Author Topic: HP3458A SRAM replacement: copy or calibrate?  (Read 4579 times)

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Offline View[+]Finder

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HP3458A SRAM replacement: copy or calibrate?
« on: October 24, 2017, 07:55:05 pm »
Why are nearly all of the posts about SRAM replacement concerned with “backing up the CAL data” so that it can be written into the new parts?

For the last few years I have been repairing and calibrating eBay gear to use in an educational program to help kids learn electronics. Our HP3458A is the lab standard and I became concerned what would happen if the SRAM batteries failed. All but a few of the posts described the process of extracting data and the expense of failing to do so and that didn’t jibe with what I had read in the HP Calibration manual. Based on my experience with repair and calibration, I find the 3458A to be the easiest of all to calibrate—yes, I have precision standards as needed—it just takes some time. Most of the hard work is built into the calibration firmware by HP.

It will soon be time to replace the SRAM’s in my 3458A as the batteries are way past their expected life, so I thought “why not just replace and run a full CAL and SCAL? Or are there ‘factory’ items in the SRAM?” I’ve reviewed the GPIB info on reading the cal data and it looks like all the adjustments are written during calibration. Am I missing something?

Thanks to advice from ‘an expert’ (xdev), I learned that I was on the right track about calibration data and that there are just few factory items that are not related to accuracy. (Perhaps as included in CALSTR?)

Not everybody has proper standards, cabling and SCAL equipment (TVCs,SG,reference meter) to perform full calibration though. Fortunately the key items for me—DCV, DCA and Ohms—require only a short piece of solid copper wire (12-14ga) for a thermal short (Zero Ohms), a precision 10VDC source and a 10kOhm resistance standard. AC calibration needs more equipment—I’ll figure that out when needed.

Right now the 3458A is working fine and in calibration, so my plan is to wait until the SRAM becomes unreliable—SRAM’s have been obtained from a reliable source—and then I’ll remove the old ones, install sockets and the new SRAM’s and start the calibration process.

Yes, the Keysight Cal Lab will be glad to help, but I’ll try DIY first.

Documentation to follow, maybe a video . . .


 
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Offline nidlaX

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #1 on: October 24, 2017, 08:55:06 pm »
Lucky you that you have access to a few thousand dollars worth of voltage and resistance standards. Those standards by themselves may cost nearly a thousand dollars to calibrate.

Calibrating the 3458A is not a matter of technical difficulty, but rather a matter of financial expenditure.
« Last Edit: October 24, 2017, 11:43:42 pm by nidlaX »
 
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Online Dr. Frank

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #2 on: October 24, 2017, 10:12:18 pm »
Hi,

I also copied the content, due to financial reasons.
If you let KS do the job, they do not replace the 3 nv-SRAMs any more, but the whole PCB for a bundle of additional bucks, that at least doubles the price of the calibration.

The calibration of 10V and 10k may be comfortably done DIY, if you have stable standards at home, and then may transfer their known values to the fresh cal-RAM by SCAL.
Frequency calibration might also be done DIY, as frequency standards are also commonly and cheaply available.

The only problem left is the AC calibration, requiring a stable synthesizer, and a set of precise thermal converters, both being very uncommon in a private lab.

That was the main reason for me for to copy (thanks to quarks  :-+).

Frank
« Last Edit: October 24, 2017, 10:15:32 pm by Dr. Frank »
 
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Offline View[+]Finder

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #3 on: October 26, 2017, 02:49:11 am »
True, the AC side of the DMM is more complicated, however the calibration process has its origin in the time of the introduction of the 3458A. Back then the measurement of RMS was analog and derived from heating a wire and measuring voltage with a thermistor to get a DC equivalent ‘RMS’. According to what I’ve read, digitized measures as in a modern scope or DMM will yield an accurate RMS voltage at various frequencies. My bench DVM is a Keysight 34465 that measures true RMS by some sort of heating process and I have a signal generator that is supposed to be able to generate a sine wave at a specific RMS voltage.

The 3458 calibration calls for AC at specified RMS voltage and frequencies, so my plan is to measure the sig gen with the 34465 in parallel with the 3458 at the various stages of the cal process.

Yes, money helps—the 34465 cost a bit, but worth every penny—but most of my gear came from eBay and repaired by hand. Over time I bootstrapped from 4.5 digits to 5.5, 6.5 finally to the 3458 also from eBay.

I had to learn a lot along the way and I’m by no standards an expert, so if there is a flaw in the thinking, let me know.

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Online Dr. Frank

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #4 on: October 26, 2017, 07:05:25 am »
True, the AC side of the DMM is more complicated, however the calibration process has its origin in the time of the introduction of the 3458A. Back then the measurement of RMS was analog and derived from heating a wire and measuring voltage with a thermistor to get a DC equivalent ‘RMS’. According to what I’ve read, digitized measures as in a modern scope or DMM will yield an accurate RMS voltage at various frequencies. My bench DVM is a Keysight 34465 that measures true RMS by some sort of heating process and I have a signal generator that is supposed to be able to generate a sine wave at a specific RMS voltage.

The 3458 calibration calls for AC at specified RMS voltage and frequencies, so my plan is to measure the sig gen with the 34465 in parallel with the 3458 at the various stages of the cal process.

Yes, money helps—the 34465 cost a bit, but worth every penny—but most of my gear came from eBay and repaired by hand. Over time I bootstrapped from 4.5 digits to 5.5, 6.5 finally to the 3458 also from eBay.

I had to learn a lot along the way and I’m by no standards an expert, so if there is a flaw in the thinking, let me know.

Thanks to all


Well, yes, at first you should definitely read the calibration manual of the 3458A, page 27 onwards.
You need to calibrate the AC mode at 2MHz and at 8MHz, at 3V, 1V and 100mV.

That is not possible with a 34465A, only Thermal Converters will do the job properly.

Even if the 3458A is quite old, it's still one of the best and most demanding instruments.

Frank
 
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Offline trle

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #5 on: October 26, 2017, 02:09:59 pm »
Hi !
I look at the forum and see that you have a knowledge for resolve my problem. Hope that you have a will to help me !?
I have a oscilloskope HP 54201A that doesn't work well, actually measured values are not good and they are constantly  changing.
Exp. when measuring frequencies, it is uneven and varies, even though the measured frequency is accurate and constant ( professional fm transmitter).
Hope that you have a time and will to help me to solve this!?
 Best regards
 Rade
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Offline View[+]Finder

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #6 on: October 27, 2017, 02:42:18 am »
One should read before trying of course.

I read the calibration manual several times before posting and the specs for the 34465 as well. In addition, I read several sources about thermal converters. Aren’t ‘DC equivalent’ and RMS the same measurement??

What does the thermal converter do other than calibrate the synthesizer’s RMS voltage at a chosen frequency? Why would a 34465 not be able to measure RMS to the same effect?

With all due respect, I’m not sure I agree with Dr Frank’s conclusion about the necessity for a TC other than when a 3225A synthesizer is used.

With thanks to all.
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #7 on: October 27, 2017, 03:10:25 am »
Unless, of course, the 34465 only goes to 300kHz for RMS. Oops! Double-check time.
Ok, time to check the RMS specs for my o’scope. Thanks
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #8 on: October 29, 2017, 07:11:20 pm »
Here's my setup for the calibration of the AC portion of the 3458A (SCAL):
1) Siglent 2042X function generator; frequency 100kHz, 2MHz, 8MHz; RMS voltage 100mV, 1V, 3V
2) Rubidium 10MHz frequency standard as ext input to the 2042X
3) R&S HMO1212 oscilloscope to monitor frequency and adjust the RMS voltage on the 2042X

Procedure:
The 2042x is connected to the HMO1212 with a BNC cable and also to the 3458A with a BNC 'T-connector'.
The 2042X is a substitute for the HP3325A function generator and the 'scope takes the place of the thermal converter. In this case the measurement of RMS voltage (DC equivalent) can be obtained from the 'scope directly and the 2042X adjusted to match the required input voltage. This will eliminate the steps associated with measuring the voltage from the thermal converter in order to correct the RMS errors in the 3325A output.

Rationale:
The accuracy and precision of today's budget-priced electronic equipment sufficiently exceeds that of equipment available when the HP3458A was introduced in 1989 to justify a modification of the claibration procedures. While it is certain that many 3458A's in use by readers of this forum are relied upon to calibrate other equipment on their bench, one would doubt that any operate at metrology levels of environmental control. In short, the 3458A I have is an 'old workhorse' still able to perform all of the tasks needed at a level of precision and repeatability sufficient to calibrate other equipment in my 'lab'. Many 3458A's on eBay have been 'rode hard and put up wet' (to continue the 'horse' metaphor) and the stories of their rehabilitation and recovery are inspiring to say the least.

In the spirit of 'reuse before recycle' it is better to have a working 3458A on one's bench and given the limitations of the NVSRAM battery life, saving the cost of a Keysight calibration is an added bonus.

If anyone wants links to the specification pages for any of the equipment mentioned, please ask. I have read through the specs, but not done an exact comparison. My guess is that the procedure above might improve on the 'thermal converter' method by an order of magnitude, particularly considering the state of gear on eBay these days.

Comments appreciated . . .
 

Offline lukier

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #9 on: October 29, 2017, 09:25:29 pm »
Here's my setup for the calibration of the AC portion of the 3458A (SCAL):
1) Siglent 2042X function generator; frequency 100kHz, 2MHz, 8MHz; RMS voltage 100mV, 1V, 3V
2) Rubidium 10MHz frequency standard as ext input to the 2042X
3) R&S HMO1212 oscilloscope to monitor frequency and adjust the RMS voltage on the 2042X

Procedure:
The 2042x is connected to the HMO1212 with a BNC cable and also to the 3458A with a BNC 'T-connector'.
The 2042X is a substitute for the HP3325A function generator and the 'scope takes the place of the thermal converter. In this case the measurement of RMS voltage (DC equivalent) can be obtained from the 'scope directly and the 2042X adjusted to match the required input voltage. This will eliminate the steps associated with measuring the voltage from the thermal converter in order to correct the RMS errors in the 3325A output.

You realise that a scope has something like 3% accuracy on the vertical? 8 bit ADC. 3458A can get down to 0.01% according to the datasheet. If you can afford 3458A then spending $100 or so on a Fluke 540A shouldn't be a problem. Fluke 540A might just about cut the muster with careful operation when doing transfers. Here https://www.eevblog.com/forum/testgear/fluke-540b-thermal-transfer-standard/ users report that 0.01% is doable.

Edit: For frequencies above 1 MHz you will need additional plugins for Fluke 540B.

The accuracy and precision of today's budget-priced electronic equipment sufficiently exceeds that of equipment available when the HP3458A was introduced in 1989 to justify a modification of the claibration procedures.

This is precision analog, not CPUs or whatnot. 3458A has the best ADC on the planet, unbeatable since 1989. Even Keysight apparently struggles to make a modernized replacement. That speaks volumes.
« Last Edit: October 29, 2017, 09:28:15 pm by lukier »
 
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Offline TiN

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #10 on: October 30, 2017, 04:42:44 am »
Quote
The accuracy and precision of today's budget-priced electronic equipment sufficiently exceeds that of equipment available when the HP3458A was introduced in 1989
You are in for a surprise. One of the reasons why 3458A is still currently manufactured and supported product since it's release in 29 years ago is simply due to precision and accuracy of it is not beaten by any modern equipment in that price range. There are handful of meters than are on par, with much higher pricetag (8508A on resistance, 5790A/B on AC voltage/current, etc).

Quote
My guess is that the procedure above might improve on the 'thermal converter' method by an order of magnitude, particularly considering the state of gear on eBay these days.
Thermal converter method is what is still currently used by many metrology labs and NMIs, together with AC voltage JJA systems (for those who can afford them). Fluke 792A is current commercial standard for AC voltage measurement for highest accuracy, still based on thermal conversion method. And 57xx series calibrators as well.  :)

I'd suggest to research the literature more. Also HP Journal covering 3458A have plenty of juicy details.
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Offline texaspyro

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #11 on: October 30, 2017, 05:28:47 am »
Sorry,  I can't stop giggling at the thought of using a scope to calibrate a 3458A...   :-DD  Maybe use an LM327 and some carbon comp resistors for the 10V / 10K cal standards?  :-//

Also,  I doubt that we will ever see a DMM with significantly better overall performance than the 3458A and today's other highest end meters.   8.5 digits is approaching the physical limits of what silicon can achieve at room temperature.
 
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Offline View[+]Finder

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #12 on: October 30, 2017, 07:40:18 pm »
Thans for the comments--glad to hear there is still humor in electronics, even if at someone else's expense.

However, several posts missed the point: my choice was between copying the contents of the NVSRAM (then loading on to the new chip) and just doing a full calibration on the 3458A once the new chips are installed. That's it. Yes, I could send it to Keysight for calibration, but how does that help someone who neither has the skill and gear to do an SRAM clone nor the funds to send to a cal lab?

Some readers (and perhaps a few posters) might have their 3458A in a controlled environment, fully caibrated to yield measurment to the full spec of a 'new' meter and up and running 24/7. That might be my goal, but falling short of perfection is not failure. Considering the comment "precision and accuracy," I would add "repeatability" as the feature that sets the 3458A apart. That is something I learned from a very smart guy on 3458A's who also was the source of my resistance calibration standards and a wealth of good advice.

For those who don't know, The major selling point of the 3458A back in 1989 was that it required only two standards to calibrate: 10VDC and 10kOhm. That was sufficient for all DC measures. The SCAL was needed only every 2 years and generally was done in a cal lab. The AC section of the 3458A--unlike my 34465A DMM--can measure RMS voltage over a range from 1Hz to 10MHz. As far as I have found in the manual, this capability is not a front panel setting, only GPIB, any help here? In any event, this measure is not on my 'must-haves' list. SCAL is needed to setup for any AC measure, hence my need for an alternative approach. BTW, the AC section is six and a half digits and who know what one should expect from an eBay thermal converter. Good luck measuring RMS with a 'heater and thermocouple' anywhere outside a temperature controlled, draft-free space.

Finally, the RMS voltage is being generated by the Siglent 2042X linked to a Rh 10MHz standard--far superior to the 3325A synthesizer--and the 10bit ADC o'scope is in the circuit to monitor and fine tune the RMS.

Here's my conclusion: disassembling the 3458A, removing the A5 board, unsoldering a few chips, adding sockets and new chips and putting it all back together will require a full calibration even if the SRAM cloning is successful. Add to this the uncertainty of 'component fondling' during assembly, thermal shock from the soldering and assistance from the family pet and a full calibration is the only option.





 

Offline TheSteve

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #13 on: October 30, 2017, 07:48:16 pm »
Removing the A5 board should not change calibration at all.
A device to read/write the nvram IC's is cheaper then the new nvrams. If you can afford a 3458A hopefully you can cough up the money for a programmer(less then 50 bucks).
VE7FM
 
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Offline View[+]Finder

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #14 on: October 30, 2017, 08:55:59 pm »
I was wrong: the 3458A display shows AC voltage as RMS. If generator output is 3V RMS, that is what is on the 3458A; 3VAC.

To Steve's comment: it is not about the cost of the programmer (I probably have one, it's been a while); it's about "Why bother?" I don't regularly use chip reading and writing software and everytime I do it turns into a day's work--the software needs updating, I can't find the programmer, WTF do I do next . . .. That's on me.
 

I have a 10V standard and a 10kOhm standard as well, so I can get the 3458A calibrated for DC no problem. I don't see the need for the cloning operation to avoid the SCAL. Anyone who is replacing NVSRAM is probably working on a 3458A that is way overdue for an SCAL and using the data form the clone is not a substitute for calibration.

Here's a thought: if the 3458A is still working (as mine is) use it to calibrate the signal generator. Hmmm . . .

 

Offline dr.diesel

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #15 on: October 30, 2017, 09:12:28 pm »
I have a 10V standard and a 10kOhm standard as well, so I can get the 3458A calibrated for DC no problem.

Are they the appropriate standards for a 3458A cal?

There is only a handful of people with these laying around at home in calibration.

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #16 on: October 30, 2017, 10:57:11 pm »
Good question. I'd say "not as good as it deserves" but good enough until I have to replace the NVSRAM. Then it gets a trip to Loveland for a real cal. I was very lucky and got a very clean and functional 3458A on eBay for under $3k, no errors other than needing a calibration.

The 10kOhm is Leeds and Northrup calibrated against a 3458A Op002 with a NIST cal cert. The 'actual' value is 10.00077kOhms over a 24hrs in a water bath. Temp was correlated with resistance--I have the papers on it from the calibrator.

My 10V is http://www.ianjohnston.com voltage standard first model. It was calibrated against a 3458A (I don't recall whether Op002) with 10.00000 at 23.1C. I have his new model on order.

The bench 34465A is in cal, but will need a calibration soon.

So to answer your question, I'm not anywhere near metrology standards, but with the tools available, I can work on ebay gear and repair and calibrate up to the standard when most of it was new 25 or so years ago.

Thanks
 

Online Dr. Frank

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #17 on: October 30, 2017, 11:15:34 pm »
If you send your instrument to Loveland anyhow, this discussion about copying or not is quite useless.

But don't miss the fact, that KS will definitely replace the A5 board with a modern one, if you won't change the nvSRAMs before.
The old PCBs have the advantage, that you can add 128kB of reading memory for free, as KS will charge you additional ~ 500 bucks for that option.

If you measure your references by your 3458A, just before you change the SRAMs, this should be good enough for a close reconstruction of the two calibration constants.. you can just write them down before the re-calibration, and compare them afterwards, how big the difference might be.

In the end, it's up to you, the advices given were the least costly options, I think.
Frank
 

Online wraper

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #18 on: October 30, 2017, 11:37:15 pm »
To Steve's comment: it is not about the cost of the programmer (I probably have one, it's been a while); it's about "Why bother?" I don't regularly use chip reading and writing software and everytime I do it turns into a day's work--the software needs updating, I can't find the programmer, WTF do I do next . . .. That's on me.
Do you realize you spent 10x more time on this tread than it would take to copy SRAM contents?
 
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Offline macboy

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #19 on: October 31, 2017, 12:31:59 am »
True, the AC side of the DMM is more complicated, however the calibration process has its origin in the time of the introduction of the 3458A. Back then the measurement of RMS was analog and derived from heating a wire and measuring voltage with a thermistor to get a DC equivalent ‘RMS’. According to what I’ve read, digitized measures as in a modern scope or DMM will yield an accurate RMS voltage at various frequencies. My bench DVM is a Keysight 34465 that measures true RMS by some sort of heating process and I have a signal generator that is supposed to be able to generate a sine wave at a specific RMS voltage.

The 3458 calibration calls for AC at specified RMS voltage and frequencies, so my plan is to measure the sig gen with the 34465 in parallel with the 3458 at the various stages of the cal process.

Yes, money helps—the 34465 cost a bit, but worth every penny—but most of my gear came from eBay and repaired by hand. Over time I bootstrapped from 4.5 digits to 5.5, 6.5 finally to the 3458 also from eBay.

I had to learn a lot along the way and I’m by no standards an expert, so if there is a flaw in the thinking, let me know.

Thanks to all
You really need to read the manuals and datasheets of your gear. Otherwise you have no idea what they are capable of.

Neither the 3458A nor the newer 34465A uses a thermal RMS converter. Actually, thermal RMS converters have always been quite uncommon, especially ones that are done right.  The 3458A has three choices of RMS measurement technique. One is using analog computation, which is incidentally by far the most common method of RMS measurement. This involves passing the AC signal through a log amp, taking an average, and passing through an anti-log amp. This is implicitly calculating the RMS value (the result is the same, but it isn't doing actual square, mean, and square-root calculations).  The 3458A's  second and generally preferred method is the synchronous sub sampling method which works well for repetitive signals up to 10 MHz. The third is the random sampling method, good for random signals.  The 34465A uses the sub-sampling methods that were designed for the 3458A. Judging solely by the datasheet, it doesn't seem to have any analog-calculating type of RMS converter, and definitely has no thermal converter.

The analog computation method is the most popular largely because Analog Devices has a set of easy to use and accurate ICs to perform this computation: AD636, AD637, etc. You can find these in many meters, both handheld and bench. If it does True RMS with more than about 40 or 50 kHz of bandwidth, the chances are very good that it uses one.
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #20 on: October 31, 2017, 02:37:26 am »
1) My purpose in starting this thread is to assist someone else who (for whatever reason) might want to calibrate their 3458A with a minimum amount of gear or replace the NVSRAM without having to clone the old one. The time I spent might help someone else spend their time doing something else.

2) Why would anyone start a thread like this without having first read the manuals? Really? More to the point, before even looking on eBay for a 3458A, I read the manuals and the 1989 Keynote article.

3) The goal is to have a working 3458A after the day arrives when the NVSRAM's are replaced. The bulk of the posts and YouTube vids left me with the impression that there was "factory-magic" in the CAL RAM that needed to be copied or the 3458A would become a brick. Thanks to this forum, I learned that was not true: a full calibration is all that is needed to put the DMM back online. No, I do not want to spend $1,000 for a new board at Keysight, hence the need to do the SRAM replacement first. I also read enough of the 3325A manual to conclude that, for the purpose at hand, it is just a sine wave signal generator. Then I read up on thermal converters--a thermocouple combined with a resistor--about as old-school analog as you can get.

Now, after trying the sig-gen/oscilloscope combo to do the SCAL, I learned a few things: first, it can be done; second, an analog thermal converter might have an advantage over the digital setup I used if the objective is to bring the 3458A fully up to spec; and finally, spending time helping others is never wasted.
 

Offline texaspyro

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #21 on: October 31, 2017, 02:55:49 am »
Get a TL866 device programmer.  They are cheap, easy to use,  and very versatile and can do the NVRAM module.   My recommendation is to first dump the cal ram using the GPIB connector.  KE5FX's gpibkit package includes my cal ram dumper program.  It outputs a listing of the cal ram contents as a text file that lists all the current settings.  It also outputs a binary ROM image.  The GPIB dump is insurance against a glitch in desoldering the NVRAM corrupting the data.

Once you pull the NVRAM, use the TL866 to read it and save the contents to another file as an additional backup.  Program the new NVRAM chip with the old data.  Even  decades old SCAL values will be much more accurate than anything you can do at home.  The same applies for the 10V/10K artifact calibration unless you have a calibrated 732A/732B and SR104 standard resistor. You REALLY don't want to mess up a 3458A by using inferior calibration standards.
 
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Offline TiN

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #22 on: October 31, 2017, 05:15:47 am »
I would be far more concerned with ADC stability (on which topic I've posted many times), than with home calibrations, leave alone SCAL. As if your meter unstable, you are in for 1300$ ADC replacement the moment you send it to KS cal.
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Online wraper

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #23 on: October 31, 2017, 11:12:53 am »
1) My purpose in starting this thread is to assist someone else who (for whatever reason) might want to calibrate their 3458A with a minimum amount of gear or replace the NVSRAM without having to clone the old one. The time I spent might help someone else spend their time doing something else.
Barely anyone has all equipment needed to calibrate 3458A. And if someone has equipment required, he/she doesn't need this tread in the first place. From what you wrote, you don't have equipment needed to properly calibrate 3458A.

Quote
Procedure:
The 2042x is connected to the HMO1212 with a BNC cable and also to the 3458A with a BNC 'T-connector'.
The 2042X is a substitute for the HP3325A function generator and the 'scope takes the place of the thermal converter. In this case the measurement of RMS voltage (DC equivalent) can be obtained from the 'scope directly and the 2042X adjusted to match the required input voltage. This will eliminate the steps associated with measuring the voltage from the thermal converter in order to correct the RMS errors in the 3325A output.
This procedure is not good even for cheap 6000 count multimeter.
Quote
that help someone who neither has the skill and gear to do an SRAM clone nor the funds to send to a cal lab?
It's relatively easy and equipment required costs <$50. Poor man's calibration is only valid if SRAM already lost it's data.
« Last Edit: October 31, 2017, 11:39:17 am by wraper »
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #24 on: October 31, 2017, 11:31:03 am »
I would be far more concerned with ADC stability (on which topic I've posted many times), than with home calibrations, leave alone SCAL. As if your meter unstable, you are in for 1300$ ADC replacement the moment you send it to KS cal.
Yep, I'd be concerned what Keysight would say about your homebrew calibration. They may decide that it drifted that much that meter should be faulty.
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #25 on: October 31, 2017, 01:12:50 pm »
I get the feeling he's not going to change his views anyway.   |O
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #26 on: November 04, 2017, 07:54:58 am »
Comments and advice were much appreciated, so if I have "thanked-you," your SCAL opinions were duly noted and need no further expansion.

Generally, opinions fell into three categories:
1) "It can't be done."
2) "It shouldn't be done."
3) "It has to be done this way." (followed by a link to software or a programmer, etc.)

Please recall my reason for the post: the choice between cloning and calibrating--and as my 3458A is working fine, the ability to calibrate after NVSRAM replacement poses fewer risks for me. (The 'cloning' would have to be done in advance, as most of the posts suggest)

Several RTFM comments--they always get my attention--sent me back to the Cal Manual for a closer read. My procedure was inferred from this paragraph from the manual:
"In the following procedure, steps 1 through 12 characterize the frequency flatness of the synthesizer and cabling configuration. The equipment setting determined from this characterization are then used in the remaining steps to precisely adjust the multimeter."

In other words, the thermal conversion and voltage measurement is required to adjust for errors coming from the synthesizer. The first printing of the 3325A was in 1978 fully 10 years before the release of the 3458A, so errors were certainly anticipated and to be managed in the cal process. Once the errors are known, the synthesizer would be adjusted to conform its RMS voltage to flatten the response across frequencies. As the cal manual continues: "you can use any value between 3V and 10V rms wherever 3V is referenced, etc." Clearly, the desired quality is not the precision of the voltage as measured by the thermal converter, but rather that the synthesizer deliver a flat response.

Even the 50-ohm resistor on the input during the calibration is needed solely to deal with the impedance mismatch between the DMM and the synthesizer. To be fair, the advantage of the thermal converter approach is that the DC side of the 3458A can be used for the critical measurements and the synthesizer spec shows much finer resolution on the voltage scale.

The express purpose was to determine whether a full calibration (with SCAL) will replace nearly all of the 253 calibration constants--certainly not as a substitute for a trip to a cal lab, but as an alternative to the cloning process described elsewhere in this forum. With that in mind and not being too concerned with perfection, my process was as follows:
1) Forget the synthesizer, thermal converters and the 50-ohm resistor; work with 50-ohm output on the source and use a 50-ohm terminator on the BNC cable to banana plug at the DMM
2) Use a modern signal generator as the source (Siglent SDG2042X)
3) Use an oscilloscope to 'characterize' the output of the sig gen in the same manner as the SCAL procedure in the Cal Manual R&S HMO1212 (16bit)
4) Apply the sine wave signal to the 3458A using the sig gen and following the SCAL procedure

Only the 100mV values needed any adjustment: the sig gen was set to 125mV to deliver 100mV. The results were better (i.e. more linear) below 1MHz when comparing the output of the sig gen with the display on the 3458A. (I'll post a table of values later.) There appears to be problem with linearity above 1MHz, but that was the case before this experiment. The DC side of the 3458A does not have any problems.
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #27 on: November 08, 2017, 07:04:07 am »
UPDATE: 'RATIO MEASUREMENT'
Improved method for adjusting the RMS voltage from a signal generator (instead of using the thermal converters, etc.) to conform to the values expected in the 3458A SCAL calibration procedure.

The 3458A can calculate a 'ratio' measurement between a known DC 'reference' voltage on the 'sense' side and an unknown voltage on the 'input' side. The EDC CR103 DC standard was used as the reference voltage (measured to 3.00000VDC on the 34465A) and the output from the sig gen was used as the input on the 3458A. The ratio measurement was selected on the front panel ('RATIO ON') and the ACV scale was used. With the sig gen on 2MHz, the amplitude 3VRMS on the sig gen was adjusted until the ratio was as close to 1.000 as possible. Without changing any settings, the 3458A was selected to SCAL 10 and 'enter' pressed. The steps were repeated for the 8MHz 3VRMS SCAL setting. Lower frequencies (<300kHz) were set using the 34465A as on the prior post, but could have been set using the method above.

Using the ratio measurement with the 3458A, a ratio of 1.000 against a 3VDC reference means that the DC equivalent (RMS) of the AC signal is also 3VRMS. This result can be obtained with precision equal to thermal converters (home lab, not metrology) at far less cost. The completed SCAL was tested against the "Performance Test Card, 90-Day Limits" for the HP3457A (the model under the 3458A) and passed all tests up to 1MHz, the limit of the 3457A. (30V and 300V tests were not included)

The equipment required to calibrate the 3458A:
DC (Cal 0)--a 10cm piece of copper wire 1mm-1.5mm for thermal short
DC (Cal 10)--a 10V 'reference standard' (I use Ian Johnston's PDVS2, but all is required is that it be a stable, known voltage that can be input at the prompt e.g. '10.000015'
Resistance and current (Cal 10.00077E3)--a precision 10kOhm resistor (I use a Leeds & Northrup NIST traceable, but a precision resistor from Mouser might work)
AC (SCAL)--a digital signal generator (I use a 10MHz Ru standard as 'clock-in', but a 4Hr warmup is OK too); a stable, low noise DC voltage source (3V, 1V, 100mV);  If the DC side of the 3458A is OK, that can be the 'meter' for all the DC inputs.
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #28 on: November 08, 2017, 10:23:15 am »
UPDATE: 'RATIO MEASUREMENT'
Improved method for adjusting the RMS voltage from a signal generator (instead of using the thermal converters, etc.) to conform to the values expected in the 3458A SCAL calibration procedure.

The 3458A can calculate a 'ratio' measurement between a known DC 'reference' voltage on the 'sense' side and an unknown voltage on the 'input' side. The EDC CR103 DC standard was used as the reference voltage (measured to 3.00000VDC on the 34465A) and the output from the sig gen was used as the input on the 3458A. The ratio measurement was selected on the front panel ('RATIO ON') and the ACV scale was used. With the sig gen on 2MHz, the amplitude 3VRMS on the sig gen was adjusted until the ratio was as close to 1.000 as possible. Without changing any settings, the 3458A was selected to SCAL 10 and 'enter' pressed. The steps were repeated for the 8MHz 3VRMS SCAL setting. Lower frequencies (<300kHz) were set using the 34465A as on the prior post, but could have been set using the method above.

Using the ratio measurement with the 3458A, a ratio of 1.000 against a 3VDC reference means that the DC equivalent (RMS) of the AC signal is also 3VRMS. This result can be obtained with precision equal to thermal converters (home lab, not metrology) at far less cost. The completed SCAL was tested against the "Performance Test Card, 90-Day Limits" for the HP3457A (the model under the 3458A) and passed all tests up to 1MHz, the limit of the 3457A. (30V and 300V tests were not included)

...

This method is a cheat only.
The 3458A will first convert the ACV to a DC reading, involving its own internal AC amplifier circuit.
As this AC amplifier is not calibrated correctly in first place, also this ratio measurement is erroneous.

Therefore, you need at first to calibrate this frequency dependency of the 3458A AC amplifier by an external AC standard for 2MHz and 8MHz, BEFORE you might use any ratio AC / DC transfer. So you can't avoid an instrument like a calibrated thermo-converter.

Btw.: The 2MHz and 8MHz specification of the 3458A  is quite mediocre, like 1.5% for 2MHz and 15% for 8..10MHz at best . So your bootstrap solution also suffers from bad accuracy, as a good TUR is required.
Therefore, the 3458A will never be able to replace a TC for AC /DC conversion due to these bad specifications..
MJTCs can have a transfer accuracy of a few ppm up to 1MHz, < 0.1% at 10MHz and < 0.8% up to 100MHz.
See Table 9.7. and 9.7e., here:
https://www.nist.gov/calibrations/voltage-measurements-calibrations#533

Frank
« Last Edit: November 08, 2017, 11:01:57 am by Dr. Frank »
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #29 on: November 08, 2017, 04:47:10 pm »
Thanks for the NIST link--I've found it better to know how calibration is done at the metrology level to understand the underlying principles and the sources of error. You are absolutely correct: this is "a cheat at best." So, being an American of the old-school, I take that as a compliment.

The objective of this thread was to find a way to avoid having to disassemble a working 3458A so that the NVSRAM's could be copied before an eventual battery failure. I'm satisfied that running all the calibration routines will put my 3458A back a condition 'as good as it was before' and that for me, is good enough.

I found your comment "The 2MHz and 8MHz specification of the 3458A  is quite mediocre . . ." most interesting: if I understand correctly, the 3458A is not so good above 2MHz, so why should I get all fussy about nailing the spec, particularly on the lower voltages? Hmmm.

The weakest link in my chain is the signal generator, in particular the granularity of the the amplitude adjustment, so the voltages are just 'close'. That is where the suggested instruments shine: the RMS voltage can be spot on. Maybe I'll go shopping on eBay . . .
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #30 on: November 08, 2017, 04:52:25 pm »
Look at ratio transformers.  Basically a variac with 6-7 decades of resolution.  But these have limits on what frequencies they can handle.  I have ESI and Gertsch ratio transformers.
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #31 on: November 08, 2017, 04:57:43 pm »

The objective of this thread was to find a way to avoid having to disassemble a working 3458A so that the NVSRAM's could be copied before an eventual battery failure.


The calram data can be dumped over GPIB without opening the case.   See my HP3458 program in KE5FX's gpibkit package.  It will output a binary image that a device programmer can use to write a blank SRAM module.  It will also output a formatted ASCII listing of all the cal constant values.  It can also dump the data SRAMs, but that is not particularly useful since those SRAMS are also use by the firmware as general purpose memory.
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #32 on: November 08, 2017, 06:20:37 pm »
You are absolutely correct: this is "a cheat at best." So, being an American of the old-school, I take that as a compliment.

..

I found your comment "The 2MHz and 8MHz specification of the 3458A  is quite mediocre . . ." most interesting: if I understand correctly, the 3458A is not so good above 2MHz, so why should I get all fussy about nailing the spec, particularly on the lower voltages? Hmmm.

The weakest link in my chain is the signal generator, in particular the granularity of the the amplitude adjustment, so the voltages are just 'close'. That is where the suggested instruments shine: the RMS voltage can be spot on. Maybe I'll go shopping on eBay . . .

Well, I had in mind, that you'd cheat, or fool yourself by this bootstrap idea, as it's a self-referred, or circular measurement.
Sorry, there was no pun intended, especially not related under any aspect to the current American president  ;)

Concerning the ACV specification, please consider, which other DMM offers that high bandwidth, up to 10MHz.
The accuracy is still at 4% up to 8MHz, and I remember many old HP TVMs , like I think the hp400E, which also measured up into that frequency region, with a similar accuracy.
So these RF specs of the 3458A are quite OK, but it really shines up to 1kHz, where it competes with the highest grade calibration instruments.

Anyhow, the ACV performance is often criticised by PMEL metrologists, for some reason...this said AC pre-amplifier could have been designed much better.

The oscillator does not need fine amplitude adjustment, any random value is accepted in these cal steps.
It only has to be stable, a few tenths of percent maybe, as a transfer is already made between different output frequencies.
I also assume, that the specification is better than these 30 year old data imply, when they had to use these in-sensitive 2mV output single thermocouple. If you use a much better MJTC, like the ones offered by NIST (SRM6001), you could proof that better accuracy to a lower degree of uncertainty.

Frank

« Last Edit: November 08, 2017, 06:25:12 pm by Dr. Frank »
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #33 on: November 08, 2017, 06:26:05 pm »
Hey, thanks 'pyro' for the link to the GPIB dump of the cal data. From bitter experience I learned not to click on the big green 'DOWNLOAD' button or run executables found on the 'net. I wrote a Python script that reads from the 3458A DC, AC, etc. and can probably figure out how to get the cal data, but if you have time, a few line of source code from your program would help.

I already figured out that the cal SRAM was the only one that really needed to be preserved (or not . . .) nice to have a confirmation, thanks. The key assignments can be recreated and I don't have any stored subroutines.

Here's my code:
import sys
import time
import visa

HP3458 = visa.GpibInstrument('GPIB0::22::INSTR',timeout=20,term_chars = "")
HP3458.write('END ON;RESET')
print (HP3458.ask('END ON;ID?'))

HP3458.write("PRESET NORM;OFORMAT ASCII;INBUF ON;TARM AUTO;TRIG AUTO;NPLC 10;MFORMAT 1;MEM OFF;NDIG 8;END ON")

HP3458.clear()
HP3458.write('FUNC 2')
#HP3458.write('RATIO ON')
data = open('datafile.txt','w')
print data
for x in range(0, 200):
    data.write (HP3458.ask('END ON')+ '\n')
    #time.sleep(1) # 1 sec
HP3458.write('END ON;RESET')
data.close()

That's using Keysight's latest VISA and Python 2.7.14

Cheers,
Donal

 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #34 on: November 08, 2017, 06:47:29 pm »
Thanks Dr. F
Now I know exactly what my wife will get for a holiday present: an SRM6001 for (only) $2,000! It even looks like jewelery, probaly comes in a nice little box.

Seriously, that is a fantastic creation--I had no idea NIST was in the builder space, much less a vendor--and a step in the same direction as the work with JJ's.

LoL on our 'Clown in Chief' I now have a serious case of euro-envy . . .

MFG,
Donal
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #35 on: November 08, 2017, 07:40:24 pm »
Latest NIST MJTC Standard is the 6002
6002a Multi-Junction Thermal Converter (Nominal input resistance 600 ? ± 15 ?)  $2,000
6002b Multi-Junction Thermal Converter (Nominal input resistance 1040 ? ± 20 ?)   $2,000
(Price for ncalibrated device)
https://www.nist.gov/sri/standard-reference-instruments/sri-6002-multi-junction-thermal-converter
Specs: https://www.nist.gov/file/376126
You may also have interest into the Fluke 972A
https://www.ebay.de/sch/i.html?_from=R40&_trksid=p2055119.m570.l1313.TR0.TRC0.H0.Xfluke+792a.TRS0&_nkw=fluke+792a&_sacat=0
BR
/PeLuLe
You will learn something new every single day
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #36 on: November 08, 2017, 08:37:53 pm »
Hey, thanks 'pyro' for the link to the GPIB dump of the cal data. From bitter experience I learned not to click on the big green 'DOWNLOAD' button or run executables found on the 'net.

GPIBKIT is open source and includes the source code.   GPIBKIT and TIMELAB are used by thousands of people over the last decade and there have never been any reports of cyber-cooties.  You can always run the .EXEs past virustotal.com and let a couple dozen different programs scan it.
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #37 on: November 08, 2017, 10:09:12 pm »
Good tip--I didn't know about virustotal
thanks
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #38 on: November 08, 2017, 10:54:37 pm »
Sorry, yep it's still the SRI 6002.
Was 1/2 the current price few years ago. But you get a rough cal report with this standard. < 1MHz is uncritical, anyhow, that's DCV, nearly.

It requires some additional apparatus, like relay switch, and a true nV DMM, for low ppm uncertainty.
« Last Edit: November 08, 2017, 11:10:25 pm by Dr. Frank »
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #39 on: November 09, 2017, 11:52:46 pm »
SCAL completed, here are the results:
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #40 on: November 10, 2017, 04:58:06 am »
Sorry, yep it's still the SRI 6002.
snip.

Yep, I'm still upset on price, regretting not to buy it when it was 998$.
Is the one on photos yours? If yes, I have some questions, as this is the one way I thought about calibrating 5790A/792A/4920M. :)
YouTube | Metrology IRC Chat room | Live-cam | Share T&M documentation? Upload! No MB limit, firmwares, photos.
 

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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #41 on: November 13, 2017, 08:00:17 pm »
UPDATE: Built-in battery backup: How long do they last?
"If it ain't broke, don't fix it" Will Rogers (just guessing)

With a working 3458A in my lab, when is the "right time" to replace the NVSRAM? "Lifetime" (well documented elsewhere . . .) is stated as 10 years, but have a look at this from the DS1230 datasheet:

The nonvolatile static RAMs constantly monitor VCC. Should the supply voltage decay, the NV SRAMs automatically write protect themselves, all inputs become “don’t care,” and all outputs become high-impedance. As VCC falls below approximately 3.0 volts, a power switching circuit connects the lithium energy source to RAM to retain data.
and this:
Each DS1230 has a built-in switch that disconnects the lithium source until the user first applies VCC. The expected tDR is defined as accumulative time in the absence of VCC starting from the time power is first applied by the user. This parameter is assured by component selection, process control, and design. It is not measured directly during production testing.

Read closely: "in the absence of VCC." This means that no power is taken from the battery while the 3458A is powered on. (Perhaps even when it is plugged in and 'off'??)

There are all sorts of lithium batteries; the one in the DS1230 are not the consumer type, they are the "really long shelf-life" chemistry as might be found in in a device that "has to work" at some time in the future and needs zero maintenance. Like, for example, a liferaft or EPIRB. The rule there is to "replace after 10 years." No exception.

As luck would have it, I happened to salvage some "replaced" 3.6V Tadiran batteries and a 9V pack from a liferaft sometime between 2001 and 2003. I figured they would be useful someday . . . and never used them. So these were at least 10 years old in 2003 (the 'rule') and possibly 25 years old today.

I measured them with my Fluke 87 (yes, it has a cal certificate from Fluke) and got 9.033V on the liferaft pack and over 3.71V on each of the four Tadiran's. (Take that as an observation, not a recommendation.)

So, if your old HP3458A on eBay really did come from "a working lab" and it was powered on as it should have been during its service, there could be many years of life left in those NVSRAM's. Oh, but if it looks like it was left out in the weather--hey, some serious science is done in severe conditions--best be prepared to work on the A5 board.

Thanks to all,
Donal
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #42 on: November 22, 2019, 08:54:45 pm »
So you're using a device containing an LM399 to calibrate a meter that uses an LTZ1000?
The Ian Johnston voltage reference is not accurate enough to properly calibrate a 3458A.

--Victor
 
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Re: HP3458A SRAM replacement: copy or calibrate?
« Reply #43 on: November 24, 2019, 11:20:53 pm »
Two years have past since my last post on the HP3458A and it is still working just fine. In the meantime I sent my 34465A meter in to Keysight for a calibration and checked the 3458A against it on it return after warmup and autocal on both. Only interested in DCV and found identical out to the display limit on the 34465A. Still the same today. I do a full cal on the 3458A (same procedure, but now with better AC and RF instruments) and the 34465 is going in for calibration next year.

I bought SRAM's for the 3458A, however I've not had to install them as the lithium battery is still keeping the memory alive during the times when the 3458A is without power or unplugged (very rare). If I have to do the SRAM's, I'll upgrade the voltage reference as well and send the meter in to Keysight for calibration. Maybe.

Couple of thoughts on meters and metrology:
There is precision and there is accuracy and then there is consistency; they are not the same. It's like "rock, paper, scissors" except precision loses to accuracy and consistency in my lab.
Unlike "new wine in old bottles," using using modern instruments to calibrate (and adjust) vintage gear makes my work easier and saves time and money.
 
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