Brymen BM079 – Strange voltage issue

[monkey_magic]

I’m hoping somebody might be able to help. I’ve had my Brymen BM079 Clamp meter a few years, it’s not see a huge amount of use – just occasionally, and has always been looked after carefully.

Several months ago I started to have some issues with testing voltages, sometimes it showed 0V when I know there should be voltage present, and on retesting it would be fine.
Initially I put it down to user error, or poor contact on the leads etc.  However, recently I went to check a 12V circuit and was getting 0V no matter how many times I checked.
I swapped to another meter which confirmed the problem is with the BM079.

I have now started to check the meter in more detail and found some very odd behaviour which is not always consistant.
I’m somewhat limited to what I can check as I no longer have access to a bench PSU so have had to make do with some odd bits I have a home.
I’ve tested with multiple sets of leads, changed the batteries etc, ruled out all the basics.

The ranges for Diode check, capacitance, resistance, temperature and EF all seem to work fine and values agree with my other meter.
The amp clamp appears to work although I only did a quick test and haven’t compared it to a known good meter.
The frequency function seems to work, although there has been a couple of times where it took an unusually long time to get a reading.

The main issue seems to be with the volt function.

Checking several voltage sources:

• 9V AC power supply - seems to work fine and agrees with my other meter.
• 1.2V DC battery - the meter will count up from 0 in 0.1V increments at roughly one count evert 0.5 seconds until it reaches 1.2V. Final reading agrees with other meter. This behaviour is very repeatable. If you remove the leads it will start counting back down to 0 at the same rate.
• 5.8V DC battery -  the meter will again count up typically from 4.5V to 5.8 volts in 0.1V increments. Sometimes it will read 5.8V immediately, occasionally it will start form 3.2V.
• 9v DC power supply - the meter will instantly display reading which agrees with my other meter.
• 18V power tool battery – instantly display reading 17.9V, then counts up further 0.5v in 0.1V increments.
• 5.2V DC phone charger – instantly displays reading, consistent and agrees with other meter.

The meter is out of warranty, I haven’t yet contacted Brymen, as I suspect any repair cost will be more than it is worth. Obviously I would like to fix it if possible.
It seems like there shouldn’t be anything catastrophically wrong as it is mostly working - but this is well outside of my level of experience and comfort zone.
I guess the next step is to take it apart although I doubt there will be anything obviously wrong.

Any suggestions on what I should look out for or where to start testing would be gratefully appreciated!
Thanks.

[J-R]

Closest I have is the Amprobe AMP-330 and it behaves the same where it counts up and down slowly.  So that appears normal.

The six voltage source tests you listed were all fine, so I would take a second look at your earlier 12V measurement to see if there is another explanation.

[monkey_magic]

Thanks, I should clarify this is not how the meter behaved when I purchased it. I've often checked batteries with it in the 1.2 - 1.5 range over the years and the meter would always display the result instantly every time, and display 0V as soon as the probes were removed. So something has definitely changed. It does appear it can still do this for some voltage sources.

With regards to the 12V source, I checked several times swapping between the two meters I had on the day and the BM079 did not register any voltage each time. As I said, I think it has been doing this for a while but intermittently which is why I put it down to leads making bad contact.

Searching for solutions I found this thread which seems similar, although I don't have any of the same problems they had with the ohm ranges.

https://www.eevblog.com/forum/repair/fluke-73-series-iii-slow-to-gt-to-reading/

[Veteran68]

Did you try with different leads?

[Kean]

With regards to the 12V source, I checked several times swapping between the two meters I had on the day and the BM079 did not register any voltage each time. As I said, I think it has been doing this for a while but intermittently which is why I put it down to leads making bad contact.

Maybe dodgy test leads?  If it happens again, swap with your other meter to see if the problem moves/changes.

[monkey_magic]

Did you try with different leads?

Maybe dodgy test leads?  If it happens again, swap with your other meter to see if the problem moves/changes.

Leads and batteries checked and ruled out as per original post, first things I checked. The issue is internal to the meter.

The problem with voltage count is 100% repeatable, I haven't timed it exactly but waiting around 10 seconds to count up to measure a AA battery. Even a cheap and nasty £5 meter can measure the voltage of a battery inside a couple of seconds!

I'm not knowledgeable on test instrument repair so looking for some pointers on what components or sections of the meter might cause this problem before I take it apart.

[_Wim_]

As you have other meters also, can you measure the input impedance of the brymen (so the resistance between the test leads when in voltage mode). This should be around 10MOhm. Input capacitance should be around 100pF according to the datasheet, but not sure if you can measure this as an LCR meter is needed probably.

[J-R]

My AMP-330 is about 90pF and 10M on the input.

I ran some more tests and the slow counting up only happens below about 1.62V.  Above that it's instant, but if you let it start going down, say from 1.65V to 1.1V, then reconnect, it sits there for a while then slowly counts back up.

Around 2V and above it's always snappy up and down.

Also, immediately shorting the leads doesn't accelerate the counting down.

So I think this is just an artifact of their filtering method and being at the very bottom of the 600.0V range. I would use a different DMM for these lower measurements anyway since the voltage accuracy specs are laughably bad at 1% + 5d with only a 100mV resolution.

I also tested my BM037 and observed a somewhat adjacent behavior where for low voltages it counts down to zero in about 1-2 seconds rather than just dropping instantly to zero.

[AVGresponding]

If you're prepared to take it apart, I'd recommend checking the pcb contacts for the rotary switch; it sounds like they might be a bit worn/dirty on the range you use the most.

[J-R]

https://www.ittsb.eu/BRYMEN-BM079-Review.html#BM079-PART4

"BM079 DC Voltage
This is a single range 0~600V.
BM079 shown major delay at measuring right away any voltage lowers than 2V.
Measurement accuracy this restored at 3~4 seconds total time.
At 2 Volt DC and above, BM079 behaves much better, the over all time for taking a measurement this goes down to 1 second.

Another somewhat unexpected behaviour is that at DCV mode, BM079 this does slow down when DCV value this instantly increase/decrease.
This behaviour simile with slow multimeter's which them have only 2.5 times display update rate.
I did my testing at the range of 12V up to 35V and therefore I would not expect to see such a limitation there."

[monkey_magic]

As you have other meters also, can you measure the input impedance of the brymen (so the resistance between the test leads when in voltage mode). This should be around 10MOhm. Input capacitance should be around 100pF according to the datasheet, but not sure if you can measure this as an LCR meter is needed probably.

I've checked with my Brymen 257S which shows the input impedance of the clamp meter to be 10.30MOhms in DCV and 10.51MOhms on ACV
I don't have an LCR meter unfortunately so don't think I can measure the capacitance.

[monkey_magic]

If you're prepared to take it apart, I'd recommend checking the pcb contacts for the rotary switch; it sounds like they might be a bit worn/dirty on the range you use the most.

I can certainly try this, although it has always been kept in a soft case and is spotless as I'm very fussy about keeping my tools and equipment clean but perhaps there is some oxidation somewhere.

[monkey_magic]

My AMP-330 is about 90pF and 10M on the input.

I ran some more tests and the slow counting up only happens below about 1.62V.  Above that it's instant, but if you let it start going down, say from 1.65V to 1.1V, then reconnect, it sits there for a while then slowly counts back up.

Around 2V and above it's always snappy up and down.

I have just tried it on a 1.5V AA battery and timed it - to get from 0V to final reading of 1.5V took just over 13.5 seconds, or as near as I could react to stop the timer.
I doubt I would have noticed a couple of seconds to register the voltage, but I am very certain it didn't perform like this previously.  I simply would have noticed before now!
I'm curious, I wonder if you could please time how long yours takes to count up to the same voltage.


I have also replicated the situation from earlier using a 12V AC SELV lighting unit attached to a 35W halogen bulb to act as a load.

The Brymen 257S on ACV shows 8.69V
The Brymen 079 on ACV fluctuates between 0.1 and 0.9V

[J-R]

I think I've figured out why you remember it being faster previously.  Change the mode from DC only to DC+AC|VFD.

[monkey_magic]

I think I've figured out why you remember it being faster previously.  Change the mode from DC only to DC+AC|VFD.

I've just tried it on this mode and the time has reduced to ~ 7 seconds. This still seems quite a lot longer than I remember, although I'm not sure how this compares with yours?
I'm pretty sure I would have previously measured only on the DC scale but I certainly could be mistaken.

Any ideas on why it doesn't measure the AC voltage correctly? I’ve just tried it on all three voltage modes and it shows zero.

The only thing I can think of is that being an electronic (dimmable) downlight driver it probably doesn't have a very clean AC waveform, but even so this seems strange it can’t see anything when other meters can.
This is not the same driver I originally had the problem with, but a similar style of unit I have laying around to try and simulate the scenario.

I always grabbed this meter in preference if I had to go out to do a quick job as it had all the functions I usually needed and I’ve never had cause to question it up to this point, but I've lost confidence in it somewhat.
I know I shouldn’t rely on it for safe isolation, but even so… 

[J-R]

Mine is nearly instant in that mode, but also it's a different model so may not be exactly apples to apples.

Maybe contact Brymen to see what they think?

[monkey_magic]

Just to bump this – sorry it has taken me a long time to get around to doing anything with this but I finally managed to make a short video to demonstrate the scenario.
I'm no youtuber, but hopefully it illustrates the problem.

I intend to send this over to Brymen to see what they make of it, but welcome any feedback first.
I'm assuming the 9V AC wallwart is unregulated which is why it is reading high.

[monkey_magic]

As an addendum to the above, I just checked the two AC voltage supplies - The 257S shows both at mains, 50Hz. While the BM079 shows both at 0Hz.

I should also say all the results are repeatable with different leads, internal batteries changed and with the meter leads piggybacked or separate. None of that changed the results.