Author Topic: BM869: Unstable, very low voltage measurements after few seconds of operation  (Read 7282 times)

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Offline jypma

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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.
« Last Edit: July 25, 2016, 08:31:41 am by jypma »
 

Offline alanb

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Have you tried changing the battery?
 

Offline jypma

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Yes, changing the battery unfortunately makes no difference.
 

Offline MosherIV

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Offline retiredcaps

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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.
« Last Edit: July 25, 2016, 05:21:48 pm by retiredcaps »
 

Offline retiredcaps

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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?
 

Offline meeder

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2 years in Europe AFAIK
 

Offline retiredcaps

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

Offline jypma

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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

 

Offline giosif

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

[Edit] Or a cold joint...

My 2 cents...
« Last Edit: July 25, 2016, 08:40:08 pm by giosif »
 

Offline retiredcaps

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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.

Quote
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.

Quote
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.
« Last Edit: July 25, 2016, 10:24:58 pm by retiredcaps »
 

Offline retiredcaps

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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?
« Last Edit: July 25, 2016, 10:46:59 pm by retiredcaps »
 

Offline Dr. Frank

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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
« Last Edit: July 26, 2016, 06:27:27 am by Dr. Frank »
 

Offline jypma

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

Quote
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.

Quote
What multimeter are you using to make this measurement?
Voltcraft VC880. It seems to measure resistors between 1M and 20M just fine.

Quote
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 readingVC880 Ohm reading (and its range)
50V / 0.260V11.12 MOhm (40M range)
5V / 0.0010V11.14 MOhm (40M range)
500V / 000.23V10.03 MOhm (40M range)
1000V/ 0000.1V10.02 MOhm (40M range)
0.5V / 52 mV9.90M (40M range)

Quote
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...
 

Offline jypma

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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?
 

Offline Kilrah

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

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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.
 
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Offline ModemHead

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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.
 
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Offline retiredcaps

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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.
 

Offline jypma

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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 :-)
 

Offline ModemHead

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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.
 

Offline HackedFridgeMagnet

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Offline jypma

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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.
 

Offline ModemHead

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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...  :-\
 

Offline MosherIV

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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.  :(

« Last Edit: July 27, 2016, 03:45:36 pm by MosherIV »
 


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