Sensible approach to HP3456A check/calibration?

[tatel]

Hi all.

I'm just a hobbyst, not an EE, much less a volt-nut. Last June I bought an HP3456A benchtop multimeter in working condition. Just because I love good quality tools.

"Only" problem would be to check it remains into specs. IIRC last calibration sticker was dated between years 2000-2010? (I'm at about 400 Kms from the device at this moment). So, I guess some kind of checking is really in order.

Odds for me to be able to find another guy with at least a 6 1/2 digits multimeter, are scarce. Same for going into any university.

I asked for quotes from four calibration labs after some Googling. First one doesn't want to do anything with HP3456A.

Second one asks for €187. They would give me an ISO certificate. They don't do any adjustments. I still have to ask about it, but it looks to me that it would be ISO certification without any data.

Third one looks better, if asked, they do "fab" cals without any certification. But I haven't received any quote yet. They say to be working on it (I asked for quotes to calibrate four different devices, just to compare with welectron).

From the fourth lab, I have just received an automated email reply so far.

Last, I could pay for a one-week lease of an HP3458. No idea how much it would cost to me, but I fear it would be on the high hundreds of euros range. I don't have any experience checking/calibrating/adjusting a 6 1/2 digit multimeter, so don't feel comfortable learning at an accelerated pace while paying for an expensive lease. That's against any hobbyist approach. I would need some standards anyway, it isn't?. So, it doesn't look as a very good idea to me. I could look at this possibility again if nothing else works, though.

Time to look at DIY stuff, then. Surely it's probably a more expensive way to go, but if I can't find a cal lab to do what I need, it looks as the best way to me. I could take it easy, and I would be able to use that stuff again in the future. At least I could check resistance and DC voltage in the lower ranges, which is what matters most to me. Or so I hope.

Conrad Hoffman's metrology mini-lab looks good. However, I guess, having a 6 1/2 multimeter means I would need one of these LTZ1000 things. I'm OK with that, I would just ask: is that fairy tale offer from TiN and others about sending to them a DIY LTZ1000 reference to calibrate still standing?

I see RN55D resistors aren't ROHS and are affected by REACH, so I fear they will get stopped at european customs. I have never ordered any non ROHS products. Actually I filter out non-ROHS anything. What do you think? Could I get these trough  EU customs? Could you recommend any ROHS alternative?

I also looked at those metrology grade standard resistors on eBay, to do just HP3456A's performance check. Once in a while I could buy one of these until getting all the seven values the manual asks for, but I would like to check my 3456 this year...

Would these Vishay z series resistors be "good enough" for a hobbyist to do the performance check? Or is it a no-no-no?

I'll gladly take any recommendation.

Best wishes.

[Wallace Gasiewicz]

Since no one else has replied.....I have two HP 3456units 
The OHM adjust on the low ohm bands on the 3456 are done with the voltage adjusts using a volt standard 
I think probably the low OHM and low Volt Scales are of more interest. or at least that is where you probably need to be accurate.

You can use resistors to "adjust " these, but I do not think it is as good,  It is probably good enough.I bought a volt standard and had that "calibrated" ....... DOWN THE RABBIT HOLE,   

The Vishay resistors are very good. but I do not see any one lower than 36 ohms....

  In My Opinion, the biggest problem for the low OHMS is getting a good FOUR wire connection. Use only good copper wires  . 
Also you have to be careful about your ambient temperature 
There are some discussions about precision resistors on this forum.  I have a suggestion. 
See if you can find an older resistance bridge on local EBay that has connections for use as a resistance box and use that for "calibration". 
Don't buy the newer inexpensive ones from China.
I bought one of these in nice shape for $20 at a hamfest:  (not too old and not too expensive, even at asking price)

https://www.ebay.com/itm/176119902021?hash=item2901906f45:g:P1IAAOSwzihlhFW-&amdata=enc%3AAQAIAAAA0LEsIbOmBD8KsFlULRNHKwVjva0cQP7PUX5StAhKkaR9VQ01m5yRlCr6hFg2Pb1Qy3%2BDp5BAVO4Q62KjLADiWb6jxGLYlsxuLE%2Bl6e8ogvn6W2K%2BzvPmmIr%2BFckUV1JcFGEIeZ8BVlFOMNMvmwurL%2FIW52lLIg9jeKGUK%2BcTHEKvG%2BQ5EGQoQ00sSGVe3Rf%2BuKZ54fkAjEPQWrJHesI9MFgy0Uq3ferIhfh4hk%2B8Qp0lTGuAIWD%2B%2Bjg9FNUOeFo65WBq3zYoy2f2ERiqff5I0V0%3D%7Ctkp%3ABk9SR8adxI2hYw
I know this is not a perfect solution but will get you  closer than you need to be....You could get the Resistance Bridge Calibrated more easily than a 3456. Perhaps there is some Lab near you???

[tatel]

Thank you very much for your answer.

We shall see if second calibration lab is able/willing to do what could be needed, and how much money they ask for. First one was not.

I already ordered one each of 100, 1K, 10K and 100K Vishay resistors, Z and S series, 0.01% and, supposedly, 1-0.2 ppm. 28-36 € each, plus VAT. For the higher Ohm ranges, just looked for the best tolerance/ppm in stock on Digikey right now. These higher range resistors are way cheaper, about 4-9 € each, plus VAT.

I know these are neither metrology-grade resistors nor what HP3456 manual calls for. Far from it. However I'm not willing to spend more on "standard" resistors. I think there's no need to go further, taking into account what my skills are and what I do as a hobbyist. I just hope they will allow me to see if Ohm ranges are out of specs  right now. At least lower value/pricier ones. I also hope, at least the pricier ones will be minimally stable to be used again in the future.

After checking resistance ranges, I will make the decision about sending the HP3456 to calibrate/adjust, if any calibration lab finally sends a quote to me. I already have a DMMCheck. It can be sent to welectron in Germany to be calibrated again if needed. That way, I'm quite confident I'll be able to see if any of my "standards" gets out of whack in the future.

If I understand it right, HP3456 has a LM*99 internal voltage reference?. I don't think DMMCheck LT1021 is better than that. So a better voltage reference would be in order. Not sure about making a LM399 reference, it probably wouldn't be good enough?

Checking against DMMCheck, my HP3456 seems to be quite good, just the 100 Ohm range looks bad, at 10 PLC 2-wire, 5 1/2 and 6 1/2 digits.

I take note of what you say about resistance bridge/box. Something like that if I'm getting it right?
https://www.ebay.es/itm/256087451550?
https://www.ebay.es/itm/175876283467?

Problem is, not only shipping from the US is crazy expensive, but that US sellers doesn't seem to like eBay's international shipping *program*. In that case, we EU buyers are also extorted a customs paperwork fee (about €50 in that price range), and, on top of it all, VAT. Far, far away from getting it on a hamfest. But I will look for it. Perhaps a bargain will appear, who knows. I already got an ebay search at work

Even if there's no bargain bridge/box, it could still be cheaper than buying about seven hundred resistors to match about 30 of these.

Last, this is what i got for 4-wire connection, I still have to do a BNC-banana adapter for HP3456 and DEER EE-5000, that stuff is lying around in some drawer right now.

https://aliexpress.com/item/32505106470.html?
https://aliexpress.com/item/1005004235132283.html?

Thank you again

Edit: link

[TUMEMBER]

Problem is, not only shipping from the US is crazy expensive, but that US sellers doesn't seem to like eBay's international shipping *program*. In that case, we EU buyers are also extorted a customs paperwork fee (about €50 in that price range), and, on top of it all, VAT. Far, far away from getting it on a hamfest. But I will look for it. Perhaps a bargain will appear, who knows. I already got an ebay search at work

Even if there's no bargain bridge/box, it could still be cheaper than buying about seven hundred resistors to match about 30 of these.

You can try here.   
https://anmar-pl.com/en/laboratorium-metrologii-elektrycznej/

There will be no problem with customs duties because it is a lab in Poland, i.e. the EU. But you would have to specify whether you are only interested in the document with amendments or additional regulations. The "paper" itself does not seem too expensive (about EUR 160 net and PLN 700). You would have to find out how much the adjustment would cost. The cost of "transportation" is probably also lower than shipping to a lab in the USA.
https://anmar-pl.com/kategoria-produktu/wzorcowanie/keithley-wzorcowanie-standardowe/

[tatel]

Thank you for the link. I will put this card up my sleeve

[donlisms]

I'm not sure I'd agree with the Wheatstone bridge approach, unless you really want to get into measuring resistance with the bridge. To me, it's a fascinating way to make potentially high resolution resistance measurements, but you either have to just take the accuracy the thing gives you as it is, or start another hobby of how to calibrate the bridge and it's elements.

The "bridge" is actually just a configuration of resistors, connections, and the detector.  It gives comparisons only.  The commercial units are a collection of all those parts in a usable form. The absolute measurement accuracy depends pretty much just on the standard resistor you're comparing the unknown to. The adjustable decades built into the box are quite convenient, but there isn't necessarily a promise that they're very high accuracy - it can vary widely, and then there's drift over the years.

Those decades are really no different than a separate decade resistor. I suspect that separately, you have a better chance at choosing something with higher accuracy. The total cost goes into the decades, rather than being split between the decades and the rest of the stuff.

Then again, with the circumstances you describe, I think you're already in pretty good shape.  Go forth and measure, and be content. 

[Kleinstein]

Checking against DMMCheck, my HP3456 seems to be quite good, just the 100 Ohm range looks bad, at 10 PLC 2-wire, 5 1/2 and 6 1/2 digits.

Those relatively cheap reference boxes may be an option. At least for a short time and with some risc of getting fake numbers they may give relatively good measured values for the voltages and resistors.
The quality of the ref. chips makes a difference over the long time and with the noise (but one can average a bit longer) and TC (so measure at a sensible temperature.

For an accurate resistance measurement one should use 4 wire mode, as especially for the small resistors. 2 wire ohm is no supposed to be accurate at 100 Ohm, it is for a quick test and > 100 K or so.

For the resistance one can compare the ranges relative to each other, so measure the 100 ohm in both the 100 ohm and 1 K range.
Another check one could do is comparing 3 equal resistors in series and parallel - which should give good 1:9 resistor steps, even of the resistors are not super accurate.  One should still have better than 0.1 % though - at least easy to check with the meter.

One can do a similar test between voltage ranges, but stable voltage references may be more tricky than stable resistors.

[tatel]

I'm not sure I'd agree with the Wheatstone bridge approach, unless you really want to get into measuring resistance with the bridge. To me, it's a fascinating way to make potentially high resolution resistance measurements, but you either have to just take the accuracy the thing gives you as it is, or start another hobby of how to calibrate the bridge and it's elements.
(...)
Then again, with the circumstances you describe, I think you're already in pretty good shape.  Go forth and measure, and be content.

Very encouraging, thank you. Of course, if, like it happened to Wallace Gasiewicz, one of these Wheatstone bridge gets across my way, on the low tens of euros price range, I'll buy it on the spot. No matter if I really need or not...

Alas, this probably won't happen anytime soon, so I'm looking at Conrad Hoffman's metrology mini-lab. Choosing between Vishay MRS and CMF series to build that Kelvin-Varley divider. I'll probably go with MRS since they are cheaper. So I could buy say, some 250 pcs of each value that needs matching.

If I'm getting it right, my 3456 should allow me to compare the resistors, no matter if the multimemer is spot on or has some drift.

[tatel]

For an accurate resistance measurement one should use 4 wire mode, as especially for the small resistors. 2 wire ohm is no supposed to be accurate at 100 Ohm, it is for a quick test and > 100 K or so.

For the resistance one can compare the ranges relative to each other, so measure the 100 ohm in both the 100 ohm and 1 K range.
Another check one could do is comparing 3 equal resistors in series and parallel - which should give good 1:9 resistor steps, even of the resistors are not super accurate.  One should still have better than 0.1 % though - at least easy to check with the meter.

One can do a similar test between voltage ranges, but stable voltage references may be more tricky than stable resistors.

Understood. Thank you for the tips, really appreciated.

[Wallace Gasiewicz]

Some Wheatstone Bridges have terminals that function as a "Resistor Box"  These terminals are labeled "R" and the terminals for the unknown are labeled "X" 
When measuring the resistance of an unknown resistor you use the "X" terminals.
That is why I included the specific example that appears in the ebay ad. 
The regular Wheatstone Bridges would not work , But this "Resistor Box" option is just an internal connection to the resistor bank.
Many of them have very good resistors that are temp stable and have already aged.I think high grade modern resistors are probably better, but these Bridges have VERY good wire wound resistors that cost a bundle when they were new.There are new resistor boxes available, The good ones are very expensive,  I just made this suggestion because it seems to me that the combo Bridge and Resistor box units appear to be a good bargain. especially since I bought mine for $20, in really good shape. Less than one good resistor.

[GigaJoe]

wonder:
 - does it has fan on back ?
buttons - loud harsh clickable or plush soft ?

before any cals , i would recommend disassemble - replace all electrolytic caps  especially PSU ,  and clean all boards  from contaminations and connectors.  usually it quite dirty after 40 years.

then run turn-on 24-7 for 2-3 weeks , or more (longer - better ) ,  then do adjustments

all adjustments manually -   so labs really not in mood to do such adjustments.
so rather - DIY reference box , and someone may willing to measure it as a reference point.

[J-R]

I am curious with regard to your goals going forward.  Are you wanting to experiment with vintage equipment or do you have specific tasks in mind?

While I assume you got a great deal on that HP3456A, I am not sure I see the value in spending money on it.  Why not save up and buy something newer with a recent calibration?

With hobbies, everyone has their own desires, of course...

[Conrad Hoffman]

GR 1432 resistance boxes are .05% but are typically better. I'd get a couple of those, check the ranges and be content. Shallcross is usually as good, if you like those better. What do you have to measure that needs to be better? Build a Hamon divider and get whatever inexpensive reference you can find and do the same. IMHO, the value in a 6.5 digit meter, for most people, is resolution, not absolute accuracy. Obviously it's fun to do as well as possible, but I wouldn't spend a fortune on it. I've got a 3455A, with fewer adjustments. It's a very handy meter if you have the space. The 3456A will do better on resistance, as it has more trims. The downside is it has more trims!

[tatel]

wonder:
 - does it has fan on back ?
buttons - loud harsh clickable or plush soft ?

To say the truth, right now I can't remember it clearly. Alzheimer? I'm moving all my workshop because of family matters, so last time I turned it on was like 6 months ago. But I would say, dead silent, so no fan, harsh buttons, and serial number begins by 2010A IIRC.

before any cals , i would recommend disassemble - replace all electrolytic caps  especially PSU ,  and clean all boards  from contaminations and connectors.  usually it quite dirty after 40 years.

It didn't look so dirty inside. Some previous owner put what I think is a temperature sensor that goes to a BNC conector implanted on the back panel. IIRC I have seen something like that on Xdevs site. My guess is previous owner cared for it. Don't remember seeing any electrolytic cap bulging -at a glance. But I'll follow your advice and eventually have a look at those caps and power rail voltages.

then run turn-on 24-7 for 2-3 weeks , or more (longer - better ) ,  then do adjustments

Understood. Indeed I checked it twice against DMMCheck between last May-July and second time got better results. See attachment. So I for sure will follow this advice.

all adjustments manually -   so labs really not in mood to do such adjustments.
so rather - DIY reference box , and someone may willing to measure it as a reference point.

Yep, I know. First calibration lab doesn't make any adjustments, no matter the device. Period. WTF?

Turning pots, it's not going to be better. Anyway I found these guys in Germany, it looks to me they are like people in this forum, calibration without certification, no AC, but adjustments are done and they aren't afraid to put their prices in the web. So I would trust them more that the marketing experts I just mentioned. Perhaps somebody here knows about them?

[guenthert]

Yep, I know. First calibration lab doesn't make any adjustments, no matter the device. Period. WTF?

Well, when turning old pots there's the risk that they disintegrate.  And if you have a cheat sheet with the deviation, you can calculate the actual value (or let the 3456A do that for you).  The calibration pots on the 3456A are of high quality and likely will be fine, but I can't blame them for not wanting to take such risks.

Turning pots, it's not going to be better. Anyway I found these guys in Germany, it looks to me they are like people in this forum, calibration without certification, no AC, but adjustments are done and they aren't afraid to put their prices in the web. So I would trust them more that the marketing experts I just mentioned. Perhaps somebody here knows about them?

these guys?  I think, we need another hint.   

[tatel]

I am curious with regard to your goals going forward.  Are you wanting to experiment with vintage equipment or do you have specific tasks in mind?

While I assume you got a great deal on that HP3456A, I am not sure I see the value in spending money on it.  Why not save up and buy something newer with a recent calibration?

With hobbies, everyone has their own desires, of course...

3456A has a very good reputation for long-term stability. If I'm getting it right, being so an old device, stability could be very good. Hence, should I become confident it isn't really out of whack, I would trust it more than any of my handheld multimeters, for a long time. I'm sure you can figure yourself what the utility of a trusty 6 1/2 is.

Last summer-fall I got some old analog scopes. Hameg 203, 303, 604, 605 in different condition. One of them works perfectly. Two other need little more than some deoxit in their pots, last one needs a serious repair I think. Also I got a really beaten, partially scavenged Tektronix 485. Its manual calls for a high-precision multimeter and that way I got into 6 1/2 multimeters, even if I'm pretty sure my Brymen 869s would be enough.

Oh, and a military HP8640B, too. I'm going to play with all that stuff to learn and to have fun. Fun is important, otherwise I'll better die. You know, when I was twenty, a good part of my time and money did go into caving, alcohol, and other, even worse things you don't speak of, specially if there any children present. But now I'm just a pureta so I need something different to get fun.

Not going further than 6 1/2, however. That's my ceiling. I swear.

But I need to know how 3456 is doing, otherwise still there will be some worms eating my liver. Of course I'm exaggerating, but can any of you see a watch that shows the wrong time without feeling slight discomfort?

DIY calibration ain't cheap, either. I can see it. Paying for having it calibrated/adjusted, if possible, looks better to me. If I need to spend €400 to get it adjusted, still it will be about the same than an used 34401 with no recent calibration from eBay. Right now there are half a dozen 34401 on sale in the EU. No one is currently calibrated, and the price range is €550-700. Of course they continue to be there because they aren't any cheap. Anything with a decent price gets sold ipso facto. But you get the picture.

You have to understand, I'm not in the US. The bargains you US guys find, look like Sci-Fi to us poor rotten fucked EU bastards. So good, so beautiful... so cheap! Alas, on eBay, you can easily see, say, $100-200 US items, with shipping costs nearing a thousand dollars. Canada is particularly bad in this aspect. In my eBay searchs, I usually filter out anything coming from outside the EU.

Best wishes

[tatel]

GR 1432 resistance boxes are .05% but are typically better. I'd get a couple of those, check the ranges and be content. Shallcross is usually as good, if you like those better. What do you have to measure that needs to be better? Build a Hamon divider and get whatever inexpensive reference you can find and do the same. IMHO, the value in a 6.5 digit meter, for most people, is resolution, not absolute accuracy. Obviously it's fun to do as well as possible, but I wouldn't spend a fortune on it. I've got a 3455A, with fewer adjustments. It's a very handy meter if you have the space. The 3456A will do better on resistance, as it has more trims. The downside is it has more trims!

You are right, of course, and I'm not worried about absolute accuracy. Well, I wouldn't mind having it, you know. But, as you said, wouldn't spend a fortune on it. That said, see my previous post about how the prices are outside the US. Particularly if you are not any EE with 20-30 years experience. Then you need to throw some more money at it.

What I do want, is to be confident that 3456 is not going to the mountain to jump like goats. It seems that this device isn't that bad. So, supposing those Vishay resistors come here, then I check my resistance ranges after a month of having the multimeter turned on, do my 4-wire measurements and it turns it's all seemingly good, then, that's it. In that case I wouldn't send this thing to any cal lab. Otherwise, having it professionally adjusted, rather than me fiddling with it without so much knowledge, doesn't look that bad to me, and I would have quite good accuracy for quite a long time BTW. At least if I have that money to spend on my hobby. That remains to be seen.

Now, I really want to thank you for that metrology mini-lab of yours. If you read the previous posts in this thread, you'll see I'm looking at building it.

Edit: I found one of these GenRads. It looks nice, of course https://www.ebay.es/itm/374719967772?

[tatel]

these guys?  I think, we need another hint.   

Understood, thank you.

[Kleinstein]

There is a good chance that an adjustment is not needed. These meters are relatively good even over a long time. Even if one would send it out for calibration it would make sense to do a DIY test as good as it can be done without spending a furtune. There are quite a few test that can be done and the test would also give you more knowledge on how to use the meter.  A first part to get (if not already there) is a GPIB interface (I don't think the meter also supports RS232) to send data to a PC. Recoding 6 digits is not that much fun with paper.

The easy test that can be done with rather little cost:
1)  check the input bias current for the 10 V high Z mode:  e.g. look at the dirft rate with sime 10 nF low leakage (PP or PS or good C0G) capacitor at the input. This is a good way to check currents to the pA range and check if the differential input impedance is high (e.g. > 100 GOhm).

2) Do a turn over test with a reasonable stable ref. source. This can be as little as well isolated (mechanical and thermal) batteries). So check if the positive and negative readings are the same. This would also include a check of the input offset. The test would make sense for the 1 V and 10 V range. Do to the difficulty getting a good source it would be tricky with 100 V (the 10 M resistance may be a bit much for batteries).

3) Check if the noise is in the expected range (e.g. check a short)

4) If one wants one could check the short range linearity (to check for some possible ADC faults) by looking at a low changing voltage (e.g. a 5000 µF range capacitor together with 10 M meter impedance). The discharge curve should be smooth and follow a smooth relatively simple curve even if no perfetly the ideal exponential (as the capacitor is not ideal).

5) Check if resistors read the same in 2 adjecent ranges (usually 4 W mode). E.g. a reasonable stable 100 Ohm, 1 K, 10K  checked in 2 ranges each.

With suitable, short time stable ref. voltages one could also check the 100 mV/1 V/10 V / 100 V ranges relative to each other. So read a test voltage (ideally both polarities) in 2 adjecent ranges each.
This readings should not be too much different and would be a rather direct check for the gain and divider steps.

This still leaves the absolute scale factors for the voltage and resistance ranges. This is something one can only check relatively to an external accurate source. The DMMCeck is a crude check, but no way good enough to do an adjustment. Chances are one would be inside the specks of the DMMcheck.  Even if not chances are good the meter could still be the more accurate one.
The comparison the DMM check also makes sense, especially if done several times (e.g. one 1-3 x a week) to look for drift. No need to use the lower resolution / 2 W mode here, only the best case, like 100 PLC and 4 W resistance for values < 1 M, and 2 W mode for 1 M (if one has a reasonable good resistor).

Even without a good absolute scale one can do quite a bit. As a bit of an extreme, for my DIY 7 digit meter I only know the voltage scale to some 0.5% and the resistance to maybe 0.02 %.

[tatel]

Thank you very much for the hints, I will put in practice as soon as I get back.

Edit: 1 MOhm, RESI 0.1%, 5ppm Digikey number 4506-MMFR5215B1M00V9-ND
10 MOhm: Ohmite, 1% 25 ppm Dgikey number    MOX750F-10ME-ND    

I know 3456's manual asks for more, would you consider that reasonably good?

[Kleinstein]

The 1 M resistor could be good enough for comparing the ranges. One may not even need it that good with a few more repeats. 0.1% accuracy is of limited use as the test with 100 K and from the 100 K range should give a much better value for the scale factor.
For a direct long term stability test 1 M and even more 10 M is not that good  anyway. It would depend a lot on cleanness and maybe leakage.

The resistors for the 3456 performance test / calibration are a different thing - they are for the absolute value to check or do the adjustment. Especially for the 1 M and 10 M range it is difficult to get very accurate long term stability. The relative comparison from 1 range to the next is relatively easy in comparison and is less demanding on the resistors.

[GigaJoe]

" temperature sensor that goes to a BNC conector implanted"

really ?   
I have some feeling you don't need to do anything with this unit .... including adjustment ....
it would be sufficient at beginning, then you figure out ,  i would say cool down inpatient to fix it (my initial advises under assumption - it scrap yard salvage unit)

[Silicium81]

The 3456A is capable of displaying the temperature with a CTN sensor, there is a 'hack' to implant a sensor internally and display the temperature by a keyboard command.

It's a good multimeter 
7,5 digits internal: https://www.eevblog.com/forum/testgear/adventures-in-retrocomputing-hacking-a-7-5-digit-hp-3456a/msg3915374/#msg3915374

Test with box resistor decade DIY:

[tatel]

" temperature sensor that goes to a BNC conector implanted"

really ?   

Yep. Not really sure it's a temperature sensor, but pretty confident that BNC shouldn't be there.

I have some feeling you don't need to do anything with this unit .... including adjustment ....

You could be right. I hope so. I wouldn't mind it, you know.

it would be sufficient at beginning, then you figure out ,  i would say cool down inpatient to fix it (my initial advises under assumption - it scrap yard salvage unit)

No, this was an ebay bid. But what I'm inpatient about isn't fixing it, but knowing how its doing. What I have been doing so far is just asking for advice and buying things to make it clear if calibration/adjustment would be in order.  I bought that thing last May, I feel it's time to make something with it.

Of course I'm willing to put my money where my mouth is, otherwise I wouldn't be here asking questions and wasting your time (and mine)

[tatel]

The 3456A is capable of displaying the temperature with a CTN sensor, there is a 'hack' to implant a sensor internally and display the temperature by a keyboard command.

It's a good multimeter 
7,5 digits internal: https://www.eevblog.com/forum/testgear/adventures-in-retrocomputing-hacking-a-7-5-digit-hp-3456a/msg3915374/#msg3915374

Test with box resistor decade DIY:

Mais oui mon pote; c'est ça que je veut bien!

I mean, don't feel any need to go 7 1/2, but having 3456 reading the same it's adjusted in the decade box. Do you have any link to that DIY decade box? Just to put another card up my sleeve.