Acceptance Testing - Page 1
Forum section: electronics / testgear

author: PixieDust
author_rank: Frequent Contributor
timestamp: September 29, 2022, 12:11:15 pm
content: Hi, as per this thread that I started a while back:https://www.eevblog.com/forum/metrology/entry-level-metrology/msg3685063/#msg3685063I purchased a used 2431L Tektronix and a used HP3478A multimeter. In the above thread I was asking for advice on calibration and the consensus was that I don't need it. I am happy with that assessment. However, I'm still uncertain as to whether the devices are fully functional.I took some time to read enough of the service manuals to know what needs to be done and what equipment is needed to properly test the functionality of the devices. I obviously don't have this equipment and was wondering about ways to get around purchasing lab grade stuff to test equipment that I paid very little for.I'm starting to learn the hidden costs of old test equipment. New stuff comes with all sorts of guarantees and expectations. With second hand stuff, all bets are off.Anyway, just wondering if there are any tips or tricks to verify the equipment? I've used both for random things here and there and so far everything has seemed to make sense and work (except (at least so far) for one of the knobs on the Tektronix which sometimes sticks and causes unwanted behaviour. If you turn the scope off and on again, things reset and it works for a little bit and randomly stops working again), but ideally I want to learn a bit more about what I got unless you guys suggest something else/better.

author: tautech
author_rank: Super Contributor
timestamp: September 29, 2022, 07:26:50 pm
content: So how much did you spend on these fine old devices not to have faith in what they can do ?The first step in acceptance is Performance Verification with is also the first step in Calibration however if you have an instrument with a mechanical fault, fix it, use it regardless and watch for the fault not to catch you out or dig deeper into your pockets to get instruments that give you faith in their measurements.So yeah point the finger at this distributor that sells bright shiny new gear that all comes with a Cal sheet for taking the piss at your predicament but do know very clearly your current position not that long ago was also mine !There was a time I spent more time repairing my equipment than enjoying the hobby and that really sucks !

author: Martin72
author_rank: Super Contributor
timestamp: September 29, 2022, 08:22:32 pm
content: Quote from: PixieDust on September 29, 2022, 12:11:15 pmFor private usage not, that´s right.For curiousity, I´ve send my new brymen 869s meter for external calibration and got it back with a 8-pages calibration report.They checked every range and found some deviations and re-adjusted them...So now I know it really instead trusting the specs blindly.In your case, in the case of the HP, a calibration would be useful to know if everything is alright.I don´t know the prices in your country, but I´ve spend under 100 bucks for it and for me it was worth it.What the tek scope concerns, hmm..I think I would buy a newer DSO instead of spending time to fix it or to be uncertain about it´s fully functioning.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 30, 2022, 01:49:52 am
content: Only you can answer this. Because you are the only one that knows your use case. And the problem is not solved by buying new equipment. Because you need to understand your use case to know how good that equipment needs to be. Do you need a 3458 to get accurate, many digit measurements of voltage, resistance or current? Or do just need to know that the measurements are in the ball park? Do you need absolute accuracy or do you need linearity? Do you need to compare measurements made months or years apart with high precision?If you don't know what you will be doing, the safest answer is to buy the top of the line new, and pay for regular calibration and adjustment. For most of us budgetary limits don't allow us to be that safe, and we have to trade the use of valuable brain cell operation for those unavailable dollars. Remember that most of the progress in electronics (starting from Faraday and up through Armstrong and Nyquist) was performed with instruments that provided only two or three digits of absolute accuracy assisted by bridges and other tools that allowed excellent differential accuracy.

author: noisyee
author_rank: Contributor
timestamp: September 30, 2022, 06:16:38 am
content: Sadly no simple tricks for hobbyists to verify an instrument properly.Most instruments have service manuals to tell how to verify, but they are not always available to public. And calibration grade instruments or standards are very, very expensive, even a second-hand one. It's almost impossible for hobbyists to do that kind of verification.Keeping some home made "standards" is always helpful. Nowadays we have easy access to some of the most high precision factory-trimmed components that are equally precise or better than our instruments. They are simply something we could trust. I always keeps a surplus GPSDO as my timing standard. Voltage reference chips, high precision resistors and capacitors come handy too.Behaviors verification is some what more difficult than performance verification. All you have to rely on is lots and lots of experience with that kinds of instruments and a good luck.After all, warranty is what you got lose for paying much less to get a decent instrument.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 30, 2022, 06:49:41 am
content: Without the instruments for it one can obviously not do a full calibration or performance verification. However there are a few points that can be tested with moderate / low costs.For the DMM this are things like offsets and zero drift, the input bias current, the same voltage in 2 adjacent ranges, some DNL tests , the turn over error, some INL test in a resistance range (chain of resistors).Even if no good enough for calibration or with even high uncertainty it also helps to compare to existing meters. This would not detect errors from a more contineous drift over time, but it could detect many cases of defects that usually cause more larger errors if they happen.For the scope the situation with the calitration is even easier. The voltage accuracy is not very high anyway, time frequency references (e.g. crystal clock) are relatively easy and dividers down to lower frequency are accurate by design. For the frequency response the simple clean square wave for the probe compensation is a first, quite good test.

author: PixieDust
author_rank: Frequent Contributor
timestamp: September 30, 2022, 01:10:21 pm
content: Quote from: tautech on September 29, 2022, 07:26:50 pmLess than 300 AUD for both instruments.Quote from: tautech on September 29, 2022, 07:26:50 pmThat explains why these chapters come right before the calibration chapters!Quote from: Kleinstein on September 30, 2022, 06:49:41 amYep, that’s the lesson I’m learning.Quote from: Martin72 on September 29, 2022, 08:22:32 pmMaybe I should do some more digging, maybe I’ll find a lab that might just test the devices or something.Quote from: noisyee on September 30, 2022, 06:16:38 amIt cost a bit, but I managed to find a service manual for the Tektronix on eBay and I found a free scanned copy for the DMM.As was suggested in the link my original post, guenthurt gave a link to a cheap Kelvin Varley project. So that will run the DMM through its DC Voltage paces. Just need all the other stuff now.Quote from: CatalinaWOW on September 30, 2022, 01:49:52 amAs james_s suggested old A list equipment is either close to spec or totally broken. In my case I’m not doing rocket science so I just need to be close. My main concern is that the units are fully functional so that I know 10 volts is roughly 10 volts etc.Quote from: Kleinstein on September 30, 2022, 06:49:41 amNot familiar enough with multimeter internals at this stage. No idea how ADC DAC works in a multimeter, also quickly looked at INL & DNL, will have to do some digging at some point to get acquainted with these topics.Quote from: Kleinstein on September 30, 2022, 06:49:41 amYep, in my original question, others said the same i.e. oscilloscopes aren’t precision devices. I’ve already played around with the probe compensation sin waves. All the sin waves I’ve thrown at it, seem to have made sense so far.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 30, 2022, 02:51:48 pm
content: Roughly ten volts? Buy a handful of dry cell batteries. By putting them in series you can get a range of voltages with two digit accuracy. Or buy one of those cheap $10 DMMs and compare your instrument with the DMM. If they agree you can be pretty confident that your instrument is "pretty close".With your Kelvin Barley set up you can push a couple more digits. But beware, you are on the road to Volt Nuttery. Nothing wrong with that, as long as you are aware that you are pushing accuracy for its own sake.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 30, 2022, 05:04:12 pm
content: For checking the scope a sine wave can be a bit tricky. It is hard to see distortion of a broken compensation of the resistive / capacitive divider. To check linearity a triangle wave is the better choice, as the eye is better in judging a line than a sine.The other good test signal is a square wave, like the compensation adjust signal. This is why it is used to adjust the compensation on the probes.A reasonable check of the vertical accuracy can be done with DC, by comparison to a DMM. Some 1-3% needs no magic there.Horizontal accuracy can be checked with a resonably known frequency, e.g. if the signal trasitions can line up with the divs on the screen.Checking the linearity is a somewhat more tricky part and only a spot check. A simple test is measuring a few resistors on there own and in series. If everything is linear they should add up. So 10 K + 10 K should be 20 K - for a 5 digit meter about within some 3 or 5 of the least digit steps. The points to check here are especially sums relatively close to the full scale and then maybe 2 or 3 resistors to make up the sum. Chances are not very high the test would fail, but it is a point the check and it would add to the confidence.A relatively easy DNL test is possible if one has a computer interface for the DMM - otherwise it gets a bit tricky, more limited. With a relatively long RC time constant one can observe a rather slow (e.g. R*C = 1000s) RC discharge and than look at the difference of consecutive readings taken at a constant rate (e.g. contineous reading at the full 5.5. digit resolution). That curve should be reasonable smooth.The best range of the 3478 is the 3 V range. So no need for a 10 V reference, more like 3 V (e.g. 2 not very new alkaline cells) or a test.A fun test to do is the Hamon divider. This can make a reasonable accurate 1:10 divider and 9:1 resistor ratio (to check the resistor ranges relative to each other).

author: PixieDust
author_rank: Frequent Contributor
timestamp: October 30, 2022, 12:32:31 pm
content: This has been bugging me so I decided to devote some more time to this. I had a flick through the performance testing/calibration section of my oscilloscope and I got frightened (way too many of all kinds of calibration instruments), so moved on to my HP3478A Service Manual. Much more palatable to a newbie. Looks like all that I need to do to confirm if the unit is in good working order or not is to connect some:- DC Volts- DC Current- AC Volts- AC Current- ResistanceGeneral-https://www.eevblog.com/forum/metrology/mini-metrology-lab-build/msg260270/#msg260270Resistance- "Resistance Standard.mp4"- "Vishay Resistor Standard.mp4"- "Voltlog #183 - Building A Resistance Standard With Vishay VHP100 Resistors.mp4"- + Search in metrology section of EEVBlog forums for “resistor”.DC Volts-https://conradhoffman.com/mini_metro_lab.html(this was recommended by guenthert in another thread - many thanks!)-https://www.eevblog.com/forum/metrology/a-stable-dcv-source-up-to-300v/-https://www.eevblog.com/forum/metrology/mini-metrology-lab-build/DC Current- "EEVblog #567 - Precision 1A Current Source.mp4"-https://www.eevblog.com/forum/blog/eevblog-567-precision-1a-current-source/msg365516/#msg365516- "EEVblog #577 - Precision 1A Current Source Part 2.mp4"- "EEVblog #579 - Precision Low Current Source.mp4"- Also found this (Haven't watched):AC Voltagehttps://www.eevblog.com/forum/metrology/ac-voltage-standard/50/https://www.eevblog.com/forum/metrology/diy-precision-ac-rms-to-dc-transfer-standard/?allSo just need to find some info on AC Current and AC Voltage info and that will cover all the bases. Looks like the HP3478A check is doable. Maybe as I learn more electronics, I'll be able to figure out how to test the oscilloscope. I'm worried about the oscilloscope the least. It came with 3 cal stickers if I remember correctly. So it has been calibrated in the past although the last known date was 1995. Other stickers looked newer but didn't have a date. The seller if I remember correctly said that it was in working order. Having used the unit, things made sense, so I'm not too worried about it.I will update this if and when I progress further with all this.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 30, 2022, 02:41:29 pm
content: These projects can improve your confidence in your meters. But if you are really concerned about accuracy you are diving into a whole new set of questions. Are you sure that each and every one of the resistors you have purchased is good? How will you calibrate your current source?The whole investigation is interesting and can eventually lead to having better accuracy than many calibration labs. But to get there you will have little time for your other electronics interests.

author: PixieDust
author_rank: Frequent Contributor
timestamp: October 31, 2022, 07:14:04 am
content: Quote from: CatalinaWOW on October 30, 2022, 02:41:29 pmI haven't got it all figured out yet. I'm not sure is the answer at the moment.Quote from: CatalinaWOW on October 30, 2022, 02:41:29 pmThe only reason why I'm going into this is two fold:1.) Buy cheap and buy twice. I have learned first hand why this is the case in electronics. But now that I have gone down the cheap route, I might as well make the most of it i.e. now that I'm looking into precision, I might as well go as far as reasonably possible.2.) The best part of cheap old equipment is that you're learning on something that is inexpensive, so mistakes won't cost a lot of money. Admittedly in electronics mistakes are more related to not getting shocked yourself rather than damage to equipment, which I'm sure was designed with these things in mind, but still.But yes, taking time from my main project with all this. But as I have found out, you have essentially two choices, the cheaper route or the expensive route. Sure I can get new, state of the art equipment straight away and get my project moving, or I can take the frugal route. Takes longer, but the end result I suspect might be cheaper. Requires a lot more operator training however and going down into the weeds. I have a sneaking suspicion that the more manual, longer route is probably the better option. You learn a lot more and that in itself is worth the effort. Whether this route is cheaper or not, I don't know. All I can say is that the expenses that I have avoided thus far are quite significant (not just in electronics). But yes, not sure if it's the smart move at this stage.