Electronics > Repair
BM786 burden current fault
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brewster:
I have an EEVBlog Brymen BM786 about 2 years old that has a problem. The burden resistance when measuring mA and uA is highly variable, especially with even slight movement of the meter. For instance, the manual implies 2.0 Ohms for the mA range and other users have reported 2.4 Ohms. Sometimes I can get results in that ballpark, but other times it shoots up to 9-10 Ohms or more, especially if the meter is tapped lightly. The resistance typically settles back to some new reading. By including another meter in series, I can confirm that the current readings vary as expected with the changing burden voltage.

I’ve tried reversing the roles with other meters (121gw and a Fluke) to check that this behaviour is only a property of this BM786. I suspected dirty fuse contacts, but I cleaned those with no improvement. Ditto for cleaning the input jacks and using different cables. I also thought of cracked traces or solder joints, but I can’t see anything (although I don’t have much experience, so I may have missed something).

Any ideas? What to look for?
Should I distrust the meter? Only on current measurement or on everything?
Thanks for any help.
J-R:
Could it be the fuse itself?
Beyond that, it seems to be 100% a mechanical fault, so keep prodding the PCB/components with a plastic tool of some sort until you can locate the source of the issue.
brewster:
Thanks for the advice. With a replacement fuse the meter became rock solid, with none of the sensitivity to movement that I reported earlier.

Although that looks like a win, it may not be the end of the story. The fuse I removed also seems just fine, with its resistance on spec and remaining stable even when the fuse is tapped sharply with a pen. So I’m left with a nagging doubt that the fuse itself was not the problem. But if the behaviour remains stable after the fuse is replaced with a new one (the one I put in there is likely a fake) I'm not going to pursue it. I probably couldn't fix a board or component failure anyway.
J-R:
Was the fuse that was in there the original?  I suppose it is possible that there is still a bad solder joint on the PCB.  Did you already inspect it with a magnifying glass or microscope?
IanB:
Remember the fuse is in a fuse holder, with a metal-to-metal interface. A layer of oxidation could build up over time, which could increase the resistance of the connection. Simply removing the fuse and reinserting it a few times might restore good contact.

But also, the fuse itself might go high resistance internally where the fuse wire is connected to the end caps. In which case a fuse replacement is the only solution.
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