this resistor is attenuated ... but isn't it too much risky like this?

Warranty is expired on my unit, so time to void this stickers, have a check inside and get some photo for your collection.
I will tear down again in next two weeks ... cause I want to install some less nosier fan inside.
So if you are curios of something else just let me know.

[mimmus78]

More photos ...

[TiN]

Anyway TiN why you used this "not even encapsulated" audio resistor?
OK it's 100K over 15K so any change in this resistor is attenuated ... but isn't it too much risky like this?

Long story, but she'll be alright. It's not really naked, there is still thick enamel coating over it.

... cause I want to install some less nosier fan inside.

New A3 too . However I'd strongly suggest against replacing fan, it's airflow speed and force are important and I'd keep them as designed.
After all it's not that noisy (compared to Keithley 2304 or Keithley 2002 series gear it's barely audible).

[TheSteve]

I find the fans in my 3458A's beyond annoying. With two of them running the frequencies from the fans seem to beat together. I can actually hear it throughout the entire house. On another floor isn't isn't loud, but I still know it is them. I am using the exact fans Keysight sells for the 3458A.

[TiN]

Than maybe your fans are just dried out / bad. Replace with new same spec/size Papst/Delta and off you go. I can't hear the unit if I just go another room and close a door

[Dr. Frank]

Dr. Frank
40K resistor from dead A1 is Vishay 301031 hermetical



40K on my 3458A's A1 is Vishay VHP101 (photos posted above). I think same is in my second unit, as it's 5 years younger.

Hi TiN,

so that's the  original resistor (i.e. technology from 1988) which you tested.
It has 1.4ppm/K only, as specified for the 3458A.
Again, I assume C- or K- foil technology inside, by selection fulfilling the tighter spec.

The VHP101 on the other hand, is different, a series combination of a C- and  a K-type resistor element, giving a 5 time better T.C., maybe.

I write "maybe", as this is not reflected in the spec.(which never improved over the version from 1989), and because it's not clear, whether the ~2005 Vishay specification of the VHP101 reflects the same components, as of from 1995 onwards, as used in the 3458As from then on.

I know, that it's a big wish, but maybe it's possible for you, also to measure the T.C. of the 40k, VHP101 resistors, on one of your working A1 boards.
This would give a better baseline about the actual specification also.

Frank

[TiN]

Your wish may come true sooner than expected. To celebrate my BD today, I didn't fight my TEA and bought a nice lot of 59 foil resistors off ebay. I mean at 1.65$/piece it's a steal. 



Hermeticals are rumored to come from Datron 4950  . One of which is 40K, so in few weeks I hope to receive these and test for TCR. If it's actually much below 0.3ppm, I'll replace original 40K VHP101 to try 

[mimmus78]

Ohh I've seen those resistors on eBay, than I thought it was better to leave to you. :-)

[mimmus78]

New A3 too . However I'd strongly suggest against replacing fan, it's airflow speed and force are important and I'd keep them as designed.

I'd like start something easy ... getting a fan with similar specs. Only less noiser.



Inviato dal mio Nexus 6P utilizzando Tapatalk

[lowimpedance]

 Neat little collection of precision goodness there  . So those hermetic resistors come 'supposedly' from a 4950 !. Or they were going to be used in one as the leads look to be never been soldered.
The 40k and the other one with shorter leads still maybe have not been used either. 
Also noted the values of the other hermetic's is a secret 

[mimmus78]

So despite what TiN think I replaced fan of my 3458a with the:

HA60251V4-1000U-A99

on paper this fan should be 13.8 CFM (0,391m³/min) and 10dBa of noise while the original part is rated at 12 CFM at whopping 26dBa (considering the fan is running at 15V I gues it will be more the 30dBa).

This unit can accept up to 13.8V but the 3458a original fan is powered by 15 V, so the idea was to run it with a couple of diode dropper to lower voltage from 15 to 14 V (the plan was also to have a little more CFM to compensate this little voltage drop).

At 14V the fan is very silent, I barely can notice the 3458a when was powered on. Unfortunately 3458a measured +17°C over the ambient temperature so this was to much difference from original fan.
So I decided to give it a push at 15V by removing the diodes and now I have 14°C over the ambient temperature that is slightly better of what I had with the original fan (14.5°C to 15°C).
Air flow and noise was increased (I guess if at 12V is 10 dBa now is around 20 dBa) but it's still acceptable and "down to the noise". Now the question is how much will the fan last with this overvoltage ...

Most probably the next time I will try to use the:

MF60251V2-1000U-A99

this has 19 CFM, still more silent than the original and same power rating than the original fan.
With this fan you will have enough margin for lowering voltage to the rated value and still keep decent air flow and noise.

[TheSteve]

For reference the fan Keysight currently sells for the 3458A is the Papst 622L - easy to get at Digikey/Mouser etc and it will assure proper air flow and is rated to handle 15 volts.

[TiN]

Resistors arrived, time to get that Keithley 7168 nanovolt scanner card busy

[View[+]Finder]

Is there a better place to discuss repairs to my 3458 option 002 reference? If not, what to do?

[TiN]

Just make own thread, like "Standalone 3458A A9 operation" or "troubleshooting A9 module".