What source to use with old multimeter calibration if I have nothing else. - Page 1
Forum section: electronics / general

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2025, 06:00:30 pm
content: I finally collected and repaired enough old multimeters that I now require an absolute voltage reference standard for calibration. Usually I might use my HP 3478A, however it is currently unavailable. My next best option would be my pretty old Tektronix DMM916.I have these meters to calibrate:2 x Tektronix DM502 3-1/2 digit with 2 volt and 200 millivolt basic ranges3 x Tektronix DM501 4-1/2 digit with 2 volt basic range2 x Tektronix DM501A 4-1/2 digit with 2 volt and 200 millivolt basic rangesCalibration requires a less than 2 volt reference, and the meters with 200 millivolt ranges also require a less than 200 millivolt reference because they reduce their reference voltage to increase sensitivity rather than using a x10 amplifier.I was thinking of buying an LTC6655 1.25 volt 0.025% 2 PPM/C bandgap reference and then using an LT5400BIMS8E-8 0.025% 1:9 resistor array to divide down to 0.125 volts for the 200 millivolt calibrations. I will pick up some OPA186 chopper stabilized operational amplifiers for buffering if required; they are not the best but they do have 5 picoamp input current and are low cost.Are there any better options that I should consider?At some future time I will design and build something better which perhaps I can send out for calibration. At one time I would have bought aPentaRefwith the proper voltages, however they are not available.

author: beenai2018
author_rank: Contributor
timestamp: January 10, 2025, 07:50:08 pm
content: My bench meter is broken currently and one of the occasions where I needed an absolute voltage ,I used the resistance functionality of my handheld to create one. ie pass a known current from the handheld via a precision resistor to get a voltage reference.Since you have multiple tek units and they have precision lower value resistors in the DC Amps range ,there may be a way to cross -calibrate against one known source (the 916).the 916 is .06%+XX on DC V but only 0.3% on resistance ranges . The tek is .05% +XX on DC V and 0.2% on DC Amps (and has a nice round 1K for the 200uA range).This is just a fun experiment while waiting on the precise parts though .

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2025, 10:39:08 pm
content: Quote from: beenai2018 on January 10, 2025, 07:50:08 pmMy DMM916 is just the best meter I happen to have available and while it has good specifications and is in excellent shape, it has not been calibrated in 20+ years and it is a handheld instead of bench instrument.The resistance calibrations on these meters depend on the basic DC volt calibrations, so I have to do the DC volt calibrations first. Then I just need a precision resistor to calibrate the ohms converter output on any range.The DM501s are calibrated against themselves and my DMM916 because I use them for temperature measurement, but one of the DM501s has just failed with high input leakage so it is time to rebuild them. One of the DM502s is out of calibration enough to notice and I could calibrate just them against my DMM916. The DM501As both have input multiplexer leakage and will be getting rebuilt and improved.Somewhere I have an ESI Kelvin Varley divider that should be good to 0.003%, but that is the problem ... somewhere. Combined with an accurate reference and some buffering, it would make an excellent calibration source over a wide voltage range.I just thought that there might be a better option than to start with a 1.25 volt 0.025% LTC6655 and precision resistor network for division, like a more accurate reference that is easily available.

author: iMo
author_rank: Super Contributor
timestamp: January 11, 2025, 08:10:12 am
content: I would recommend you to go for LM399 or ADR1399 when targeting 4.5digits meters adjusting. Perhaps buy 2-3 of them, put it in parallel, with an opamp and the LT5400 4x10k divider you will get something around 10.5V out. You will need to calibrate it after some 3-4months burn-in.Avoid the epoxy case references, like the one in the PentaRef..PS: an example below with LT5400-3 divider

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 09:52:18 am
content: Quote from: iMo on January 11, 2025, 08:10:12 amThe problem with that is that the initial accuracy of the LTC6655 is 0.025%, or 312.5 microvolts out of 1.25 volts, while the ADR1399 is 275 millivolts out of 7.05 volts, or 49 millivolts out of 1.25 volts, making the initial accuracy 150 times worse. I do not have any way to do the initial calibration of a LM399 or ADR1399 based reference standard, and if I did, then I could do the same with a simpler LTC6655 based reference standard, or more likely an LT1021 or similar.There are some calibrated 10 volt reference standards I could buy, however the calibration voltages I need are below 2 volts and below 200 millivolts, so any likely divider would ruin the accuracy without itself being calibrated. I could work my way around this by matching a set of precision resistors to make a divider, and I might do this to replace the expensive LT5400BIMS8E-8 with something that performs even better. 40 0.1% 15ppm/C resistors cost the same as 1 LT5400BIMS8E-8 so I need to look into that.

author: iMo
author_rank: Super Contributor
timestamp: January 11, 2025, 10:07:37 am
content: Well, using off-the shelf ref chips for direct 4.5digits adjusting might be an extremely difficult exercise..1.9999V means 100uV resolution, for that you would hardly find a such a chip (with say a better initial precision/accuracy as 50uV, not counting dividers errors, if any)..

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 11:28:33 am
content: Quote from: iMo on January 11, 2025, 10:07:37 amThe specified basic accuracy of the DM501 is 0.1%, and of the DM501A is 0.05%. While not ideal, the LTC6655 is specified to have an initial accuracy of 0.025%, so better than either meter.Hmm, I could pick up a0.01% 5 volt VREF-01for $35 if I can trust my homemade precision divider.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 11, 2025, 12:54:54 pm
content: no offence but can the @op find some fluke 343 341, valhalla references etc ... at some point you need good references voltages in the 4 5 6 digitsfluke 787 any gizmo who can shoot out very good references ... sure you have the lm399 projects here for sale but it 10v precision, you dont cover a full range ...you had the IAN johnston now :https://www.wrytech.eu/sure they cost a lot but ...

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2025, 01:12:45 pm
content: There are indeed a few relatively accurate reference available. Not just the LTC6655. There is also Ti's REF70xx, ADR45xx and max6126 with similar accuracy. It is still not a good accuracy ratio to really get the full accuracy.The older DMMs may want a voltage like 1.8 V and thus likely some divider (e.g. 3/4 of 2.5 V or 1/3 of 5 V) needed.1.8 V reference are available (e.g. max6070), though less common and not with as good an accuracy.The ref35180 could be an interesting low cost (some $2 with 0.05% accuracy) option to check on a DMM with 2 V range.

author: tggzzz
author_rank: Super Contributor
timestamp: January 11, 2025, 01:19:00 pm
content: Quote from: David Hess on January 11, 2025, 11:28:33 amDo you have a KVD, or would it be easier to build a Harmon resistive dividerhttp://conradhoffman.com/HamonResistor.html?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 11, 2025, 01:48:06 pm
content: I was going to suggest the Hamon but got beat to it! It might be better to find your ESI. It's not so small that it's lost under some paperwork, so get digging! With that you should be able to ratio up or down wherever you need to go, from the best source you can buy or borrow. What's your general location?

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 02:31:31 pm
content: Quote from: tggzzz on January 11, 2025, 01:19:00 pmQuote from: Conrad Hoffman on January 11, 2025, 01:48:06 pmI think my ESI KVD is still in my storage unit in St. Louis, so I do not know when I will be getting it. I am now in New Hampshire. I will do another search for it because it might be here.I am thinking of doing something similar to a Harmon divider, but without the trimming because I can accept other division values and can take advantage of chopper stabilized amplifiers to remove loading. Oddly enough I have a pair of ESI bridges which can operate in nulling mode but will probably take the easy route and just measure the resistances under controlled conditions.Update: If I start with 5 volts, then 3 equal resistors can produce 1.6667 volts, and then 6 equal resistors can produce 0.1852 volts.

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 02:33:26 pm
content: Quote from: Kleinstein on January 11, 2025, 01:12:45 pmThe LTC6655 and 1.8 volt Maxim parts have the virtue that I can use them directly for the basic DC range calibration without a divider. Of the two, the LTC6655 should give a better result despite its lower voltage.

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 02:36:25 pm
content: Quote from: coromonadalix on January 11, 2025, 12:54:54 pmI could do that but I tend to be miserly, and I am not sure how I could verify the performance of a used calibration source.For a long term project I was thinking of duplicating the operating concept behind thePentaref, but with better accuracy and higher voltage. It is apparent from photos what it is doing to generate multiple calibrated outputs.

author: Andreas
author_rank: Super Contributor
timestamp: January 11, 2025, 02:39:49 pm
content: Quote from: David Hess on January 11, 2025, 09:52:18 amHello,initial accuracy means adjustment accuracy at the fab done at 25 deg C with short pulses (avoiding self heating).You get additional ageing (until the device gets into your lab)Soldering shift (according to data sheet up to 0.025% with a reflow profile; probably more by hand soldering).Temperature drift due to self heating. (the LTC6655 will heat up considerably due to rather high idle current of 5-7 mA)And additional drift due to humidity (especially when using the plastic package).Even when the inital accuracy is a 3 sigma value you will need a additional calibration after soldering to get a TUR of >4:1.with best regardsAndreas

author: tggzzz
author_rank: Super Contributor
timestamp: January 11, 2025, 04:05:12 pm
content: Quote from: David Hess on January 11, 2025, 02:36:25 pmHow can you verify the drift of a new calibration source?I calibrated my very old zener source by taking it to a Maker Faire in Hannover. The nearbyPTB(=NIST / NPL) brought along an HP3458 thattheyhad calibrated the day before. Good enough for me

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 04:10:08 pm
content: Quote from: tggzzz on January 11, 2025, 04:05:12 pmMy HP 3478A is still packed away in a box in St. Louis or I would be using it in some capacity in this exercise.Temperature drift is easy enough. Measuring aging takes a more stable reference. Hopefully one has a not so accurate but much more stable reference which has been in use for a long time to make a comparison against.If a buy a VREF-01, then it comes with free recalibration for 2 years if I pay for the shipping.

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 04:21:09 pm
content: Quote from: Andreas on January 11, 2025, 02:39:49 pmI selected the LTC6655 because it was the best I could do, but I did the math and I can make the needed precision dividers to use a 5 volt precalibrated source. So unless I find something better, I will use a 5 volt VREF-01 and some matched precision resistor sets, and at least two chopper stabilized amplifiers although normal precision operational amplifiers with offset nulling capability could work also. I estimate this will get me down to 0.015%.Is there a better reference than theVREF-01out there at a reasonable price? There are piles of these things coming out of China, including LM399 models, but I am dubious.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2025, 04:46:08 pm
content: Voltage references are standard parts are usually no more accurate. The soldering stress alone limits how much initial accuracy makes sense.There are a few reference boards (e.g. LM399 or AD584 based) with supposedly measured individual voltage(s) available at an afordable price. The principle accuracy can be OK (e.g. 100 ppm range). The problem is that there are also some where the measurements are fake. So one would still want some extra check at least. It could still be worth the gamble in combination with an LTC6655 or similar.

author: iMo
author_rank: Super Contributor
timestamp: January 11, 2025, 05:16:08 pm
content: Quote from: iMo on January 11, 2025, 10:07:37 amThis entire Metrology Section deals with your problems, basically..

author: bdunham7
author_rank: Super Contributor
timestamp: January 11, 2025, 05:16:17 pm
content: Quote from: David Hess on January 11, 2025, 04:21:09 pmNot that I would have confidence in. The VREF devices that Doug Malone sells are pretty carefully aged and characterized. My VREF-10s are stable to within a few ppm after several years.What is the input impedance of the meters you want to calibrate? The VREF products will drive a pretty high output current so you can use a pretty low resistance divider and skip the buffering stages.

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 05:38:16 pm
content: As an aside, classic Tektronix instruments have a lot of Pozidriv screws. I finally got tired of struggling with them with a Phillips screwdriver and picked up a Wera driver and Pozidriv bit set. This made things so much easier.Wera's smallest driver is smaller than I expected, but works great. I will pick up their larger driver later.https://www.amazon.com/dp/B003GDIQIQhttps://www.amazon.com/dp/B003KN3G4Q

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 05:58:52 pm
content: Quote from: bdunham7 on January 11, 2025, 05:16:17 pmThese meters all have 10 megohm inputs. The essentially Harmon dividers I am considering will use 15 kilohm resistors and I am going to test out OPA186 chopper stabilized amplifiers which have a 4 picoamp input bias current. If they are not sufficient, then I have plenty of other suitable precision operational amplifiers.

author: iMo
author_rank: Super Contributor
timestamp: January 11, 2025, 06:06:52 pm
content: Btw., what ADC chip do your 4.5digits TEK meters use? I have here several 2V 4.5d modules with the ICL7135 (with 7 segm leds) and they work surprisingly well (with LM358-1.2V reference).PS: ok found it - 501 with a discrete ADC, 501A with LD120 and LD121A (we discussed this chipset here, a multislope basically)Quote

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 11, 2025, 07:50:23 pm
content: Whatever path you choose, if you really care you will have to find a traceability path. Calibrate one of your instruments. Buy something new. Whatever. A low cost but slow approach is the USA Cal Club which can get a reference with some traceability to you every few years. You can then bootstrap whatever path you have taken into the voltage range of choice.

author: G0HZU
author_rank: Super Contributor
timestamp: January 11, 2025, 08:36:49 pm
content: The other option is the 'do nothing' option and just carry on using the meters in their current state?There's enough of them to cross reference in terms of confidence factor for everyday use. Unless you actually have a need for a traceable calibration for a particular task, then I don't see much practical point in all this.Maybe just wait until you get the 3478A meter and use that as a house reference. Even if the 3478A is a bit out of spec it shouldn't really matter for typical home/hobby use. Absolute accuracy generally isn't as important as range linearity and meter stability and having several (good quality) meters can boost confidence in this respect.

author: tggzzz
author_rank: Super Contributor
timestamp: January 11, 2025, 08:56:55 pm
content: My 3468 has not changed one digit in 7 years. 0.014% low but still in spec.

author: David Hess
author_rank: Super Contributor
timestamp: January 11, 2025, 10:56:48 pm
content: Quote from: iMo on January 11, 2025, 06:06:52 pmThe DM501 relies on the Fairchild 3814 for the digital logic. The high frequency inverter is clocked from the ADC clock and under some conditions, the integrator detects the inverter noise causing missing codes. See page 95 of volume 3 of the Analog Devices Data Conversion Handbook for a short discussion of this problem in synchronous charge balancing ADCs. Only one of my DM501s displays this problem and I am going to try and fix it.Update: It is more likely that the comparator is doing the detection.The DM501A uses the Siliconix LD120/LD121A run-up run-down integrating chipset which should perform better than the Intersil dual-slope integrating ADC, but suffers from two flaws. The input multiplexer is vulnerable and often damaged, which is the case with both of my DM501As to different degrees. The other flaw is a mistake in the design itself where the low common mode rejection of the integrated CMOS input buffer is not corrected, leading to poor linearity. The LD120 was replaced with the LD122 which removed the integrated buffer allowing an external precision low input bias current operational amplifier, typically an AD542, to be used. Later DM501As have this modification, and I am planning on implementing it on both of mine, but using much better parts. Intersil prevented this problem by correcting the error from limited common mode rejection as part of the automatic zero cycle.The DM502 uses the 3-1/2 digit version of the Siliconix chipset where the linearity flaw is not significant.Ultimately these are not very good multimeters, but their small form factor allows lots of them to be used at once and I enjoy the challenge of improving their performance.

author: iMo
author_rank: Super Contributor
timestamp: January 12, 2025, 09:36:45 am
content: I have here a Hungarian DMM based on the 120/121A, but they implemented only 3.5digits in it. Also I got spare 120/121A for it, so would be great if you may share your experience with the improving the 120 analog part then..

author: Kean
author_rank: Unknown
timestamp: January 12, 2025, 11:43:10 am
content: Quote from: David Hess on January 11, 2025, 05:38:16 pmSlightly OT but if you get a larger Wera driver you might want to consider the ratcheting 816 RA.https://www.amazon.com/dp/B004VMWZV0If you want ESD safe, then you would get the 816 R ESD, but I have not seen an ESD-safe ratcheting one.I have a few too many Wera (& Wiha) tools...

author: David Hess
author_rank: Super Contributor
timestamp: January 12, 2025, 09:21:09 pm
content: Quote from: G0HZU on January 11, 2025, 08:36:49 pmOriginally I was going to fix the leakage in one of my DM501s and replace the multiplexer and buffer in my DM501A, but then I bought another working DM501 and non-working DM501A, and noticed that at least one of my DM502s is off by more than 2%, so I decided it was time to do something about calibration.Quote from: Kean on January 12, 2025, 11:43:10 amI might do that, but in my experience ratcheting drivers have an annoying amount of slack, and I have seen reviews of the Wera drivers saying this.Quote from: iMo on January 12, 2025, 09:36:45 amI do not think the linearity problem is significant with only 3.5 digits. You can compare the LD120 and LD122 datasheets to see exactly what Siliconix changed to correct the problem in the original design. They also published an application note discussing some details which I am attaching below.There are more application notes about this whole series of chips in the 1982 Siliconix Analog Switch and IC Product Data Book if you are interested. It was too large to attach here.

author: iMo
author_rank: Super Contributor
timestamp: January 12, 2025, 09:38:33 pm
content: Yup, I have all the app notes from Siliconix incl. the product catalogue with perhaps 100 pages dedicated to the 120/122/121A. We had adiscussionhere (with the newopamp buffering)on it some time back.

author: G0HZU
author_rank: Super Contributor
timestamp: January 14, 2025, 01:03:53 am
content: QuoteThanks. 2% is a big error, so I would expect something to have failed or have been damaged. So some service work may be required ahead of any calibration.I've got various DMMs here from low cost handhelds to some nice 6.5 digit bench types. All of the 3.5 digit handheld meters are typically within about +/- 0.1% when I test them and none of them have been adjusted in at least 30 years.If I explored all voltages across all ranges the accuracy probably won't be within +/- 0.1% but I'm not too fussed either way. However, they all appear to be very stable over time. I think that if a (decent) DMM drifts enough to be out of its spec limits then this would be a concern for me and it would probably need more than a quick calibration using voltage references and some resistive dividers to provide high confidence that it is OK.

author: David Hess
author_rank: Super Contributor
timestamp: January 16, 2025, 04:38:34 pm
content: Quote from: G0HZU on January 14, 2025, 01:03:53 amI rebuilt the two DM502s a couple years ago but did not check their calibration except against other meters. They mostly get used to monitor voltage and current on my bench supply where a 2% error would not be noticed until now. The usual failure is input leakage but that is not the case here, so I will take a close look before I do the calibration. Thanks for pointing this out.

author: David Hess
author_rank: Super Contributor
timestamp: January 16, 2025, 05:07:22 pm
content: I received an email from voltagestandard.com saying that my reference will be shipping shortly, so I am preparing an order from Mouser for the parts I will need.I will likely use 49.9k 0.01% resistors for the Harmon dividers.The operational amplifier selection is more interesting. The AD8677 and LT1012 compare favorably with the best chopper stabilized parts but cost more. Other OP-07 style precision operational amplifiers with nulling capability have too much worst case input bias current. Various chopper stabilized parts fail either because of high offset, or high input bias current. The low cost OPA186 is acceptable, and I may use it just for cost reasons, but there are 4 others in my list which should perform a little better despite higher input bias current.I did a broadband noise analysis but am not including it because its significance is questionable. I am doing a low frequency noise analysis now, but have to calculate some of the datasheet values.Update: the 5 volt reference showed up today. One DM502 is off by 1.4%. My Tektronix DMM916 is right on, or at least within its narrow specifications.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2025, 06:21:23 pm
content: The dividers have impedances in the 10 to 20 kilohm range, so including current noise but not resistor noise because it is insignificant, the 0.1 to 10 Hz noise in nVpp is shown below, along with the maximum total error.OPA180 189 nVpp 113 uVOPA182 125 nVpp 11 uVOPA186 751 nVpp 12 uV Lowest Input Bias Current, Highest Voltage NoiseOPA188 240 nVpp 53 uVOPA189 120 nVpp 9 uVADA4522-1 321 nVpp 9 uVLTC2057 217 nVpp 10 uVLT1012 500 nVpp 9 uVAD8677 266 nVpp 21 uVSo several will work down to 10 microvolts for a 4-1/2 digit meter, but would require a different circuit with offset trimming, even for the chopper stabilized parts, below that.

author: iMo
author_rank: Super Contributor
timestamp: January 19, 2025, 09:30:40 am
content: Quote from: David Hess on January 16, 2025, 05:07:22 pmWhy so large value resistors for your Hamon divider?

author: David Hess
author_rank: Super Contributor
timestamp: January 19, 2025, 03:26:13 pm
content: Quote from: iMo on January 19, 2025, 09:30:40 amThere are a couple of reasons. Remember that the output impedance of the divider is much lower, 10 to 20 kilohms in this case, so thermal noise is not an issue and current noise is reasonable. At the moment 49.9 kilohm 0.01% 15 ppm/C resistors are available at low cost, and higher resistance limits errors from self heating when some are in series and some are in parallel resulting in unequal voltages.Update:This design has to be able to calibrate a 4-1/2 digit meter on its 200 millivolt range, or 10 microvolts per count. The worst case offset error barely meets that requirement without offset null adjustment, and the low frequency noise easily meets that requirement. For 5-1/2 digit calibration, the noise is still low enough, but staightforward offset null calibration would be required to get 1 microvolt accuracy. The 49.9 kilohm standard resistance would need to be lowered for a 6-1/2 digit calibration to reduce noise.

author: tggzzz
author_rank: Super Contributor
timestamp: January 19, 2025, 03:56:29 pm
content: Quote from: iMo on January 19, 2025, 09:30:40 amMany KVDs are 100kohm input resistance and, of course, a varying output resistance.The varying output resistance means that no current can be taken, which restricts the loads to being null-voltmeters or electrometers or modern DVMs with "infinite" input resistance. It also means that there is little benefit to lowering the resistance.Of course very few (any?) DVMs have "infinite" input resistance for 100V and 1kV ranges; usually the inputs are 10Mohm resistive dividers. I do have one old "DVM" with infinite resistance at 1kV, a Fluke 893A.Having a high input resistance is a benefit when the input is 1kV. Even 100kohm means that 10W is being dissipated in the first stages of the KVD, hence the resistors are often kept in oil. Yes, the "DVM" description is correct but misleading

author: MiDi
author_rank: Frequent Contributor
timestamp: January 19, 2025, 07:35:30 pm
content: Quote from: David Hess on January 17, 2025, 06:21:23 pmMaybe this overview helps:

author: David Hess
author_rank: Super Contributor
timestamp: January 19, 2025, 08:05:02 pm
content: Quote from: MiDi on January 19, 2025, 07:35:30 pmI have much larger overviews but limited myself to 24+ volt chopper stabilized parts and bipolar parts which have offset null capability. There are some precision JFET parts which could be used, but their input offset drift is 10 times higher and they lack offset nulling.Modern parts lack offset nulling. This can be worked around by adding a second operational amplifier to buffer the trimming network, and is what is necessary for chopper stabilized parts. Offset nulling will be required for 5-1/2 digits and higher.

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2025, 01:01:10 am
content: I have been comparing my two DM501As to make a parts list for refurbishing them before calibration, and I found more damage on the new-to-me broken one. Those are the low leakage glass diodes protecting the ohms converter and the one on the right is fried. That is not good news since it must have been quite a large overload.Maybe a previous user was following Dave's instructions about discharging capacitors using ohms mode.I found discrepancies between my units and the service manual that I have, so I ordered a more complete manual from Artek Manuals.

author: munorc
author_rank: Regular Contributor
timestamp: January 29, 2025, 06:46:41 pm
content: I have been following this. Lots of recommendations for various voltage references.If I have something like an accurate 10V reference, that serves a small range.I ask since I have a stack of HP 3478A 5-1/2 digit DMM's. One still has its calibration, but the other have lost theirs since the battery has died. I have KVD options to present lower voltages. What is the path to calibrating the upper ranges? The 3478A needs an accurate calibration voltage at 30mV, 3V, 30V and 300V. AC Calibration needs only one, 3v at 1kHz.In the old days, the only reference was a standard cell. That combined with a stable source, a volt box and a precision potentiometer was all that was required to calibrate low to high.

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2025, 09:46:02 pm
content: Quote from: munorc on January 29, 2025, 06:46:41 pmI bought a 0.01% 5 volt reference as discussed and am going to use Harmon dividers to produce accurate voltages for 200 millivolt and 2 volt calibrations.A Harmon divider can also be used as an attenuator in the feedback loop of an operational amplifier to multiply the input to a higher voltage. So in this way an accurate higher voltage can be produced from a lower voltage reference. In my case I will make a calibration voltage for a 20 volt range from my 5 volt reference. This could be extended to higher voltages.Any divider could be used in this way, so a Kelvin–Varley divider could be used as the feedback network of an operational amplifier to produce an accurate higher voltage.

author: tggzzz
author_rank: Super Contributor
timestamp: January 29, 2025, 09:52:32 pm
content: Quote from: David Hess on January 29, 2025, 09:46:02 pmOr there's the null-voltmeter technique. Use a KVD to divide down an unknown (but stable) high voltage, and twiddle the division ratio until the output is the same as your reference. Determine "the same" by connecting a 2% analogue meter between the KVD output and your reference, and have zero current flowing through that meter.From the division ratio you can calculate the actual value of the high voltage.Don't dissipate too much power in the KVD, though

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2025, 10:37:29 pm
content: Quote from: tggzzz on January 29, 2025, 09:52:32 pmThe usual caveats apply when extending to higher voltages. Besides safety, power dissipation in the high side resistance will increase temperature causing an error from the temperature coefficient of resistance. At high voltages, the voltage coefficient of resistance will become a significant source of error.In place of a Harmon divider, a series connection of several matched resistors will cancel part of these errors because every resistor will have the same dissipation and voltage.I made a concerted effort to find my ESI Kelvin–Varley divider but it must still be in my storage unit in Missouri.

author: munorc
author_rank: Regular Contributor
timestamp: January 29, 2025, 11:19:16 pm
content: So the JR 107 KVD is rated to 1,100V, and is protected by a 1/100A fuse. Still not something I want to connect 300V+ to.The other choice I have is a 3 decade Dekapot KVD.I have a L&N Volt Box rated for 750V on the max side. and intended to feed a 1.6V potentiometer. Probably a better path to compare to a low voltage source. Of course, a Volt Box is another element that required calibration and its history is unknown.I have a couple of tube era power supplies, a Lambda Model 71 500 volt and a Fluke model 407C 555V. The manual for the Fluke actually outlines its use to calibrate voltmeters, using a differential voltmeter (which I do not have).

author: David Hess
author_rank: Super Contributor
timestamp: January 31, 2025, 01:10:02 am
content: Quote from: G0HZU on January 14, 2025, 01:03:53 amI checked over my "bad" DM502, and I think the single turn trimmer potentiometer for the basic 2 volt range is the problem. It was very finicky when adjusted, so probably needs to be replaced, along with other single turn trimmers. Just to be clear, the ADC has two native input ranges, 200 millivolts, and 2 volts.The ADC seems to have missing codes which should not happen with only 3-1/2 digits, but I will need something more sophisticated to check it out in detail. These multimeters use those crummy edge wipe Texas Instruments sockets, so I may preemptively replace the sockets in the analog section. When I rebuilt the power supplies a couple years ago, I replaced the aluminum electrolytic capacitors, but only the edge wipe sockets for the unobtainable dual regulator IC.

author: David Hess
author_rank: Super Contributor
timestamp: February 27, 2025, 06:25:58 pm
content: Started work on the three DM501s. The new one I bought works fine and only requires calibration. One of my old ones suffered from input leakage, and now my second old one also suffers from input leakage. The leakage is zero at room temperature, but rises to about 2 nanoamps once the multimeters warm up which is 20 times too high.I pulled one of the leaky ADC boards apart and after testing decided that the leakage is either in the input buffer, or the printed circuit board. I doubt the input transistors on two different versions 5 years apart failed with high leakage, so I steam cleaned the board. The leakage was still present, so I finally swapped the ADC between the working DM501 and the bad one, and the leakage stayed with the bad one. So it is the main boards which have leakage and not the ADC boards.I am pulling the main boards, will steam clean them, and then bake them out.I am going to try steam cleaning first to avoid immersion because parts like trimmer resistors and capacitors may not like it. I picked up the more recent Bissell steam cleaner for this and it seems to work well:https://www.amazon.com/dp/B0DCHBC8JJThe photo below shows the DM501s before they warmed up. The two on the right will reach about 20 counts and the newer one on the left will remain at zero. The oldest one, in the middle, was assembled in 1974.

author: Vtile
author_rank: Super Contributor
timestamp: March 05, 2025, 07:17:54 pm
content: I was wondering... If the resistance measurement internally is voltage referenced, could one use known precision resistor to go to the voltage reference adjustment if the meter. I doubt thought, resistance accuracies are usually pretty poor in all meters pricy or cheap doesn't usually matter. Racal Dana had at least one model that were almost better on resistance than voltage though.

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2025, 09:30:03 pm
content: Quote from: Vtile on March 05, 2025, 07:17:54 pmThe DM501 may be a special case. The DM501 ohms converter relies on the ADC's negative reference, and the ADC also has a separate positive reference. That makes two trimmers for the references and one trimmer for the ohms converter current source. An external resistance could only calibrate one of the voltage references, and would rely on the internal resistance and negative reference calibration.The DM502 and DM501A use a separate reference and calibration for their ohms converters. An external resistance could be used to calibrate either the ohms converter or the ADC, but it takes an external voltage reference and resistance to calibrate both.A better design uses a single reference for the ADC and ohms converter, so the ohms measurement is made ratiometrically with a comparison to an internal precision resistor, with the exact reference voltage being unimportant. In this case, an external resistor cannot be used to calibrate the reference, and there may not even be a separate resistance calibration.The continuing saga of my three DM501s is here:https://www.eevblog.com/forum/metrology/repairing-tektronix-dm501a-and-how-to-clean-a-board/Two at this point are repaired with DC and ohms calibrated. The current and AC ranges should be good but I have not messed with their calibrations. The third and oldest DM501 appears to be suffering from a massive amount of leakage on its main board which will require extreme cleaning.