Author Topic: Brymen BM789  (Read 68476 times)

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Online bdunham7

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Re: Brymen BM789
« Reply #175 on: September 20, 2021, 09:25:15 pm »
None of the asian design multimeters I can find are using a DC-blocking cap, except the BM869s.

My BK 391A definitely had a blocking cap and measured infinite DC impedance on ACV.

According to other tests here, the BM 78x series has a DC blocking cap as well for ACV above the mVAC range.

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline floobydust

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Re: Brymen BM789
« Reply #176 on: September 20, 2021, 10:43:27 pm »
BM789 - I don't see a DC blocking cap in any PC board photos.
 

Online bdunham7

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Re: Brymen BM789
« Reply #177 on: September 20, 2021, 11:04:29 pm »
BM789 - I don't see a DC blocking cap in any PC board photos.

You have to give the Dave credit for posting photos of the guts--not too many manufacturers do that anymore.

I also had a hard time with that, although I think it is likely to be the lower of the two components I have rectangled here:

« Last Edit: September 20, 2021, 11:06:08 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online joeqsmith

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Re: Brymen BM789
« Reply #178 on: September 21, 2021, 02:22:05 am »
Look close and you may notice the part designator for the larger component shows R.  I believe this is one of the shunts but to be clear, I have not spent any time trying to trace it out. 

Online bdunham7

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Re: Brymen BM789
« Reply #179 on: September 21, 2021, 03:44:09 am »
Look close and you may notice the part designator for the larger component shows R.  I believe this is one of the shunts but to be clear, I have not spent any time trying to trace it out.

I did see that designator originally, but without one in hand its hard to know if it means the white rectangular thing that looks a lot like a capacitor or not.  I don't know where else it could be except perhaps up masquerading as a PTC.  I doubt they'd let high voltage get up north of that switch anywhere.  I do see a C1 on the back that looks to be in the right spot, but it looks too small?  Perhaps SMT HV capacitors are more advanced than I know.

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline floobydust

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Re: Brymen BM789
« Reply #180 on: September 21, 2021, 04:37:08 am »
At the bottom it's shunt R49 plate resistor MPC 72 or 73 I can't see all the text.
At the top box, the film cap looks small and one end connects directly to the DMM IC analog switches, so I think it's a filter that gets switched in/out maybe for VFD's. It's near the low voltage business on the board.
You can discern a bit from the switch video too https://www.eevblog.com/forum/testgear/bm786-switch-issue/msg3515584/#msg3515584

I'll follow traces on C1, for DC-block it needs to connect to the rotary switch which is miles away it does at S1A1, S2A1 but not shorted out for the majority of functions like DCV, ohms etc. so I'm still skeptical.

The user manual mentions for 600mVAC "Signal peak absolute values, including DC bias, less than 1000mV peak" which I take as a 1V max. so no cap.
« Last Edit: September 21, 2021, 04:50:54 am by floobydust »
 

Online bdunham7

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Re: Brymen BM789
« Reply #181 on: September 21, 2021, 04:51:45 am »
[ Specified attachment is not available ]
I'll follow traces on C1, for DC-block it needs to connect to the rotary switch which is miles away it does as S1A1, S2A1 but not shorted out for the majority of functions like DCV, ohms etc.

Actually it looks to me that C1 is within the borders of the switch cover, above the + jack and to the left of the PTCs.  I tried looking at Dave's videos regarding the switch problem where he has it torn apart, but they were mostly too closeup.  I'm betting C1 is it.

Edit:  C1 is right under where the switch contacts for the ACV range would be.  R49 is in fact a 1R resistor. I'm guessing the reason for the combined mV system is that they were just out of switch positions due to the design not being 360-degree capable

« Last Edit: September 21, 2021, 05:20:05 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online joeqsmith

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Re: Brymen BM789
« Reply #182 on: September 21, 2021, 09:19:45 pm »
I believe that's it but again, have not looked.   I gave a little demonstration on using a 100 year old blow torch to do some surface mount soldering and had pulled this capacitor from the board.  It ended up stuck on the one pad.  It would be easy enough to measure the value and trace it out. 

10000pF at a kV+ seems doable in this package. 
https://product.tdk.com/en/system/files?file=dam/doc/product/capacitor/ceramic/mlcc/catalog/mlcc_commercial_highvoltage_en.pdf

Offline NeutrionTopic starter

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Re: Brymen BM789
« Reply #183 on: September 23, 2021, 03:36:24 pm »
OK so we now know that there is at least a positive aspect of the DC coupling, so if Brymen could manage to adress the "overrange problem", it could be safer on the mV scale that some other meters.

I was trying to characterize the meter to know above which level I can not trust it anymore on the AC mV scale. In the worst case scenario when the width of the squarewave is just above the declared minimum of 350us this safety limit can be as low as 400-430mV. Above which the meter starts to show BS values. So it is not necessarily has to reach the 650mv displayed value to have something way higher real value. Even with 50% duty cycle It starts to show invalid values above 500mV.

Unfortunately we still don't know what happens, if we just raise the voltage to lets say 50V. Maybe it will overrange. What is this lowest safety marging with the 869?
The other question is, what could be the lowest safety margin with some kind of completely  different waveform? Could it go even further down? Would be really nice to know.

Anyone has a newer firmware than the 08?

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.    :-DD    I could run others but will wait and see what bdunham7 comes up with.
What was the exact waveform here?


And regarding the AC coupled mode:

Similar to the ipact wrench, or cordless drill tests where they are getting attached to eachother to see which is the stronger, we might see multimeter fights as well in the future! :)


It is really interesting that this issue surprises even experienced engineers. Might there are tonns of equipment which are getting damaged all the time because of this regulary, just people are not avare of the cause. Wonder how long they keep the charge.

 

Offline 2N3055

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Re: Brymen BM789
« Reply #184 on: September 23, 2021, 06:35:31 pm »

It is really interesting that this issue surprises even experienced engineers. Might there are tonns of equipment which are getting damaged all the time because of this regulary, just people are not avare of the cause. Wonder how long they keep the charge.

I don't find it interesting if experienced engineers would be surprised by this.
Thing that would come to my mind would be more like: they should know better.....

Don't get me wrong, I'm not talking about making mistakes. But if experienced engineer doesn't understand this, maybe he/she is not as experienced as they think. At least not in measurement and measurement instruments basics..
 

Online bdunham7

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Re: Brymen BM789
« Reply #185 on: September 23, 2021, 08:02:32 pm »
I don't find it interesting if experienced engineers would be surprised by this.
Thing that would come to my mind would be more like: they should know better....

If they hadn't run into it or thought of it, well that's me and a lot of other people.  If they don't understand it once tested and explained, that's different.  I would have thought that if it was an issue, the DMM manufacturers would take steps to alleviate it--which is completely straightforward and doable-- but apparently they haven't in all cases.  You do need a fairly unusual series of steps to make all that happen, so I doubt it is a regular occurrence.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fungus

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Re: Brymen BM789
« Reply #186 on: September 23, 2021, 08:14:36 pm »
Hands on hearts:

How many people knew about this and took precautions before reading this thread?

 

Offline floobydust

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Re: Brymen BM789
« Reply #187 on: September 23, 2021, 08:24:33 pm »
After the oscilloscope AC/DC coupling switch went kabang and shot out sparks, I had to think WTF just happened and spend time figuring it out because I couldn't afford to damage precious gear.

With ACmV readings, it's the situation where your measurement is out to lunch and you have to dig to find out why.
Is the test equipment damaged, or buggy software, used incorrectly, or a misunderstanding about what it can do? A lazy engineer will just sluff it off.
 

Online joeqsmith

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Re: Brymen BM789
« Reply #188 on: September 23, 2021, 09:21:35 pm »
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.    :-DD    I could run others but will wait and see what bdunham7 comes up with.
What was the exact waveform here?

I have no way to know exact.   From attached, it was roughly a 0.649 VRMS sine with a 2.84VDC bias.

And regarding the AC coupled mode:

Similar to the ipact wrench, or cordless drill tests where they are getting attached to eachother to see which is the stronger, we might see multimeter fights as well in the future! :)

I doubt it as the meters themselves should easily handle it. 

Offline NeutrionTopic starter

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Re: Brymen BM789
« Reply #189 on: September 24, 2021, 11:22:19 am »
Hands on hearts:

How many people knew about this and took precautions before reading this thread?


I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.

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.    :-DD    I could run others but will wait and see what bdunham7 comes up with.
What was the exact waveform here?

I have no way to know exact.   From attached, it was roughly a 0.649 VRMS sine with a 2.84VDC bias.


Just because the problem is here generally the opposite,(showing something which is much less than it is) so I was wondering whether the sinewave might got too high frequency.
Because if not, than it means that the mentioned lowest safety limit could be as low as 0 Volt... Than there is no way of knowing whether because of DC bias an overrange situation occured.

With my little offset squarewave, below 400mV AC displayed value one can be on the safe side, but it seems that it is not the case with other waveforms. Like some small AC riding on some DC offset.



 

Offline 2N3055

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Re: Brymen BM789
« Reply #190 on: September 24, 2021, 12:16:12 pm »

I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.


Well, I don't think measuring milivolts and measuring VFD connected equipment have anything in common.
 

Offline NeutrionTopic starter

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Re: Brymen BM789
« Reply #191 on: September 24, 2021, 12:26:30 pm »
Charging up the meter tipically occurs at V AC in Brymen and both mV and VAC in Fluke it seems. So I was not talking about the missing overrange indication on the Brymen. 
 

Online joeqsmith

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Re: Brymen BM789
« Reply #192 on: September 24, 2021, 12:39:34 pm »
... so I was wondering whether the sinewave might got too high frequency.

All of those were done using a 60Hz sinewave.   


I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.

Well, I don't think measuring milivolts and measuring VFD connected equipment have anything in common.


I've never really given it any thought.  When I use an external block, out of habit I will discharge it.  When working above 24V I tend to take more precautions.

Much of the time when I need a meter, I am not using the AC modes.  Normally I'm not working on a mix of technologies at the same time where the internal block would even come into play.   If I am troubleshooting power electronics, I normally start with it unpowered.  When I get to where I am starting to bring it up, I am using a scope.   For logic, I'm using PC/JTAG, scope, LA.   

So, I doubt I will go out of my way to change any habits but it's certainly something to be aware of.   

Now, could Dave's 121GW diode test take out a sensitive device?   

Offline 2N3055

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Re: Brymen BM789
« Reply #193 on: September 24, 2021, 12:44:37 pm »
... so I was wondering whether the sinewave might got too high frequency.

All of those were done using a 60Hz sinewave.   


I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.

Well, I don't think measuring milivolts and measuring VFD connected equipment have anything in common.


I've never really given it any thought.  When I use an external block, out of habit I will discharge it.  When working above 24V I tend to take more precautions.

Much of the time when I need a meter, I am not using the AC modes.  Normally I'm not working on a mix of technologies at the same time where the internal block would even come into play.   If I am troubleshooting power electronics, I normally start with it unpowered.  When I get to where I am starting to bring it up, I am using a scope.   For logic, I'm using PC/JTAG, scope, LA.   

So, I doubt I will go out of my way to change any habits but it's certainly something to be aware of.   

Now, could Dave's 121GW diode test take out a sensitive device?

Agree..
Usual process is systematic, and there is a natural progression how one goes about troubleshooting things.

As for 121GW I don't know, but MTX3293 has 26V/10mA range in diode measurements.
I'm sure there are many sensitive devices that could be damaged by it.
 

Offline NeutrionTopic starter

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Re: Brymen BM789
« Reply #194 on: September 24, 2021, 01:17:06 pm »
But than also designing your own thing, and knowing all the guts of it (I suppose you Joe mostly design stuff) makes you to to take different measures than starting to measure around in something which you have not much idea about, maybe not even a schematics is aviable. And there could be also some hazardous failure modes, which in theory should not happen but they do sometimes.

Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.
 

Offline 2N3055

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Re: Brymen BM789
« Reply #195 on: September 24, 2021, 01:46:12 pm »
But than also designing your own thing, and knowing all the guts of it (I suppose you Joe mostly design stuff) makes you to to take different measures than starting to measure around in something which you have not much idea about, maybe not even a schematics is aviable. And there could be also some hazardous failure modes, which in theory should not happen but they do sometimes.

Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.

It is irrelevant if you are designing thing on your own, or you are repairing someone else's design.
If you are repairing something unknown, first order of business is to make it known, at least to the point to be able to understand where are the dangerous parts.  Any part of circuit that can become dangerous in any scenario (even the fault far outside normal operating parameters) should be permanently treated as dangerous.  Not to protect your meter but your life..

BM869 question I answered 3-4 times already. Yes and no.
 

Offline NeutrionTopic starter

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Re: Brymen BM789
« Reply #196 on: September 24, 2021, 02:11:44 pm »
I run through the topic, but only found this. But here you write about incorrect nubers, but not necessarily 0.


But does the 869 correctly shows "OL" for AC in this scenario?  Did you try to switch polarity?

With the 869  it makes sense to have everything on the same switch position, but with the single display if there is no way to show overrange they certainly could put in a separate AC mV position.


I think we have a language barrier here.

BM869S has 2 different positions for DC and AC mV, but both are nevertheless still DC coupled .

It does show OVL if you apply larger AC voltage. If you combine it with DC, that combined goes over range it also behaves funny.

For instance 1V P-P 400HZ square wave, will show correct 500mV RMS. If I add more than 0.6 V DC to it (offset) it will start showing wrong numbers. Funny enough, I can add -2,3V offset before it starts making problems. So it is not symmetric.




 

Online joeqsmith

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Re: Brymen BM789
« Reply #197 on: September 24, 2021, 02:16:02 pm »
But than also designing your own thing, and knowing all the guts of it (I suppose you Joe mostly design stuff) makes you to to take different measures than starting to measure around in something which you have not much idea about, maybe not even a schematics is aviable. And there could be also some hazardous failure modes, which in theory should not happen but they do sometimes.
Typically when I need to repair something that is new enough that I can't get schematics and it's actually something that is worth the time to attempt to repair, I start by writing down every part number of the ICs and pull the data sheets.  Then I start to trace out the design.   Most of the time, I have some idea where to look based on the symptoms and so I am only looking at small sections of the design. 

Just because schematics are not available for free off the internet doesn't mean they are not available.  I have written companies before and have been able to obtain schematics and service manuals. 

Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.
Does it really matter?  I saw it as more a gimmick than anything useful.   

Offline NeutrionTopic starter

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Re: Brymen BM789
« Reply #198 on: September 24, 2021, 02:45:26 pm »
This is a good throughout approach, but often one does not have the time for it, and have to start to measure around to get an idea. Of course it's not ideal.


Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.
Does it really matter?  I saw it as more a gimmick than anything useful.   


No, but I am interested. And the 869 is the competitor to the 789 within Brymen. (And actually not much of a competition in this price/quality range anywhere else...)

But generally it matters with the 789 because first I thought, that below a given displayed mV AC level I could feel myself "safe" but it seems that it is not the case.

I don't know how fast it could work in FW but if the meter would only switch internally to DC measurement, it should only check if its above 1 V DC in any any direction, and than it could display OL.

We will see.


 

Online joeqsmith

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Re: Brymen BM789
« Reply #199 on: September 24, 2021, 03:56:17 pm »
...
But generally it matters with the 789 because first I thought, that below a given displayed mV AC level I could feel myself "safe" but it seems that it is not the case.
...

Feel Safe?   It's rare I work on anything where I'm concerned with safety.  In those cases, I never feel safe!  :-DD  I'm also not playing with mV.  :-DD


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