Brymen BM789

[bdunham7]

As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me?

Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models.  The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc.  I believe it is a requirement.  As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult.  Input protection circuitry is not that hard to make and in any case, is a well known quantity.

[Caliaxy]

...
In every single other mode (DC mV included), the meter displays “.OL” when overloaded, hence the reasonable expectation to do so in AC mV as well.
...

Unfortunately the "reasonable expectation" argument doesn't work here, for a variety of reasons, but I suggest you read the excellent post by 2N3055, here:

https://www.eevblog.com/forum/testgear/brymen-bm789/msg3686746/#msg3686746

Again, you and the OP are assuming that this was an oversight by the Brymen engineering team, or a bug in the firmware of the BM789. I don't think that's the case at all.

And as has been stated many times: if one so much as spends 45 seconds to read the User Manual and use the DMM accordingly, this issue will never come up.

I did read that post (and all the other posts in this thread). I find it excellent and educative (like most 2N3055's posts) but a little bit exaggerated (the part with 10000V on mV scale...). I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.

Maybe "design fail" is too harsh. What would this be then? A price-motivated decision? It would be great if a good meter from a reputable brand would not display false information when overloaded. Grateful to learn that it might. It might not be easy to do it right. If this is understandable in lower cost meters, in Brymen's case, even their flagship model (BM869S) behaves the same way. But their flagship meter is also positioned at a very competitive price point... In A brands, the cheaper meters seem to behave similarly but at least the higher end ones behave correctly. Maybe this is the difference between A brands and B brands, and these are the kind of trade offs one should be aware of.

Again, no drama here, we can all live with this. I don't expect things to be perfect. The fact that you need to press a button 6 times to switch from AC mV to DC mV bothers me more than the DC offset issue raised here, but I can live with that too if I understand that it saves on the cost (and, of course, I'm free to choose to pay the difference if I want better).

Out of curiosity: what's your meter? Does it behave the same way? If yes, were you aware of it before this thread?

[Neutrion]

Anything they couldn't cram in there? They documented it. What else could they do?

It might be possible to make it say 0L, we'll see. I'd bet heavily against them being able to make it measure correctly though.

I don't want it to read correctly above the specified limit, only to show "OL" if overloaded.

But it is kind of amazing to see people who never ever measure anyting else than what they expect, this would be my goal as well to get to this level.
But for those who are not perfect yet, an example again:
User starts measuring correctly on the V AC scale,  to see what is the level of the signal like.
Meter shows 200 mV AC, so user proceeds to the mV scale because he needs  better accuracy.
(Up until this point I think we all agree that he is doing right...)
Than he sees the signal raising to 600mV. And there he can not be sure whether it is 600mV or maybe 325V because something went wrong.
I will do some tests to see what is the highest reading which one can trust. But it depends both on frequeny and duty cycle. Possibly also on waveform, but I can not easily test anything else than a squarewave.

If we could find out whether there is any limit up to which the meter could be trusted, than it means that it could be set as overrange limit in the FW as well. But than we would lose counts.
But I hope to get a more elegant solution.

[bdunham7]

But I hope to get a more elegant solution.

People have mentioned an external blocking capacitor, but to me that just seems like an opportunity to short something out or get zapped.  After all, the reason I have CAT-rated meters, shielded test leaks, insulated alligator clips and so on is to avoid the accidental zapping of either hapless components or myself.  So here's an idea!

Some of Fluke's older but still produced meters lack the Lo-Z function.  This feature, also known as Ghost Voltage or Stray Voltage eliminator, works by lowering the input impedance of the meter with a parallel PTC so as to short out very high impedance voltage leakage that you don't want to measure.  So, Fluke has an add-on unit, with nicely shielded jacks and plugs, that can be inserted between the test leads and the meter to add this functionality.  Note the price! 

https://www.fluke.com/en-us/product/accessories/adapters/fluke-sv225

Brymen (or someone) can introduce the same thing only put a 1kV blocking capacitor in the unit instead of a PTC.  They can call it the 'ripple master' or TrueAC or DC Block...or whatever.

[2N3055]

As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me?

Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models.  The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc.  I believe it is a requirement.  As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult.  Input protection circuitry is not that hard to make and in any case, is a well known quantity.

Ok, so your point is that it should survive.
Ok, valid point.
And it will, because it was tested it will. Survival is not in question here.

But, if the point is that it should survive,  and also show voltage is too high for the range and that there might be dangerous voltages and that you should change range?
On every range

So here is a brain teaser :  in your own words it will survive in any range including Ohms, right.  And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....

That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.

You want failsafe?
There are are also Duspol type voltage tester devices. I have BM75EU from that type. Guess what I use when poking around electric installations, power sockets, fuses and anything 240V AC ? It also has vibration warning, RCD tester, and works even when batteries die. LCD with precise measurements and resistance/continuity stops working, but voltage detection still works. That is regulatory requirement for that type of devices.. User safety first. Best accuracy 1.5% DC, 2.5% AC.

BM789 is a electronics meter, very, very nice one. High resolution and accuracy, lots of functions. It even has "fluke like" diode test, continuity with screen blinking, it will remember settings..
It is probably best meter in the class. Even if you don't look at the price.
But it is not failsafe device for field use, it is not explosion proof, it is not IP87...
It requires operator to operate it with intelligence, otherwise garbage in , garbage out.

Don't get me wrong. I do understand that completely failsafe device probably could be made.
You would end up with a device that would be a fancy version of that Duspol type device.
No current measurement. They are inherently unsafe. Slow auto ranging/switching (because now it has to not only range but also decide which one is it supposed to do)...No high bandwidth, for better RF resilience. etc etc..

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.
In a  way the instrument designer expects me to do it. That guarantees good results.

Whoever doesn't like that, I have a snarky Open Source zealots like comment : " If you don't like, it make your own!".

[joeqsmith]

3055, funny post...   I want that James Bond meter.    Keep in mind that the OP just expects it to display over range, not draw the bath water for them.     
 

As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me?

Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models. The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc.  I believe it is a requirement. As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult.  Input protection circuitry is not that hard to make and in any case, is a well known quantity.

Ok, so no luck on your end.  All your meters behave as expected while sweeping the frequency, amplitude and offset.   Not at all what I would expect but I can try some of my meters and see how they behave.     

Just a friendly reminder that you may remember one of the members here damaging a 121GW PCB.  Dave suggesting that the lower voltages could not damage the meter the way that was shown.    So I setup an experiment using a DC voltage with enough current that the with the low voltage clamps engaged, it would not load down the supply.  100s of mA lethal stuff.     Basically the idea was if the switch was rotated with the input live, we could draw an arc and have enough current to burn the board. 





Be careful. 

[floobydust]

16.1 REASONABLY FORESEEABLE MISUSE
"No HAZARDS shall arise if adjustments, knobs, or other software-based or hardware-based controls are set in a way not intended, and not described in the instructions. Other possible cases of REASONABLY FORESEEABLE MISUSE that are not addressed by specific requirements in this standard shall be addressed by RISK assessment (see Clause 17). "

But the certifiers can screw up too and just re-test the function that was being tested - not all the others, so side effect failures can sneak through.
If a DMM doesn't display a hazard when it's expected, i.e. on ACV and not functioning- that's unsafe. After a transient test for example, it's expected to still display HV if present and you are intending to measure that. The owner's manual can give coverage for many misuse scenarios.


The Brymen IC is familiar to us, study the PCB a bit. For these chips, true-RMS is calculated by a tiny DSP section, which removes the DC component from the result.
So A/D reading that module's output will never know if there is a DC overload unless you switch the mux and look pre-RMS for that A/D clipping.
I can't see any rectifier though, the op-amp if used can also saturate and you have no way of knowing like in the 121GW. The PCB shows provisions for an entire IC/subsection maybe for True-RMS (four tantalums).
A firmware challenge is correctly looking at the quantity to auto-range (can't on a fixed 100mV range...) so it can display OL, can get missed.

[bdunham7]

So here is a brain teaser :  in your own words it will survive in any range including Ohms, right.  And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....

Actually in that case they say 'OVERLOAD' or OL.  Which is exactly what they are seeing--voltage above their range limit.  I don't expect them to do more than tell me that they can't get a reading, what I don't want to see is a normal-looking but incorrect reading.

That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.

I'm not after that.  I'm only asking that the meter do one of two things--give me a reasonably correct answer or say "I don't know".  The only fail is when it gives me a plausible but completely wrong answer.  I'm sure that you could probably make any meter fail this at some point, so it just comes down to what one thinks is reasonable and what other similar meters are capable of.

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

That's not true, the F87V has a 600 mVAC range, it is just doesn't have a separate switch position for it. 

When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.

OK, it's the Fluke 189.  Which is why they go on eBay for three times the price of a new BM789!   

[2N3055]

Anything they couldn't cram in there? They documented it. What else could they do?

It might be possible to make it say 0L, we'll see. I'd bet heavily against them being able to make it measure correctly though.

I don't want it to read correctly above the specified limit, only to show "OL" if overloaded.

But it is kind of amazing to see people who never ever measure anyting else than what they expect, this would be my goal as well to get to this level.
But for those who are not perfect yet, an example again:
User starts measuring correctly on the V AC scale,  to see what is the level of the signal like.
Meter shows 200 mV AC, so user proceeds to the mV scale because he needs  better accuracy.
(Up until this point I think we all agree that he is doing right...)
Than he sees the signal raising to 600mV. And there he can not be sure whether it is 600mV or maybe 325V because something went wrong.
I will do some tests to see what is the highest reading which one can trust. But it depends both on frequeny and duty cycle. Possibly also on waveform, but I can not easily test anything else than a squarewave.

If we could find out whether there is any limit up to which the meter could be trusted, than it means that it could be set as overrange limit in the FW as well. But than we would lose counts.
But I hope to get a more elegant solution.

Do you work on thermionic valve amplifiers?
That is, for instance, place where you can have 100mV AC combined with 300V. On audio amplifiers you can have 10mV riding on 80V...
But most of those high DC voltages are static working point related, i.e. don't change much or at all.
Most of them might move a bit with thermal bias circuits doing their stuff.

I cannot find scenario where it suddenly goes from 100mVAC+0 DC to 100mVAC+50V DC.
If that happens during testing, that means you switched to mV too soon, before you really figured out what voltages you have here.. Back to the square one..

Honestly, I understand you would like it to be smarter than it is. I have nothing against it. It's just I really rarely see that kind of scenario. Because of measurement practice that was beaten into me many, many years ago with moving coil, manual range meters. One wrong move, bum, you don't have a meter anymore.. I know todays meters survive pretty much anything, and that you can switch between functions without disconnecting them etc.. But I still don't do it. And I am rewarded with a fact that i don't see those fringe cases where meters behave weirdly.
I'm not saying you're wrong, but pointing out that you CANNOT trust meter blindly EVER. Measure 3 times, cut once.

While we are talking real world scenarios, when, for instance, repairing something, my first troubleshooting start with meter in volts (autorange) and checking power busses. If they look kinda OK,  I fire up scope. And check those power busses again with a scope to see if they are clean.
Meters are utterly useless to see actual SIGNALS. They come in handy when I first see, with a scope, there is a clean DC voltage on a point and IF I want to see precisely what it is.
To add insult to the injury, most of the time scope measurements will do good enough job, and it can measure dozens of parameters simultaneously.
Meter is more used for continuity, resistance, and when more accuracy is needed. Which also is a moot point when you have 16bit 0.25% accurate scope..
My method is to poke around circuit to understand it. That way there aren't much surprises.

Every repair job is actually reverse engineering. That or you simply stab in a dark and change random components, or just change them in a bulk.

When you're designing your own circuits, there should not be such unexpected surprises. Otherwise you're doing something wrong.
If there is a scenario where I suspect that (because of the circuit) there can be malfunction that can inject some weird stuff, i connect two meters, one measuring main parameter, other one monitoring for error conditions. Or watch it with a scope. Truth is you should always go back and forth and repeat measurement, varying methods, until you're sure you're measuring the right thing.

Best,

[bdunham7]

Ok, so no luck on your end.  All your meters behave as expected while sweeping the frequency, amplitude and offset.   Not at all what I would expect but I can try some of my meters and see how they behave. 

No, I'm just referring to their ability to withstand without exploding.  Getting any of them to read 'wrong' is easy with a combination of high crest factor and offset.  I even had my 8846A reading what should be a DC signal as the wrong polarity, until I turned on the analog filter and it started behaving.  Finding a signal that baffles something like the F189 but not the the Harbor Freight one is the trick.  I'm down to my last HF meter anyway....

Edit:  We would have to agree on what the second meter should be so that any results can be replicated.  So far, the 8846A has easily been beaten "Harbor Freight freebie outperforms Fluke's best bench meter!" because it starts by default with the analog filter off.  A 0-1V 5% 1kHz test signal, which should read ~50mV, is 15.4mV on the 8846A, 46.4mV on the HF.  The 46.4mV is more or less correct because if you turn the analog filter on or use a 189/289, that's what you get.  I was going to suggest the 189 as the target because you have one and they're well regarded, but I'm finding it very difficult to fool. 

[2N3055]

So here is a brain teaser :  in your own words it will survive in any range including Ohms, right.  And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....

Actually in that case they say 'OVERLOAD' or OL.  Which is exactly what they are seeing--voltage above their range limit.  I don't expect them to do more than tell me that they can't get a reading, what I don't want to see is a normal-looking but incorrect reading.

That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.

I'm not after that.  I'm only asking that the meter do one of two things--give me a reasonably correct answer or say "I don't know".  The only fail is when it gives me a plausible but completely wrong answer.  I'm sure that you could probably make any meter fail this at some point, so it just comes down to what one thinks is reasonable and what other similar meters are capable of.

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

That's not true, the F87V has a 600 mVAC range, it is just doesn't have a separate switch position for it. 

When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.

OK, it's the Fluke 189.  Which is why they go on eBay for three times the price of a new BM789!   

It will show whatever it shows when resistance is too high. Like with infinite resistance. Or in a case of BM869, it will start blinking trying to autorange.
Not at any moment it will give indication that you connected VOLTAGE to the inputs. That is my point.

Without that kind of Auto select mode it cannot know that what you connected to it is wrong. Except in current mode where you have fuse as an indicator.

As for F87V I apologize, I meant mV DC. My bad. And you know, despite that what I meant, but never mind. I said at the beginning, this happens only if you mix AC and DC.
If you feed it only AC (AC coupled like F87V) there is no problem and will nicely show OVL.

I don't trust used old stuff that much. Not even Fluke, especially when I saw with my own eyes what people do to them.

In my book both BM869 and MTX3293 come pretty close to fantastic... Each in it's own right.

By the way MTX 3293 cannot have that problem because it is single input, single function, and is AC coupled from uV to kV. In AC+DC is DC coupled but uses both to autorange.

[2N3055]

I did read that post (and all the other posts in this thread). I find it excellent and educative (like most 2N3055's posts) but a little bit exaggerated (the part with 10000V on mV scale...). I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.

Thank you for the nice words and sorry for the drama.. I tend overexaggerate with comparisons to prove the point works at any scale, my bad.

[bdunham7]

It will show whatever it shows when resistance is too high. Like with infinite resistance. Or in a case of BM869, it will start blinking trying to autorange.
Not at any moment it will give indication that you connected VOLTAGE to the inputs. That is my point.

OK, true--but that would be an expectation and feature that go beyond what is required and what both the OP and I would expect, which is that the meter simply not display a plausible but grossly wrong number.  It's not just a safety issue, although we like to talk about those dramatically.  Suppose I'm measuring a single-ended square wave like the OP at 300mVAC and it jumps up to 900mVAC?  I expect to see 'OL', not 640mVAC.  I don't think that's unreasonable.

As for not trusting old stuff, I'm in the opposite camp, although I may agree if it is old abused junk.  To me 'new' means 'experimental'.  I start to trust things after a decade or so of flawless service.  And I've been proven right more than once...

[Caliaxy]

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

You got it wrong. Like BM789, Fluke 87V does have an AC 600mV range (and a DC 600mV range), but they are not on the same rotary dial position. The AC 600mV is on the AC V position, which starts auto-ranging from 600mV up, whereas the DC 600mV range has its own rotary dial position, all for itself. The DC V mode starts auto-ranging from 6V up. All AC V ranges (including 600mV) are AC coupled, so the DC offset is irrelevant, unlike in BM789.

What 87V doesn't have is a combined AC+DC mV mode, hardly useful for many but plagued by the DC offset issue (which I'm starting to believe is a ubiquitous problem across most brands).

[AndrewBCN]

...
I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.

Maybe "design fail" is too harsh. What would this be then?
...

Seems to me it's quite simply this:
1. Don't read the BM789 User Manual (or pretend that you didn't read it).
2. Select the ACmV 600mV range even though you know full well you are going to exceed the maximum measurable voltage for that range
3. Carefully select a large DC offset that will saturate the DC coupled ACmV input circuitry of the DMM, thus preventing the detection of an overload condition.
4. Bitch about it on the EEVblog. 

[joeqsmith]

Ok, so no luck on your end.  All your meters behave as expected while sweeping the frequency, amplitude and offset.   Not at all what I would expect but I can try some of my meters and see how they behave. 

No, I'm just referring to their ability to withstand without exploding.  Getting any of them to read 'wrong' is easy with a combination of high crest factor and offset.  I even had my 8846A reading what should be a DC signal as the wrong polarity, until I turned on the analog filter and it started behaving.  Finding a signal that baffles something like the F189 but not the the Harbor Freight one is the trick.  I'm down to my last HF meter anyway....

Edit:  We would have to agree on what the second meter should be so that any results can be replicated.  So far, the 8846A has easily been beaten "Harbor Freight freebie outperforms Fluke's best bench meter!" because it starts by default with the analog filter off.  A 0-1V 5% 1kHz test signal, which should read ~50mV, is 15.4mV on the 8846A, 46.4mV on the HF.  The 46.4mV is more or less correct because if you turn the analog filter on or use a 189/289, that's what you get.  I was going to suggest the 189 as the target because you have one and they're well regarded, but I'm finding it very difficult to fool. 

I tried a few.  The Gossen I have is 300,000 count and there's no mV range.   So I can't count that one. 

121GW,  UNI-T UT61E+, Fluke 107, CEM DT9939 all have problems which I would expect.  I'm just using a sine.   Again, for me, no big deal. 

For a list of the meters I still have, you can use the spreadsheet and see beyond the free HF and 189, if we have any other common ground.   That or post a list of what you have. 

But I hope to get a more elegant solution.

People have mentioned an external blocking capacitor, but to me that just seems like an opportunity to short something out or get zapped.  After all, the reason I have CAT-rated meters, shielded test leaks, insulated alligator clips and so on is to avoid the accidental zapping of either hapless components or myself.  So here's an idea!
....

If working in CAT III, I am dealing with AC mains.  The meter I use has no mV and clamp inputs.  It can't read resistance, capacitance, continuity...  I don't go stringing caps around and there would be no reason for it.  If the mains start having a DC bias, all hells breaking loose.    Still, it would be trivial to mount a cap in an insulated housing with proper connectors.  I've never felt the need to do it.   

***
Added pictures.   

[bdunham7]

That or post a list of what you have. 

Other than stuff that can be dismissed as too old (which is most of my stuff including me...) I have a Fluke 116, 189, 289, 323 and 8846A.  I also have an HP 34401A, 3478A and several HF-level meters including the free model.  That is all unless I've forgotten something.

If the mains start having a DC bias, all hells breaking loose.    Still, it would be trivial to mount a cap in an insulated housing with proper connectors.  I've never felt the need to do it.

True, but I would think of something mains-derived, like a power supply, where a rectifier failure could change the picture quite quickly.  I don't need to do that either because I have meters with their own blocking caps.

[joeqsmith]

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested.  You bet.  Do I care, other than for a small bit of entertainment, not at all.        I could run others but will wait and see what bdunham7 comes up with.   

[AndrewBCN]

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

You got it wrong. Like BM789, Fluke 87V does have an AC 600mV range (and a DC 600mV range), but they are not on the same rotary dial position. The AC 600mV is on the AC V position, which starts auto-ranging from 600mV up, whereas the DC 600mV range has its own rotary dial position, all for itself. The DC V mode starts auto-ranging from 6V up. All AC V ranges (including 600mV) are AC coupled, so the DC offset is irrelevant, unlike in BM789.
...

No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That means the Brymen has a high resolution, separate dial position for manual range 600mV ACmv which you can choose to use for high resolution ACV measurements - exactly as 2N3055 wrote. And yes, they should add in the BM789 User Manual a warning that this range requires thinking... 

[AndrewBCN]

I can see the headlines tomorrow:
Electronics Engineer Exposes Major Design Flaw In Hundreds of Multimeters
Proves That Most DMMs Cannot Be Used To Measure Ripple And Noise In PSUs

[bdunham7]

I could run others but will wait and see what bdunham7 comes up with.

Don't look at me!  All I have so far is a Fluke advertisement.  I was unable to get any of my meters other than the HF freebie to make an error with this signal (I used 1kHz)  They all read correctly up to a point then displayed an overrange indication.  Even the CenTech P37772 more or less got it right, however that doesn't have a mVAC range, but it does have 2 VAC.  The HF freebie made a similar error at a  higher AC voltage, which was needed since its lowest range is 200VAC.  You know, since it is intended primarily a mains and service panel meter.

So the goal is to fool a 'good' meter with a signal that the HF meter can read correctly, whether by proper operation or luck.  We really need to use the DC ranges, because of the lowest AC range being 200V.  I've tried low duty cycle DC pulses, which defeated the 8846A w/o analog filter on, but I have been completely unable to fool the 189, other than going beyond bandwidth (I had a 50Hz 0.01% duty cycle pulse, for example).  So if anyone can come up with a signal that the 189 gives a 'wrong' result on in the DC range....

[floobydust]

I can see the headlines tomorrow:
Electronics Engineer Exposes Major Design Flaw In Hundreds of Multimeters
Proves That Most DMMs Cannot Be Used To Measure Ripple And Noise In PSUs

Na. We have the technology! Like a built-in IQ test

[bdunham7]

No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That's preposterous.  The bottom 10% of a 60,000 count range is not equivalent to a full 6000 count range, especially when you are talking about TRMS.  And the 87V has a 'hi-res' mode, but I don't have one so I can't tell you the details of that.  But it's OK to be silly if you like--this is EEVBlog after all.

[bdunham7]

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested. 

So it appears to completely clamped yet using only the calculated AC component to determine whether to display OL, if I'm understanding things correctly.  Or did you have to dial that in more carefully?  Could you try and replicate the OPs issue?  I'd say a 0-1V 50% 200Hz signal and then vary the amplitude up and down and see if you can get it to 'hang' at one value, and if it changes when you swap the leads.

[Caliaxy]

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested.  You bet.  Do I care, other than for a small bit of entertainment, not at all.        I could run others but will wait and see what bdunham7 comes up with.

Well, I'm not bdunham7 but, as mentioned in some of my posts above, most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S. Fluke 189, 89IV and Agillent U1282A seem to work fine. I don't care much either, but it's good to know. I almost always use an old HP analog meter for AC mV measurements. Overloaded it a few times with voltages higher than the selected range, but its needle is still intact