BM786 burden current fault - Page 1
Forum section: electronics / repair

author: brewster
author_rank: Newbie
timestamp: August 11, 2024, 05:41:54 am
content: 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.

author: J-R
author_rank: Super Contributor
timestamp: August 11, 2024, 09:29:56 am
content: 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.

author: brewster
author_rank: Newbie
timestamp: August 19, 2024, 10:20:12 pm
content: 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.

author: J-R
author_rank: Super Contributor
timestamp: August 20, 2024, 07:22:09 am
content: 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?

author: IanB
author_rank: Super Contributor
timestamp: August 20, 2024, 07:29:13 am
content: 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.

author: brewster
author_rank: Newbie
timestamp: August 21, 2024, 10:47:13 am
content: Thank you both for assistance. Some answers:The fuse was the original SIBA 400mA. The meter had not been opened since I bought it from Dave's online shop. I inspected the solder joints on the fuse holder and nearby components with a magnifier but without seeing anything obvious.Earlier I had cleaned the contact surfaces on the fuse and holder, but only gently with IPA to remove grease or surface dirt. The meter has lived in a benign environment since new, so I didn't think of oxidation. However, that might explain why the problem appears to have gone away after further switching of the fuse. I'll pop the fuse out and in again and if necessary replace it. I've ordered one of Dave's ASTM fuse kits, which are now in store again.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2024, 12:34:26 pm
content: This can be the problem:Quote from: brewster on August 21, 2024, 10:47:13 amSlightly greasy contacts are good. the grease coating prevents oxidation and after cleaning the contacts probably start oxidizing more, which results in bad contacts. Mechanical pressure pushes the grease aside and also seals the contact area from contact with oxygen.The switch contacts under the dial are sometimes also coated on purpose with a bit of (silicone) grease.

author: J-R
author_rank: Super Contributor
timestamp: August 22, 2024, 03:56:02 am
content: Given how tightly the fuse fits in the holder and the metals used, I'm doubting oxidation.Maybe flaky test leads. I recall some complaints of Brymen's having this issue due to the crimp of the wire to the probe tip.

author: EEVblog
author_rank: Unknown
timestamp: August 22, 2024, 04:50:34 am
content: Dodgy test leads?

author: floobydust
author_rank: Super Contributor
timestamp: August 22, 2024, 05:39:11 am
content: If I'm not mistaken, all shunts are in series. I would also consider a bad solder joint on the shunt(s) R40, R49, ferrite bead L10 or L17 cracked, and fuseholder clips, the jacks...The through-hole parts give them a good visual check for not having enough solder or a fractured solder joint. Just use another multimeter on continuity while poking the BM786 internals.

author: brewster
author_rank: Newbie
timestamp: August 22, 2024, 05:42:55 am
content: To save confusion I note that I'm not using the Brymen leads (which I have otherwise found excellent) but my own constructed cables. I have tried to exclude the leads from consideration by switching two other meters into the same cabling setup and finding no problem with them. It was only after I changed the fuse in the BM786 that the problem went away.Of course, it is hard to vary one part of a setup without affecting other parts. I have not completely excluded the possibility of an intermittent fault in the leads, but if the problem recurs I will replace the entire cabling setup and retest.

author: EEVblog
author_rank: Unknown
timestamp: August 22, 2024, 06:25:28 am
content: Quote from: brewster on August 22, 2024, 05:42:55 amIn that case, as above, visual inspection of PCB and joints. But I wouldn't rule out a mechnically dodgy fuse either.

author: brewster
author_rank: Newbie
timestamp: August 30, 2024, 06:48:21 am
content: Solved - I think! To recap, I found that popping in a different fuse seemed to remove the problem. I also found that the original fuse was immune to much sharper treatment than the modest movement that would upset the meter when that fuse was installed. That doesn’t make much sense, except as suggested by a couple of replies it points to a mechanical fault.I decided to replace the fuse again because the one I put in there as a temporary measure is likely to be a fake. Then I noticed that one end of the fuse holder does not make firm contact with the fuse. A gentle bending closer together seems to have fixed it.Thanks all for assistance.