BM869: Unstable, very low voltage measurements after few seconds of operation

[jypma]

My BM869 has recently started to act up when measuring voltage. Of course, it's just a few months out of warranty (2.5y old, bought from TME).

Measurements are unstable, and values are much too low (~50%). But only after it's been on for a few seconds.

For example, consider measuring a full NiMH cell :

•   The cell measures 1.3680V with another meter
•   I connect the BM869 to the cell while off
•   Now I switch the meter to voltage mode
•   It'll read 1.3624V for a few seconds
•   Then, I'll see the value slowly decreasing, until it keeps hovering between 0.70V and 0.90V. It keeps going up and down there, not settling on a value.

Measuring my bench supply at 5V starts out at 5V (when switching on the BM869), then after a few seconds drops and hovers between 3.74V and 4.09V.

Turning the meter off and on consistently repeats the above.

Resistence measurement seems to be working OK. Haven't checked current yet. Switching test leads and new battery makes no difference. Manual range makes no difference.

Might the internal voltage be acting up? Maybe some self-heating in the first few seconds of turning it on? I've emailed Brymen, but they don't know what it might be and say I should send it in for repair. I'd like to avoid sending it in for unknown costs...

Any help will be greatly appreciated.

[alanb]

Have you tried changing the battery?

[jypma]

Yes, changing the battery unfortunately makes no difference.

[MosherIV]

Worth having a look at Dave's repair of a BM325

https://www.eevblog.com/2016/05/28/eevblog-884-eevblog-bm235-multimeter-repair/

[retiredcaps]

A number of questions.

Q1) Has this meter been subjected to high voltage (i.e > 240V AC)?

Q2) Please post clear focused pictures of both sides of your pcb so we can look for obvious damage.

Q3) With another meter, measure the input impedance of the BM869 when it is set to DCV mode. I'm expecting around 11M ohm. Report your reading.

Q4) Your blue MOVs should measure open circuit resistance with another meter.

Q5) Your grey PTCs in heatshrink should measure 1k ohm with another meter.

Q6) Your power resistor, grey with red band, should measure around 1k ohm with another meter.

4, 5 and 6 can be measured in circuit.

[retiredcaps]

Of course, it's just a few months out of warranty (2.5y old, bought from TME).

I thought Brymen meters only have a 1 year warranty?

[meeder]

2 years in Europe AFAIK

[retiredcaps]

2 years in Europe AFAIK

Thanks for clarifying.  I did not realise there were country differences in terms of warranty.

[jypma]

Yes, all of EU has a "mandated" 2 year warranty (I'm in Denmark), even though companies do try to make you believe otherwise.

Here are the answers to retiredcaps' questions:

1) No high voltage. I dont recall even ever heaving measured mains voltage with it. It starting acting up a few weeks ago, during low voltage stuff.
2) I created some pictures.
3)  Input impedance (without battery, in DCV) fluctuates between 5M and 7M Ohm. With battery and turned on, between 7M and 9.5M.
4) MOV's measure as open circuit.
5) PTC's are 1.10K and 1.12K
6) Power resistors are 0.888K and 0.897K

[giosif]

For such a sudden malfunction, I'd suspect dirty input connectors or dirty/damaged range selector...

[Edit] Or a cold joint...

My 2 cents...

[retiredcaps]

3)  Input impedance (without battery, in DCV) fluctuates between 5M and 7M Ohm. With battery and turned on, between 7M and 9.5M.

The above is a problem.  With the battery in and turned on, it should be at least 10M ohm, probably 11M ohm when all the input protection is accounted for.  I don't have BM86x, but my Greenlee DM200A which is a Brymen is 11M ohm input impedance on DCV.

As giosif mentioned, you might want to check the range switch pcb underneath the rotary knob.  You could have some dirt/debris causing some leakage current.  Clean with 91% IPA and let it dry thoroughly.

4) MOV's measure as open circuit.

What multimeter are you using to make this measurement?  Some of the cheaper meters only can measure up to 2M ohm and a MOV that reads 4M ohm would be bad.

5) PTC's are 1.10K and 1.12K
6) Power resistors are 0.888K and 0.897K

5 and 6 look close enough.  They are likely good.

[retiredcaps]

Q7) Put battery in BM869 and turn onto DC mV range.  Using another meter, measure the input impedance of the BM869.  It should be 10M ohm.

Q8) Measure a 1k ohm resistor.  What does the BM869 show?

[Dr. Frank]

3)  Input impedance (without battery, in DCV) fluctuates between 5M and 7M Ohm. With battery and turned on, between 7M and 9.5M.

That reading will depend on Ohmmeter and on 869 DC range used (V or mV, and different manually chosen ranges).
If Ohmmeter creates overflow on 869, its range protection is active, and will create random Ohm readings.

I use 34465A as Ohmmeter, readings for reference:

BM869 range / compliance volt reading     34465A Ohm reading (and its range)
50V / 05.073V                                               10.101 MOhm (10M range)
5V / 2.6503V                                             011.111 MOhm (100M range)
5V / OL                                                       11.107 MOhm (10M range)
500V / 005.02V                                                10.006 MOhm (10M range)
1000V/ 0005.01V                                     9,9993 MOhm (10M range)

0.5 V / OL                                                6.617M (10M range)
0.5V / OL                                                  009.989 M (100M range)

Frank

[jypma]

Thanks for all the suggestions! Here are my latest findings. Unfortunately, no closer to a solution it seems.

Clean with 91% IPA and let it dry thoroughly.

Done (both sides). Didn't seem to make a difference; I also didn't see any contaminants.

What multimeter are you using to make this measurement?

Voltcraft VC880. It seems to measure resistors between 1M and 20M just fine.

Q7) Put battery in BM869 and turn onto DC mV range.  Using another meter, measure the input impedance of the BM869.  It should be 10M ohm.

My previous measurements were off due to auto-ranging. I redid the measurements with both meters in manual range mode. Using manual range on both, I get stable readings for all ranges:

BM869 range / compliance volt reading	VC880 Ohm reading (and its range)
50V / 0.260V	11.12 MOhm (40M range)
5V / 0.0010V	11.14 MOhm (40M range)
500V / 000.23V	10.03 MOhm (40M range)
1000V/ 0000.1V	10.02 MOhm (40M range)
0.5V / 52 mV	9.90M (40M range)

Q8) Measure a 1k ohm resistor.  What does the BM869 show?

• It shows 1.0010k , stable, when on the 5K range
• However, on the 50K range, the value drops after a few seconds, like the voltage ranges. I.e. it will initially show 1K, then after a few seconds drop and start fluctuating between 0.6K and 0.8K.

I did notice that the plastic lower blast shield around the fuses was only held down with 1 screw instead of two. I think that will have put extra force on the PCB every time I disconnect a lead. Still, this doesn't feel like a mechanical problem to me: the meter provides the correct value in the first few seconds on every range.

Would it help to show you a video of the symptom? BTW, still no reply from either TME or Elma, whom I both asked for repair options...

[jypma]

By the way, now that I put it back together the speaker no longer works... and I can't find it on the PCBs, where is it?

[Kilrah]

Must be in the back case, with springs making contact with it from the PCB...

[retiredcaps]

I can only think of two more tests to do.

Q9) Set your DC power supply to 3VDC.
Set BM869 to DCV auto range.
Connect BM869 probes to power supply.

Using this picture from

http://www.118volt.it/recensione-multimetro-brymen-bm869-bm867-p2/

you can clearly see the precision resistor network. 

Put your VC880 black probe on the BM869 COM jack as ground.
Put your VC880 into DCV and the red probe on each part of the resistor network. 
Since there are no schematics, some readings will be n/a.

What I'm expecting is to see is 300mV on at least one of the legs of the resistor network.

If yes, then we know that the 3V is reaching the precision network properly.  If the BM869 LCD display is show something like 2.4V, then we can assume the problem is somewhere closer to the digital side?

Q10) Carefully examine the area underneath the shields for debris, dirt, etc.

[ModemHead]

I'm a big fan of functional testing such as suggested by retiredcaps. But if you're trouble-shooting with a standard 10Meg DMM, any measurements you take on the downstream side of the divider can be loaded down by that DMM.

The usual value for the input resistor on a modern auto-ranging DMM is 10Meg. This will be the top of the divider.  When the divider is in 1/10 mode, the bottom resistor will be 1.111Meg. (Hence the 11.11Meg input impedance.)  With the 10Meg of the trouble-shooting DMM in parallel with the 1.111Meg bottom divider resistor, the ratio is altered from 0.1000 to 0.0909.  So with a 3.000V input, the expected measurement would be 0.273V.  If the DMM under test were operating correctly, it should display 2.73V as well.

[retiredcaps]

But if you're trouble-shooting with a standard 10Meg DMM, any measurements you take on the downstream side of the divider can be loaded down by that DMM.

Yes, I forgot about this little detail.

[jypma]

A big thanks to retiredcaps and ModemHead for their suggestions! And thanks to Kilrah, yes the speaker only works after all the screws are fastened (the springs don't make contact otherwise).

I hooked up retiredcaps' suggested measurement setup for the resistor network.

• I found a resistor network pin that measures 275mV with the VC880. Touching it makes the fluctuating measurement of the BM869 to go slightly down.
• I measured the same pin with my DS1042E scope as well (claims 1M input impedance). Says 242mV, so I reckon its input impedance is a bit higher.
  

I now think the drop & fluctuations happen after the resistor network, as its voltage remains stable while the display does change.

I reckon it's new multimeter time, since just shipping this thing anywhere + Danish diagnosis labour costs + new board(?) will probably equal a new meter, yet I'll get 2 years warranty with a new one. I just kinda expected a multimeter to live more than 2,5 years :-)

[ModemHead]

If the voltage divider output is constant but the reading is changing, then my short list of suspects would include the A/D converter itself, or the voltage reference.  The Italian page that retiredcaps posted says the reference is a REF43G.  If you can find it, it should be easy to check it's output for a constant 2.5V.

[HackedFridgeMagnet]

Have you read this thread? May be of interest.

https://www.eevblog.com/forum/testgear/brymen-bm869s-rejecting-fresh-9v-batteries/

[jypma]

Thanks HackedFridgeMagnet, I hadn't spotted that post yet. However, seems my protection diode is fine, and the meter pulls 3-9 mA at 8V. And it does accept fresh batteries :-)

I measured the voltage reference, it's a stable 2.5V all the way. So that leaves the ADC... which again I can't find. Could it be part of the magic BTC AD-85-4 chip? It'd be really nice to be able to measure what goes into the ADC.

[ModemHead]

Yeah, I would assume the "BTC" part is a multi-function chip with the A/D in it.  The tracks from the precision resistor network should lead right to it.  I don't believe there are any data sheets available for these Brymen parts, so trouble-shooting around it would be a giant fishing expedition.

Speaking of fishing expeditions, you can always look for datasheets on the off-the-shelf parts.  Power/gnd pins and voltage regulators provide a means to check the power supply rails for funny stuff.  If the negative supply (with respect to analog ground/COM jack) were acting up, the logic would probably still run fine, but the A/D might do weird things.

As you can probably tell, I'm running out of things to suggest... 

[MosherIV]

I would suggest tracing the signal path from the resistor network to the BTC AD-85-4
Check for any dry joints on components in that path.

If there are any cap on the signal path - worth a check, both value, esr and if the component is broken.
(In Dave's video - the fault was a broken component that passed at the factory but failed in the field)

Finally, check if there are any dry joints on the BTC AD-85-4 itself.

There are not many reports here of these meters failing, so I would assume that this should not happen.
Unlikely to be killed by ESD since the meter has good protection. So either a bad component or the BTC AD-85-4 has gone bad. 

[retiredcaps]

I would suggest tracing the signal path from the resistor network to the BTC AD-85-4
Check for any dry joints on components in that path.

In addition to checking the connections, if all else fails try cleaning the entire PCB with 91% IPA.  Some people are reluctant to try this because they believe or think it does nothing, but once in a while, IPA does some magic.

https://www.eevblog.com/forum/repair/fluke-1507-insulation-tester/msg982359/#msg982359

Give the area around the BTC chip a good scrub with a clean toothbrush and use a sharp probe or dental pick and maybe gently drag it into the BTC pins to potentially dislodge foreign debris/dirt.

Because I purposely buy used and abused multimeters for learning purposes, cleaning with pcb with IPA is just part of my standard process.

[retiredcaps]

I reckon it's new multimeter time, since just shipping this thing anywhere + Danish diagnosis labour costs + new board(?) will probably equal a new meter, yet I'll get 2 years warranty with a new one. I just kinda expected a multimeter to live more than 2,5 years :-)

If the repair of this meter fails, you can always posted it on ebay for parts?  Maybe someone will want the LCD if he/she dropped and smashed theirs.  Alternatively, put in away in the "fix later" pile.

Now that Brymen is becoming more popular, the number of failed units are starting to rise.  Just yesterday

https://www.eevblog.com/forum/buysellwanted/usa-source-for-brymen-meters/msg991261/#msg991261

Two Brymen BM257s failed with calibration error

https://www.eevblog.com/forum/testgear/brymen-257s-lost-calibration-data/?all

Dave's BM235 has at least two failed units with bad soldering of a component on the input jacks.

[jypma]

I just checked, the resistor network's output pin indeed goes directly into the BTC. And I measured that pin/net's voltage to be stable, so it can't really be a cap on that line. Visual inspection of the BTC doesn't show any dry joints.

To me, that inplies next indeed checking the VCC rails that go into the BTC. That's going to be tough, since the meter doesn't turn on without the screen present (and the headers conventiently aren't 0.1"); I had to solder a modwire on just to get the Vref to measure.

I think at this point, the time investment of further research vs. likelihood of it being repairable (or of course having fun learning something new ;-) is leaning towards just getting a new meter...

[Hydrawerk]

You can keep the old DMM for spare parts. Maybe only the IC is broken.

[retiredcaps]

You can keep the old DMM for spare parts. Maybe only the IC is broken.

The Brymen IC is unobtainable from Brymen.  And the OP might think

"Fool me once, shame on you"
"Fool me twice, shame on me"

Unless the Brymen BM869 is the only multimeter in the entire world that meets his requirements and since he hasn't used it on mains, why spend another $250 USD or more when a $50 USD meter might do?

[retiredcaps]

BTW, still no reply from either TME or Elma, whom I both asked for repair options...

The feedback on TME that I have read here is that they are very quick to ship and orders arrive quickly, but after sales or warranty service seems  to go /dev/null.

For this reason and possibly high return shipping costs from Canada to TME, I would never buy anything from TME.

[jypma]

I guess the meter is going into the "fix later" bin for now. At some point I'll order some 2mm pitch headers, make a cable to connect the boards, and do more live testing. I've also not been able to get the RF shield off... is that just a matter of applying heat to each of the contacts in turn and keep wiggling it?

FWIW, I've actually had pretty good (within warranty) service from TME on other items. I order from them a lot. In many cases I received a refund or replacement after only a textual description of problems. Of course, being in Denmark, I don't have a lot of choice: they're one of the few electronic suppliers that ship from within the EU (importing any >$12 item from outside EU costs another $25 in "import handling" in DK...)

Service after warranty apparently is hard to get, but it was going to be economically challenging anyways, giving high shipping costs (and generally high wages) from Denmark.

So I was kinda just going to get a new BM869... but before I do get fooled twice, if anyone can recommend a cheaper alternative, do give a shout. These are the features I've regularly used over the time I've had it:

• Temperature, capacitance, diode, continuity
• 500.000 count mode (show cap, battery discharge)
• VDC+VAC dual display (showing ripple on power supply)
• Very happy for its insanely fast update rate

[cs.dk]

I know shipping to Poland is a deal killer.. However, Biall which is the importer in Poland may give you a quote on a new mainboard.. Probably not worth it though, but an email is free to send
http://www.biall.com.pl/

I surely hopes that my Brymen meters will last longer than 2,5 yr.

[HackedFridgeMagnet]

What about the old freezer spray, could that help isolate the problem?

[EPTech]

Hi There,

Seems like you get the abnormal behavior when switching to higher ranges right, like you did with the resistor measurement. Is a 47k resistor measurement in the 50k range also drifting?

Besides the chipset there is considerable silicone on the board. Would you check whether one of those happens to be a sample and hold. I expect te sample and hold to be in the chipset, along with the ADC, but this design may be an exception. If there is a sample and hold in the path, it may be drifting, due to a bad capacitor (polypropylene) or a bad SH chip itself.

Good hunting.