Fellow Norwegians: Got access to precision gear?Begging to calibrate HP 3456A :) - Page 1
Forum section: electronics / general

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 07, 2012, 05:17:42 pm
content: Hello,I scored this old 1980s bench multimeter on ebay. It is damaged for repair. So I got it cheap. As some of you may know, I am a newbie, so I ask many dumb questions. (But I tryonlyask the ones I cannot find the answer to on google.) Here is another one: A big portion of the circuitry has a massive black shield. Is it safe to open that? Underneeth there are several pcbs stacked on top of eachother, and I can spot a long wide metal can down in there.There is also a second shield covering the input line wiring saying: Warning: Line voltage is connected to components under this shield even when the power switch is in the off position. What is safe, and what is safe to touch?Now to the multimeter:The 7-segment display is displaying: ------------- whilst on. I think it is supposed to display "OL". I ran the self-test, and the meter found error 3, the service manual says:QuoteAnyone have experience with these old meters, or ideas? Otherwise I will just crack on, on my ownThanks for your time!Marius

author: Unknown
author_rank: Unknown
timestamp: February 07, 2012, 08:20:47 pm
content: Quote from: king.oslo on February 07, 2012, 05:17:42 pmMost of the warnings state that if you connect a dangerous voltage to the inputs, these voltages will also be present in the circuit below this shield. Just don't connect it to anything like mains voltages while having the covers of, limit yourself to stuff like resistors, 9V batteries, lab supplies or function generators. I would probably avoid any dangerous input voltages until you've fixed the existing issues. You should be able to troubleshoot the current issue without any inputs or even with shorted inputs.Because the input impedance at low DC voltage ranges is very high (specified as > 10 Gohm), contamination like finger prints can cause issues. If you have to solder in this area, be sure to clean thoroughly to remove all flux and other contamination. The service manual may contain a procedure for this.Quote from: king.oslo on February 07, 2012, 05:17:42 pmI don't recall this circuit. The service manual should give you an idea what's under it. Probably the mains transformer and fuses. I don't believe this instrument has a stand-by power supply or any high-voltage supplies. I wouldn't touch the stuff on the primary side of the mains transformer unless the instrument has been unplugged for some time and all caps were discharged (not that I'd expect any high voltage caps in this instrument).Quote from: king.oslo on February 07, 2012, 05:17:42 pmI've not seen this issue before. The basic idea (I'm sure there's a simplified schematic in the service manual) is that there are two micro processors in this instrument. One is connected to the front panel and GPIB interfaces, and is at ground potential. The other is floating and connected to the guard (LO) input terminal. These two processors are connected via transformers. See the theory of operation section in the manual about in-guard logic and isolation logic.I would follow the procedure described in the referenced sections 8-A-34 and 8-B-6 of the service manual (in the 03456-90004 version), they should help you localize the problem. These tests should be fairly safe, since they only involve the digital logic and not sensitive analog circuits or potentially dangerous power supplies. Just make sure you're shorting the correct pins.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 07, 2012, 08:35:06 pm
content: Quote from: alm on February 07, 2012, 08:20:47 pmAlm, that is a great reply. Thank you so much. I am surprised (and impressed) that there are people which has experienced this specific about test equipment.I will crack on this evening. Thanks. Wish me luck!M

author: Richard W.
author_rank: Regular Contributor
timestamp: February 07, 2012, 11:44:08 pm
content: Good luck, be careful and be patient, it maybe needs some time to find the fault.As alm said, thats very high impedance stuff.Try to hold any pcbs just on their edges, avoid any dust, grease, dirt or pizza on the pcbs and inside the instrument.Flux residue can be problematic in such circuits, so a bottle of pcb-cleaner is helpful.I'm not familiar with this paticular meter, but on IMG_3232.jpg you can see the precision voltage reference. It's a LM299.It's responsible for the meters accuracy. If the meter can't be repaired, you can build a nice standalone voltage reference.(http://webench.national.com/an/AN/AN-161.pdf)(http://webench.national.com/an/AN/AN-184.pdf)

author: Unknown
author_rank: Unknown
timestamp: February 08, 2012, 12:46:47 am
content: Only the analog circuits are high impedance. The digital and power supply circuits are much less sensitive. This does not sound like an impossible repair, the digital circuits should be fairly standard parts, and the 3456A is fairly popular on the used market, so spares may be available.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 08, 2012, 09:22:59 am
content: Thanks guys.I have narrowed the problem down to the inguard logic.M

author: saturation
author_rank: Super Contributor
timestamp: February 08, 2012, 01:19:46 pm
content: What are the 1st 4 numbers of the serial number? Its separated by the next set by an A, such as 2201AXXXXX.I ask because I track the reported failures on these DVMs, and they are pretty consistent unless the unit was abused and the inputs overloaded.This will help others repair them as at least those reported seem to be in the same sections.On the 2015A series PROMs and firmware corruption.On the 2201A power supply capacitors.Keep us posted with your progress, I have 4 myself, 3 are working and the 4th needs the PSU troubleshooted.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 08, 2012, 02:10:36 pm
content: Saturation, it is 2201A11016.The caps you are referring to, are they the through-hole monsters at the back of the meter, under the black shield? As seen in the bottom of IMG_3230.jpg? What's the issue with them? ESR? I don't own a ESR meter yet.Marius

author: saturation
author_rank: Super Contributor
timestamp: February 08, 2012, 04:25:18 pm
content: Hello Marius, great. Luckily no, but most folks who report the failure do not mention which of the caps fail in the PSU. For example:http://www.antiqueradios.com/forums/viewtopic.php?t=70957&view=nextMy last repair only was a 100uF 50V cap, in the manual labeled as A10C8.What I've observed is that eBay units pulled from a working environment are more reliable, the ones seem to be more of risk are 'mothballed' for ?? years, then if restarted some electrolytic caps may simply need to be reformed but aren't, leading to the failure. The 2015A series has multiple subtle to not so subtle revisions up to the I think its lower than 2201A04999.Do you know anything about the history of your unit, did it fail in use or was it stored for a period, tested then failed?IIRC to decode serial number: 2201A04999 for example:20th year since 196001 monthA = America, i.e., made in USA04999 = 5 digit serial numberAlm said it best. One other thing is simply disconnect the power cord and turn the unit ON to discharge those huge caps; that's the residual voltage the manual talks about when the unit is off. To be extra sure check the rails with a DMM before touching any boards.Note, the failures on the PSU generate a plethora of codes, including # 3 like yours. It may not be that section at all. As caps have a physical appearance of failure or maybe hot from high ESR, it'll pay to peruse the PSU section and eyeball or touch any bad caps just by probability. What tends to happens is the cap isn't dead, but its erratic operation could supply irregular power to the board it powers, leading to a specific error message that isn't a PSU fault as suggested in the 3456a troubleshooting guide.Quote from: king.oslo on February 08, 2012, 02:10:36 pm

author: Unknown
author_rank: Unknown
timestamp: February 08, 2012, 05:24:48 pm
content: How hard would it be for a DMM to just measure its own voltage rails as a self test?Measuring voltage and ripple on all power rails (the manual will probably have instructions), especially those related to the in-guard digital circuitry, sounds like a good idea. If the ripple is fine, the capacitors are probably not the cause of your problems.I believe the year/month encoded in the serial number just represents the date of the last major change, there may be several years between this date and the actual production date.

author: saturation
author_rank: Super Contributor
timestamp: February 08, 2012, 06:54:43 pm
content: You're right alm, its a paradox: it may not have a reliable comparison to serve as a standard to test itself against, like an old saying a 'catch-22'. Of course, an old reliable standby is you power it up, and nothing lights up is the simplest test but only works in a total failure, rather than malfunction. IIRC none of the 3456a self tests are a test of PSU integrity.This is an important thread, IMHO as there hasn't really been a discussion about servicing this venerable DVM, which I think is the best in the series before the 3458a. Given how generally prolific it is on eBay, it something others could consider as a bench meter, in a nonproduction setting.Quote from: alm on February 08, 2012, 05:24:48 pm

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 08, 2012, 07:11:19 pm
content: Guys. +5V supply on the microprocessor A30U13 is VMAX= 5.04. But VPPis 4.6V!Pictures are coming. Tell us what you think.MUPDATE: I measured the frequency of the dips in the voltage: they are 108Hz and 92Hz

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 08, 2012, 07:31:05 pm
content: Pictures!IMG_3234.jpg: All of a sudden, the 7segments went from displaying ---------- to OL. I tried measuring voltage some DC voltage through using the meter. But it didn't respond. I tried rebooting the meter, then it went back to -----------.IMG_3235.jpg: Something in this area is constantly producing a clicking sound whilst power is on. It sounds like and old HDD. Couldn't tell which component was making the sound.IMG_3236.jpg: A humming sound was emitted from this area. The whole underside of the meter. Perhaps the high voltage stuff? No idea where it came from. It sounded like it was emitted from the whole surface. Any ideas?IMG_3240.jpg: The measurement of the 5V supply to the microprocessor. This must be causing the malfunction, for sure?IMG_3241.jpg: The girl I am living with is asking to come to entertain her. I just want to be here to look at this. The construction of this thing is the finest I have ever seen. The top and bottom cover comes off with a single screw. All the boards are gold plated and the interior is just breathtaking.IMG_3241.jpg: Another look at the side of the meter where the humming sound is emitted from.Kind regards,Marius

author: saturation
author_rank: Super Contributor
timestamp: February 08, 2012, 08:19:46 pm
content: "Sounds" like you're getting a good clue, the trail is hot. Transformers tend to normally hum a touch, but there should be no AC on DC rails. I think there is enough gold in the old HP devices to pay for the device, maybe more, once its reclaimed.

author: Unknown
author_rank: Unknown
timestamp: February 08, 2012, 10:24:51 pm
content: Does the clicking sound like a relay? I can imagine relays being unstable with this kind of power supply. The meter contains a fair number of relays for tasks like range or function switching.This kind of power supply can definitely cause a faulty in-guard logic operation, the troubleshooting procedure wasn't completely off the mark after all. The exponential decay of the scope shot probably represents a cap being discharged through a constant(ish) resistance. Normally filter caps are sized so the discharge takes much longer than the time between cycles, so the voltage only falls a few percent or so between AC cycles. The ~100 Hz frequency is consistent with rectified 50 Hz AC. The frequency doubles due to rectification.I would look into the big caps on this rail, the smaller caps are for localized bypassing of high frequencies, the 100 / 120 Hz should be filtered by the big filter caps. Try measuring capacitance and/or ESR. You'll have to desolder at least one leg for this measurement. Putting a similar value in parallel and observing if the problem goes away should also give you an indication.Be sure to also check other caps of similar types/brand. If they were manufactured at the same time and were exposed to the same temperatures, chances are that they're also dried out.

author: Richard W.
author_rank: Regular Contributor
timestamp: February 08, 2012, 11:22:52 pm
content: QuoteThese metal cans in the lower third, right of the middle of the pictures with 6 pins and the turquoise caps look like reed relais.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 09, 2012, 12:06:38 am
content: My friends,I have made some progress. Perhaps I fixed it. You be the judge. I found the cap (A10C5 - that blue monster), but I couldn't lift a leg because the clearance between hole and leg was too small. I tired using hot air to keep the five legs hot whilst trying to pull it out, but I gave it up after trying for quite a while.Eventually I just cut the legs, and soldered in a new cap. This cap is not 4000uF like the old one. It is just 2200uF, but it was closest 50V cap I could find.I powered it up. And it seemed to work. The humming sound was gone too. And the clicking sounds? Alm, I think you are right. The clicking is gone too, it only occurs whenI measure components or voltages. Yes!I ran the self-test. The manual says the meter will display a negative integer if the test fails, and "100 "if the tests pass. It displays neither. It only displays +1.8.8.8.8.8.8.8.8.+8 whilst the LEDs are on. When the LEDs go out. It displays nothing. This bothers me a bit.VDC measurement seems to work, so does 2 wire resistance. Not checked math, 4 wire or VAC yet. I wanted to ask your opinion about the self-test.Here are some photographs.IMG_3243.jpg: Testing to see if a new new fixed the problem...IMG_3244.jpg: ...and finally it powers up. The sounds are gone. Here measuring a 22.6ohm 1% resistor.IMG_3248.jpg: Size comparison between the blue monster and the new friend.What do you think about the self-test?M

author: Unknown
author_rank: Unknown
timestamp: February 09, 2012, 12:26:13 am
content: I would add another 2200 uF cap in parallel before you reassemble it. No need to fuss about tolerances with electrolytics, but almost a factor two too low may be a bit much. Also check if the extra pins of the old cap were used for circuit connections, Tektronix liked to use them to complete some ground connections. You may need to solder a few jumbers if there are no PCB traces. Make sure you jumper the correct holes.The +1.8888+8 output is normal, reread the self test section of the manual. It should display +1.888... and light all LEDs, go blank, and start again without showing any other numbers like -3. It outputs 100 on theHP-IBbus.Seems like my like it should work fine now, although I would also check ripple on the other rails (maybe they're just marginal) and probably do a rough performance verification (check if all functions and ranges give reasonable values).

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 09, 2012, 12:33:42 am
content: I am heading to bed, but I will do the things you suggested tomorrow.One thing I noticed was that the readings were fluctuating a bit. The resistance was fluctuating between 22.140 and 22.138 and 22.139.Thats all I can report for now. Good night.M

author: saturation
author_rank: Super Contributor
timestamp: February 09, 2012, 01:57:38 pm
content: Congratulations! Much has happened in the past evening, and it was a blow by blow repair job. Glad to hear its up and running and that reports on 2201A series malfunctions are consistent: the PSU is common culprit.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 09, 2012, 03:01:00 pm
content: Saturation, when I discovered it was a 2201A, I found myself filled with skepticism about the PSU claims. I wasn't thinking I could be so lucky to find that this was the case. Murphy gets me each time.But when I measured the rails I wasn't believing my eyes! I swapped the cap, and I was dancing around the room until 1:30AM! Haha! I caught alm's reply, then I passed out on the bedToday I finalized the work, and cleaned it inside out. It looks great now. Only problem is the reset and guard-switches. Somebody has spilled something sticky on them, so after pressing them down I have to manually pull them back out. But I couldn't figure out to get in there to clean them without taking everything apart. Do you know a convenient way? If you tell me I will post some pictures of doing the jobSometimes we get lucky (but very rarely), so when I got lucky it feels great!Oh, and any ideas on how to get it checked/calibrated reasonably?Kind regards,Marius

author: Unknown
author_rank: Unknown
timestamp: February 09, 2012, 06:08:06 pm
content: Commercial calibration is probably going to be very expensive because it uses all manually operated trimmers, you can't just hook it up to a calibrator, start the procedure via GPIB and walk away. Unless a component that influences the accuracy (eg. voltage reference, divider resistors) is changed, or it was treated harshly, I would expect it to be quite close to specs. It's not unheard of for these instruments to still be in 1 year specs after many years without calibration.Even a cheap voltage reference, like those made by Geller Labs and voltagestandard.com, is almost an order of magnitude less accurate than the 1 yerar accuracy of the 3456A (23 ppm on the 10 VDC range), so you can only use them to prove that it's out of specs, not that it's within specs. Voltagestandard.com sells a $200 reference that has a similar accuracy as the meter (still not the 3-5x TUR that would be required for calibration). This would only allow you to check a single range. In my opinion it's not worth the money.I would check if all ranges give reasonable values (a 1% 1 kohm resistor might read 1.009 k, but should not read 2 kohm). Voltage dividers can be blown up by high voltages, for example. If it's close, than I would assume it to be correct until you have reasons to believe otherwise. It's hard to think of a situation where absolute accuracy is critical, and these instruments are known for good linearity and transfer accuracy.

author: saturation
author_rank: Super Contributor
timestamp: February 09, 2012, 09:44:37 pm
content: I'm happy for you. Yes, I know this dance, I've done some myself. Before you jump into calibration you'd best be sure nothing else will fail, so I'd leave the unit powered on for at least a full 3 days, if not a week. During this time do the manual's recommended performance check to insure all the ranges are working.The unit may not be far off calibration, but even if it were, the precision should be intact. You can spot check ranges as alm suggested.You can search the archives for my older posts on DIY calibration.For a non-official cal, the easiest thing to do is a transfer. You'll need a known calibrated 6.5 digit DMM at minimum [ better if 7.5 or better] stable sources of DCV, resistance, ACV, including variable frequency --- AC is the hardest to find. Get as close to the recommend test value mentioned in the manual, but it needn't be metrology grade, all it need be is stable enough for minutes during a transfer. Make sure your room conditions are similar to when the cal'd DMM was done.Just as example to do a 1V DCV range, I used a 1.25V NiMH battery, which say reads 1.234 567 V with the calibrated DMM, then quickly read it with your 3456a, then adjust the 3456a pots to read identical to the good DMM. Move back and forth between the 2 DMMs say 3x to insure the reading is sync'd. After you've completed the whole DCV function you can randomly check any DCV between both DMMs, to see they are in sync from 1uV to 1kV and touch up as needed.If you get one of those low cost references as alm discussed, a problem is you have only several tests voltages, it may not cover values the 3456a requires. There is also a question if they truly live up to their ppm accuracy. But in the end, the DIY is better than no-cal at all, assuming the DMM is grossly out of adjustment and one isn't going to pay the $100-200 to have it calibrated by a standards traceable lab.There are sources of error during a transfer that will affect your values, such as reading uV, milliohm kV, megaohm etc., require metrology technique to pull off, but it will be noticeable because the transfer values will not be stable or the comparison between the cal'd DMM and the 3456a will not be close.Quote from: king.oslo on February 09, 2012, 03:01:00 pm

author: Unknown
author_rank: Unknown
timestamp: February 09, 2012, 10:26:02 pm
content: Quote from: saturation on February 09, 2012, 09:44:37 pmI doubt whether acquiring stable sources spanning all ranges and functions is going to be cheaper than a commercial cal. How do you generate high voltage, high frequency AC with any accuracy? Most hobbyists probably won't have equipment for DC voltages above 100 V or so. Most 6.5 digit DMMs will have inferior accuracy compared to the 3456A, so calibration will be worse than factory spec.Quote from: saturation on February 09, 2012, 09:44:37 pmI would always follow the procedure in the manual. You may have to adjust the range for 0 V first, then for gain, then for linearity, for example. You may also have to adjust the other ranges after adjusting DCV, I don't recall the exact procedure. To compensate for noise and drift, you can connect meters in parallel and average a lot of readings, but this makes the procedure more tricky. Of course this doesn't work very well with resistance.Quote from: saturation on February 09, 2012, 09:44:37 pmI try to avoid doing adjustments without the proper equipment unless I can prove it's out of cal. Improvised equipment and techniques are likely to produce inferior results than the original factory cal, so it's not unlikely that you'd make it worse by adjusting it to incorrect values.

author: saturation
author_rank: Super Contributor
timestamp: February 10, 2012, 01:42:11 pm
content: In reply to alms many good points:DIY is only a consideration if its grossly out of cal given many concerns you raise. The question will be leave it be, or align it against a cheaper and less accurate non 3456a 6.5 digit remains with the reader.Variable frequency high voltage AC source are indeed hard to find, as I mentioned, particularly with stable outputs needed to make measurements, but luckily the AC sections can be adjusted separately, as can all the other sections of the 3456a, you needn't cal what isn't off, but that's not the official mantra, that's DIY approach. You needn't do all at once, too.DCV source to 1kV can be found easily, but you have to want to find them. I use biomedical grade electrophoretic supplies, as I posted earlier in eevblog; it produces a stable output down to 1-10mV up to 3kV. and are dumped in quantity in the US. I obtained 2 for $10 each, $12 more each for shipping. When new, such supplies are worth $8k.To cal the 3456a, even a metrologist will need to interpret the 3456a procedures to match it to modern equivalents today, since most of the equipment in the manual is obsolete; for example nulling voltmeters are replaced by a far much more stable direct output cal stations.Over the past 2 years I've collected equipment specifically to serve as standards and about 6 months ago had to pleasure to compare it against a traceable 3456a, and not only were my DIY adjustments in spec, they had tighter spreads within the allowable range of variation.Further, in post I made on eevblog on my review of the 1272a handheld DMM that was shipped by Agilent to me for review out of cal, I used DIY procedures to put it back into cal. Agilent subsequently sent me another 1272a to review, and now comparing both DMMs side by side, they were both in cal but the Agilent version had much tighter results, closer to the 3456a, that I had also DIY. I adjusted the 1272a to compensate for those errors, and now everyone is tight together.Noise, drift, temp, proper leads, etc. are all part of consideration during the measurement. One doesn't connect the 2 DMM together to avoid interactions between them, since one can't be absolutely certain the DUT isn't still faulty. So each measurement is done separately, and multiple times. For resistors, you cannot handle them, to avoid causing them to drift due to temp, and megaohm values are best shielded etc.,Metrology is a field on its own, and as alm and my rebuttal suggest, is involved even as DIY. You needn't jump into it if a device is still in or close to spec. However, learning how to do it will give you great control over your equipment. There are many techniques that are not part of everyday electronics work.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 10, 2012, 04:11:50 pm
content: Ok, so I shout out to my fellow Norwegians:Are there anyone in Oslo whom has access to highly accurate equipment whom is willing to help me put this meter into spec???I checked a local company. They asking NOK4500 to do a cal. That is circa USD750. I cannot afford that.Kind regards,MariusPS: saturation, the meter has been powered on for 2 days now. Still going strong

author: saturation
author_rank: Super Contributor
timestamp: February 10, 2012, 05:49:05 pm
content: Hello Marius:You may want to join and post too to folks at volt nuts, to find someone near you who may be able to help with DIY or formal calibration at low cost:http://www.febo.com/cgi-bin/mailman/listinfo/volt-nutsthe other helpful board is HP group:http://tech.groups.yahoo.com/group/hp_agilent_equipment/They have all been helpful in getting tips to keep my stuff up and running.Quote from: king.oslo on February 10, 2012, 04:11:50 pm

author: Neganur
author_rank: Unknown
timestamp: February 10, 2012, 09:46:15 pm
content: Quote from: king.oslo on February 10, 2012, 04:11:50 pmNot Norwegian (but Danish), I have access to two Fluke calibrators here at the uni in Helsinki. The 1 Gohm range may be problematic since I think to remember 100 Mohm being the maximum when I cal'ed my two HP3478's. Would have to check.You could also try to contact your local uni and ask if they have any. Sometimes people are nice enough to offer help (it's more a time issue than not wanting to help).

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 10, 2012, 10:03:53 pm
content: Quote from: Neganur on February 10, 2012, 09:46:15 pmNeganur, will do.I do think the Uni of Trondheim has better equipment. As far as I know, Uni of Oslo doesn't do any engineering, but I will phone and ask tomorrow anyway.Do you have access to equipment in Denmark? My girlfriend is Danish, so we travel there all the time.Kind regards,Marius

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 11, 2012, 12:01:45 am
content: Ok, I have made some progress.Tell me what you think.An engineer with access to the standards replied to an email of mine. He told me I could bring the meter to his institution next week. He said he could compare my meter to the standard.Am I mistaken in thinking that if I record how far my meter is off from the standard, then when I get home, I can calibrate any difference then (given that the temperature is the same, and that I can access suitable DC voltage and AC voltage source at home)?Thanks.Kind regards,Marius

author: saturation
author_rank: Super Contributor
timestamp: February 12, 2012, 03:02:11 pm
content: You are making quick progress, Marius. A delayed adjustment is not recommended.There are other phenomena that cause instability: humidity, barometric pressure, cables used, type of metal on the ends that contact with both devices,touchingthe cables, or just adjusting the pots, just to name afew.If you've never done this before and are not fully aware of the other phenomena that affect sensitive measures, the only way you'll see your approach has trouble is that after adjustment the reading of the standard and your DUT are not in sync down to the last digits, say uV, and the values are unstable.When the standard and the DUT are together, at least some variables are controlled, as they are now exposed to the same issue simultaneously.For example see Dave's video inside a cal lab, and what the cal guy says to Dave:You can zoom to 42:30 to see how cal's are done.In analog components, adjustments to switches and pots need to 'settle' as shown in the video when Dave turns the knobs. Temp stability is crucial: at the start note the last digits bobble about until it 20 minutes later, it ~ 9.999920 then is is ~ 9.999965, which is in spec for 30ppm, similar to the 3456a. However, 20 min is just a practical guideline, in experimentation I find 4+ hours is typical, and leaving it overnight is a simple way to get it fully stabilized before adjustment. This is why the 'cal guy' says 'take it home and never turn it off'.The cal guy mentions 'test uncertainty ratio' of ~ 3.5:1 which is similar to what to expect between the 3456a measured against a 3458a, the best DMM now available. TUR means the "accuracy of the DUT / accuracy of the standard". The ideal is 4:1.When you use smaller ratios, such as against another 6.5 digit DMM, it gives a TUR of 1:1. To offset this ratio, the value you adjust, and the reading of the standard must be 1:1 to reduce uncertainty to something like a TUR of 3.5:1 or better. The only way to insure that is when both the standard and the DUT are together to tweak the adjustments to keep a 'tightest' variation between them.Quote from: king.oslo on February 11, 2012, 12:01:45 am

author: Unknown
author_rank: Unknown
timestamp: February 12, 2012, 04:21:18 pm
content: Quote from: saturation on February 10, 2012, 01:42:11 pmFor a proper voltage reference with good short-term stability, I would agree. For improvised sources, I'm not so sure. Even after warm-up, short-term (5 minute) fluctuations of some signal sources/power supplies I tried were 50 ppm and 200 ppm, but the fluctuations were closely correlated between various multimeters. Averaging can correct for short-term fluctuations, but sometimes there's a long term drift component, even after hours of warm-up. Averaging samples taken during the same time period works around this problem, but requires parallel connections.See the attached image for some actual data. Green is a 16 sample moving average of blue, and cyan the same for black. This was to emulate the transient response of red. There are some minor differences in transient response, but apart from that the values are very close. Note that these three meters came from different parts of the world, and two of them were last calibrated at least ten years ago.What kind of interactions are you talking about? A defective DUT having a much lower impedance, loading the voltage source? A DUT putting out a bias current much larger than specified? Unless you're talking about huge amounts of current or very low voltages, two parallel connect meters should see the same voltage.Quote from: saturation on February 12, 2012, 03:02:11 pmAnother 6.5 digit DMM is likely to be worse than 1:1. For example, 1 years specs for HP 3456A compared to HP 34401A are 23 ppm:35 ppm, or about 1:1.5. A 34401A calibrated within the last 90 days would be a little better than 1:1, and a 24 h calibration interval would give you 1.5:1. Much better instruments are needed if you want to calibrate the 3456A to 90 days or even 24 hour specs (8 ppm for 10 VDC).

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 12, 2012, 09:13:25 pm
content: Quote from: ErikTheNorwegian on February 12, 2012, 08:26:52 pmThanks Erik,That is great.It'd be great if you pass me your phone number. Mine is 402 402 00!M

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 13, 2012, 01:57:25 pm
content: Hello there,I was invited to a company - NEMKO. They compared the DMM to their standard. The results were okay, but I had wished it was better. Their calibrator was something like 8 or 9 digits.Without much of a warm up, this were the results:Standard - 3456A0VDC - 0.000001VDC - 1.0001210VDC - 10.00021100VDC - 100.004100VAC (50Hz) - 102.5400VAC (50Hz) - 40310R - 10.02310k - 100.0021R - 1.091Calibrating the meter against their standard was circa NOK3000-NOK4000 ~ USD500-USD700, depending on how long they would need to complete the job.When I accept ErikTheNorwegian's offer, is it possible to adjust only the AC and resistance, or will this upset the DC calibration?Thank you for your time.Kind regards,Marius

author: Neganur
author_rank: Unknown
timestamp: February 13, 2012, 04:32:04 pm
content: If the meter is anything like the other HP's, the most important cal is the DCV and DCA since the Ohms measurements are derived from those values.ACV and ACA usually have a very poor tolerance compared to the DC ranges and are for sine wave only.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 13, 2012, 04:34:02 pm
content: Quote from: Neganur on February 13, 2012, 04:32:04 pmThe meter hasn't got current-measurement.M

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 18, 2012, 02:52:03 pm
content: I am preparing myself (and the 3456A) to get adjusted up. I checked out the official calibration procedure for this meter, but it seems to require the use of instruments which I don't have (heck, some of them I have never even heard of).Are there more up to date procedures available from anywhere? What would you do if you were me?Thank you for your time.Kind regards,Marius

author: Unknown
author_rank: Unknown
timestamp: February 18, 2012, 03:39:24 pm
content: I would try to arrange for an hour or so of warm-up, I believe this is specified in the manual, and an ambient temperature close to in your lab. Accuracy is usually specified for calibration temp +/- 5°C, so too much difference in temperature will reduce accuracy. I'm not sure about the magnitude of the warm-up drift, you can easily test this yourself by hooking it up to a stable source, turning it on and see how the reading changes during the first hour or so.Regarding your quick performance verification:Sure you didn't miss a zero for the 1 VDC reading? The current value is out of spec, while 10 VDC and 100 VDC are comfortably within 1 year spec (about 24 ppm plus a few counts). It also has only less digit than the other values.The AC results seem to be out of spec, assuming the filter was enabled (I don't remember if it's enabled by default).Were the resistance measurements taken in 5.5 digit mode? Resolution should be 100 uOhm in the lowest range. 10k value appears to be off by an order of magnitude, that seems unlikely. What was the integration time? For 0.1 PLC, which would match the 5.5 digit resolution, it's allowed 0.01% + 14 mohms of offset in the 100 ohm range, so it would still be close to specs. In 1 or 10 PLC mode, it'd be clearly out of spec, assuming correct procedures and cables were used.What sources are you planning to use to do the adjustment? Adjusting AC voltage from the mains voltage + transformer, for example, may not give very good results, since its stability may not be better than the ~2% accuracy you have now. You should be able to check the stability of the source by measuring its variance. First measure the intrinsic noise by shorting the inputs with a piece of copper around the binding posts and measure the variance (math -> stat, let it measure for a while, then recall variance to get the variance and recall mean/count for mean and number of samples) to get a baseline. Then do the same with your source. If the standard deviation (square root of the variance) is not significantly lower than the factory spec, your cal will be less accurate than factory spec. If it's in the same order of magnitude or higher than the current error, it's probably not a good source to adjust to. You don't need accurate values if you have another calibrated DMM, adjusting the trimmers to 0.984165 V works just fine. Try to stay close to the ~1.2x ranges, 1.3 V would not be very optimal, for example.You apparently need a stable 1 VAC/1 kHz source and a bunch of accurate 1 Kohm - 10 Mohm resistors. My experience is that 1/4 W metal film resistors are horrible for this purpose, the value would change noticeably due to heating by the current from the DMM. Standard resistors or precision wirewound resistors appear to work much better, if only due to their much larger thermal mass. I wouldn't worry too much about the required equipment. Even a real cal lab might just use a DMM calibrator and reference DMM.I don't expect any change to DC calibration after adjusting AC or resistance. If you check the adjustment procedure, they first adjust DCV, then R, then ACV. If DCV would be influenced by ACV, the procedure would be wrong. Resistance calibration could influence ACV calibration, as ACV is performed after resistance calibration. This seems extremely unlikely to me, since these circuits usually have very little in common apart from the circuitry also used for DCV.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 18, 2012, 04:14:44 pm
content: Alm, thank you. I am filled with humility by your evident experience.The recordings I did at the lab were done in a hurry, so perhaps I got a reading wrong. It sure looks like that is the case.I thought I would make my 1VAC 1KHz source using PIC mcu and a transistor.Circa what current should I expect the meter to draw from the circuit during measurement in ac and dc?Thank you for your time.Kind regards,Marius

author: Unknown
author_rank: Unknown
timestamp: February 18, 2012, 04:40:10 pm
content: Almost no current, especially at low frequencies. Impedance at DC is > 10 Gohm <= 10 V and 10 Mohm at higher ranges. AC impedance is something like 1 Mohm with probably < 100 pF of capacitance is parallel, or an impedance of more than 500 kOhm at 1 kHz, so the peak current would be something like 2 uA.No idea if you will achieve the required stability with a basic oscillator, but I don't see any harm in trying. The aim is good short-term (minutes, not hours) stability. I would try to get a sinusoidal signal, a square wave will have all kinds of nasty harmonics. Note that you need a sine with an amplitude of sqrt(2) volts to get 1 Vrms. Since the frequency is constant, just a sharp band pass filter tuned to your frequency may be enough, if your frequency is stable. This would also remove the DC component. At least use a $0.2 or so crystal as clock source for the PIC.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 18, 2012, 07:58:00 pm
content: I have a rubidium oscillator.Is it's short term stability suitable?M

author: Unknown
author_rank: Unknown
timestamp: February 18, 2012, 08:19:04 pm
content: Frequency isn't critical at all (I don't think HP bother to specify a frequency tolerance for the 1 kHz source), but varying by a few percent as would be possible with the internal RC oscillator might be an issue, and a crystal only costs a few pennies. It's the amplitude accuracy that counts.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 18, 2012, 08:36:28 pm
content: Ok, which method would you use to build a suitable ac source?M

author: Unknown
author_rank: Unknown
timestamp: February 18, 2012, 08:52:53 pm
content: I got decent result with a HP DDS function gen after allowing it to warm up, standard deviation of a 1 Vrms/1kHz sinusoidal signal was about 10 uV over the time span of five minutes (almost 400 samples). Main limitations was that it did not go beyond 7 Vrms.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 18, 2012, 09:16:23 pm
content: Which one?M

author: amspire
author_rank: Super Contributor
timestamp: February 18, 2012, 09:38:14 pm
content: One method I have used in the past to make a stable AC voltage reference is to used a stepped sinewave generated from cmos switches connected to, say, a 10V DC reference IC, and to then pass it through an op-amp low pass filter to turn it into a sinewave with about 0.2% distortion.The trick was the stepped waveform I used had no harmonics before the 9th from memory, so I was able to have the filter 3db point set well above the sinewave frequency, and so the filter stability didn't affect the output voltage much.You could use a micro to generate the clocks for the waveform. Then you need the dc reference, four matched and stable 10K resistors and a couple of opamps to make the filter.To calibrate it, I have an old Fluke 540A thermal transfer calibrator that is accurate to about 0.01%, but the output has pretty good accuracy based on the DC reference voltage only.I did this design for 50/60Hz but it is not hard to scale it up to 1KHz fixed frequency. I can dig up the circuit if you are interested.Richard.

author: saturation
author_rank: Super Contributor
timestamp: February 18, 2012, 10:42:08 pm
content: More challenges ahead, much has happened since the last posts!Some thoughts on various issues raised on this thread to this point:IIRC, the 3456a AC range is not stellar; the LSD variation is so high that the final accuracy isn't much better than a Fluke 87V. I checked the high range AC against line voltage and compared it against the Fluke 87V, and most other ranges using a DDS generator too. Note, at line voltage, I'm limited only to 60 Hz, whereas with the DDS, I could check output with the variable frequencies required. I made no adjustments, just checks.There is no 'zero volt' range, i.e., so I presume its 0.1VDC. If so, it reads out with an exponent to a max of 7 digits:+100.0000 -3note the last digit is 100nV. If you zero out the input with a shorting bar to give you lowest noise, I typically get < 500nV at equilibrium. Uncertainty of the LSD at this range is is 20-30 counts at 6 digits, so the accuracy is limited to 2-3uV, but you can get resolution to 100nV range.'Zero volts' however is tested per range on the 3456a by shorting the inputs, and what you see is the offset voltage.Quote from: king.oslo on February 18, 2012, 08:36:28 pm

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 19, 2012, 12:33:55 am
content: Quote from: amspire on February 18, 2012, 09:38:14 pmRichard, please let us see it!Saturation, in this context, what is the LSD? If I google the term, I only find information about psychedelic drugs.Thank you for your time.Kind regards,Marius

author: saturation
author_rank: Super Contributor
timestamp: February 19, 2012, 02:51:52 am
content: Good graph and comments. In reply:1.Yes, these all are short term sources; < = single digit minutes of stability; thus its important to have the "standard" DMM side by side with the DUT to check for drift. The variance function can help confirm drift is occurring, but since it doesn't readout in real time you'll need to rely on LSD variations.Yes, electronic sources are noisy for this purpose.I used stacks of good quality NiMH batteries, : 8 cells ~ 10.0VDC etc.,, to compare 1 & 10 VDC ranges. There is a slow 10 uV drift down the more batteries are stacked [ each cell isn't at the exact same potential as others in the stack, so stronger charge the weaker ones ] but hold long enough to compare the DUT and the standard down to the uV. To reduce voltages further I use a Kelvin-Varley voltage divider.For the 100VDC scale, I used as many batteries as I had then I reconfirmed the reading at full scale with an electronic reference to 100VDC and then to 1kVDC, comparing both DUT and standard meters continuously. It isn't as stable as the battery, but a point is to keep the DUT and standard as tight a reading possible to reduce error, see #3.2.I'd insure all DMMs connected together are first working properly individually, particularly after a repair. It may not be accurate without calibration, but the repaired meter should be precise. Once the user is satisfied all is well, then connecting them together should not be a problem unless you are concerned at the uV level.On low voltages, once other low level issues are stable: temperature, Seebeck, etc., you're still left with the leads picking up EMI, so the less leads are involved, and the shortest they can be, is best. To reduce pickup, all uneeded additional connections are avoided.3. There are problems even if the calibrated meter were identical to 3456a in accuracy, giving a TUR of truly 1:1. The theory of how this uncertainty is minimized is that each measurement comparing the DUT and the standard must have as little difference as practical, but tighter than allowed by the manual. This is done by reducing the limits of a specification that the meter is allowed to vary, the idea of a guard band.http://www.agilent.com/metrology/pdf/guardband_beginners.pdfFor example, at 24hrs, at 10V the acceptable range is 9.999 90 to 10.000 10; this is 8ppm + 2 digits. To adjust it with a TUR of 1:1, the acceptable range must be reduced >= 4:1, which is range spanning 2ppm. So the acceptable range after adjustment is now at least ~ 10.000 00 to 10.000 04 relative to standard used. The Agilent papers details how to make a more accurate calculation of the new limits, but its easier to keep the difference nearly nil between the DUT and the standard, so you can keep making as many subtle adjustments until the DUT and standard are measuring as tightly as possible.A simple test after adjustment is to measure the same same transfer reference simultaneously, say at 10V. After some time, both meters should have ~ same mean and variance.Given the 3456a must already be out of cal to warrant adjusting it, which would be best, leaving it out of cal, or approximating the 34401a? The 3456a precision in unaffected. Regardless of TUR ratio, the best that can be achieved is the accuracy of the lesser unit: if the standard is better than the DUT, the best you can achieve is the spec of the DUT; if the standard is less than the DUT, then at best the DUT will be as good as the standard.Quote from: alm on February 12, 2012, 04:21:18 pm

author: saturation
author_rank: Super Contributor
timestamp: February 19, 2012, 02:53:13 am
content: Sorry Marius, least significant digit; the right most digit.Quote from: king.oslo on February 19, 2012, 12:33:55 am

author: amspire
author_rank: Super Contributor
timestamp: February 19, 2012, 07:24:51 am
content: Dug up the circuit and did a quick LTSpice sim (file attached).You will notice that the first two opamps are just voltage followers. I think I was using up all four opamps in the packages, but it would be possible to reduce it to 3 opamps.The micro needs to generate the 1KHz waveforms from a 30KHz interrupt. Alternately, it is possible to just get a 30KHz clock and a 8 stage cmos shift register (instead of the 74HC04 in the circuit) to get the four phased squarewaves. Power the 74HC04 (or one of the other CMOS families) or the cmos shift register mentioned above from a 5V reference IC like a Ref02. C58 removes the 2.5V DC offset from the sinewave and the opamps are powered by +/- 5V supply. remove c58 and R92 as long as the opamps have a +7.5/-2.5V supply or higher, and then you would put a capacitor on the output. You need to make sure that the opamps stay in their linear range, and you may want to turn one of the first two opamps into a gain stage to boost the sinewave to 10VAC. That would need a supply of at least +/- 20V for the opamps.The 100K resistors can be anything between 10K and 100K, as long as they are well matched in value. Matching to 0.1% would be nice, but 1% is not a disaster.The filter has the -3db point well above the 1KHz point. Its response should be pretty flat and stable at 1KHz, and the gain should be unity.There are lots of other ways to generate sinewaves from square waves, but this one is a good compromise for low clock, simple resistor and capacitor values, and high stability.Richard.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 21, 2012, 02:00:24 pm
content: I have been working on the calibration voltage source, but nothing is complete so I got nothing to show you yet.I was inspired by Richard, so expect no major differences from the circuits he suggested. So a big tank you goes out to him, and the others whom has helped me.Thank you.Here are some photographs of the funny keyboard that the instrument has. I was cleaning the keys yesterday, and loved the way it has been designed.Disassembly of the front panel is done by six screws. Great construction.Thank you for your time.Kind regards,Marius

author: lowimpedance
author_rank: Super Contributor
timestamp: February 21, 2012, 10:35:04 pm
content: Quote from: king.oslo on February 21, 2012, 02:00:24 pmThese switches were very common on HP gear of this vintage from x-y plotters, counters and so on.There is a 'gold finger' contact and a matching gold pad. I had a problem with these switches on a 3456a, not with the contacts but the little leaf spring jumping out of its seat, (at the base nearest the pcb!), when pressed. Most annoying when you just reassembled it !!!!. Found if you turn the spring over and re insert it it stayed put (i suspect since its stamped out of sheet of spring steel there is a small burr left which helped to bite into the plastic body of the switch).Nice to see quality gear like this brought back to life, enjoycheersJohn

author: amspire
author_rank: Super Contributor
timestamp: February 21, 2012, 11:13:49 pm
content: Marius, good luck building the circuit. If you can just get an AC source that is stable with your meter's resolution, that is an achievement. It is not easy getting really stable AC.To get a micro to generate the correct four phased 1Khz outputs is pretty easy - just have a table of 30 4 byte entries, and every 30KHz interrupt, output the next element on the table to pins. I have attached the table.It would be great to see the results.Richard.

author: king.oslo
author_rank: Frequent Contributor
timestamp: March 19, 2012, 11:20:55 am
content: Hello everyone,I am simulating the AC reference circuit in LTSpice. It outputs a beautiful AC sine wave.Work is progressing well with the DC references too. Some parts blew up, so I will have to buy some new ones before I can proceed. I will post photographs and perhaps a video if I find reason to do so.Richard, is it possible to drive the ac circuit directly with square waves from the micro rather than using the logic chips? Problem is I have never used a logic chip in my life, so I would happily save time by outputting the four square waves directly from the micro into C58 capacitor with straight wire.Which components need to be powered by DC reference ICs and which can be powered by regular voltage regulators?Thanks.M

author: amspire
author_rank: Super Contributor
timestamp: March 20, 2012, 03:59:02 am
content: Quote from: king.oslo on March 19, 2012, 11:20:55 amYou can try outputting from the micro directly - it is a cmos output so it could be good. Try to avoid any heavy loads on other pins of the micro, and power the micro from a voltage reference IC.A way to test to see what stability you can get from a microprocessor output is to set up an output to output PWM and have it going to a 10K resistor followed by a 1uF ceramic cap to ground. Do a 90% PWM and check the DC stability of your output, Try other ratios like 50%, 10%. The sinewave stability will be about the same as the PWM DC output stability.Richard.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 20, 2013, 02:33:55 am
content: Hello world,I am looking at this again. I built this up a few days ago, but tonight I thought I wanted to have a go myself tonightAttached is my circuit and FFT for the 1khz AC source. Problem is I got very little experience, I have read up on some theory, but my intution is not developed enough to get a sense from eyeballing the frequency domain of how badly my 6.5 digits will fluctuage. What do you think of this circuit and most importantly: the FFT?First, will you kindly look at my FFT and tell me what your gut feeling is? If you can spot something which would yield better results, please let me know. How are stable commercial AC calibrator circuits designed?Thank you for your time.Kind regards,Marius

author: saturation
author_rank: Super Contributor
timestamp: January 20, 2013, 01:00:44 pm
content: The 1kHz signal used for calibration is pure sine wave, this FFT is very noisy. If its not a pure sine wave, adjusting the true RMS calibration will be off even if you have the amplitude of the waveform stable, which is not easy to do.Even if your 3456a is off factory specs for calibration, its probably far better than any lesser digit DMM you have, so go carefully in adjusting it, once done it cannot be undone since you have no good reference to refer too.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 20, 2013, 06:14:07 pm
content: Dear saturation,It is noisy, but it should be pretty stable noise, don't you agree?If I connect this reference to an in cal 3458a and my dmm, and adjust my meter to match, should work right? As long as the integral of the waveform is stable, the readings shouldn't fluctuate, because this is a true rms meter. Is not this correct?Thanks.M

author: Unknown
author_rank: Unknown
timestamp: January 20, 2013, 06:26:37 pm
content: As long as their bandwidth is the same or the signal doesn't have any energy beyond the bandwidth of the meters.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 20, 2013, 06:28:02 pm
content: Quote from: alm on January 20, 2013, 06:26:37 pmGood point. I will check this. Thanks.M

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 20, 2013, 07:13:12 pm
content: ThanksM

author: robrenz
author_rank: Super Contributor
timestamp: January 20, 2013, 07:29:15 pm
content: king.oslo can have my beerI will probably never make it to Norway"But You helpt me from having that flickering text flashing from the bench! You made my day! Ever come to Norway, on beer is on me!"

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 20, 2013, 07:52:49 pm
content: Quote from: robrenz on January 20, 2013, 07:29:15 pmThanks man!Flickering Christmas-tree peter-pan benches are no goodthanks for your supportM

author: saturation
author_rank: Super Contributor
timestamp: January 20, 2013, 08:04:11 pm
content: Yes, assuming the noise sources are stable and you are comparing it against another meter with similar bandwidth as the 3456a, as alm stated.Alas, the 3458a has a much wider bandwidth than the 3456a and has 3 modes for measuring true RMS, all of which are far far better than the 3456a. Unless you use a pure sine during calibration, it introduces additional uncertainty.But Erik has come to rescue, it reads like has everything you might need!Quote from: king.oslo on January 20, 2013, 06:14:07 pm

author: amspire
author_rank: Super Contributor
timestamp: January 28, 2013, 02:54:17 am
content: Quote from: king.oslo on January 20, 2013, 02:33:55 am