Author Topic: BM869: Unstable, very low voltage measurements after few seconds of operation  (Read 7281 times)

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Offline retiredcaps

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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.
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.
« Last Edit: July 27, 2016, 05:03:29 pm by retiredcaps »
 

Offline retiredcaps

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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.
 

Offline jypma

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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...
 

Offline Hydrawerk

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You can keep the old DMM for spare parts. Maybe only the IC is broken.
Amazing machines. https://www.youtube.com/user/denha (It is not me...)
 

Offline retiredcaps

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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?
 

Offline retiredcaps

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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.

 

Offline jypma

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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
 

Offline cs.dk

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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.
 

Offline HackedFridgeMagnet

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

Offline EPTech

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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.

Kind greetings,

Pascal.
 


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