EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: January 24, 2025, 09:20:42 pm
content: Repairing a customers Brymen BM786 multimeter, and investigating the cause of the InEr error message on the input jack alert feature.Plus a giveaway to a local youngster.00:00 - A returned BM786 Multimeter with InEr error message03:23 - Split jacks and input alert circuit05:19 - In-Circuit probing TIP and Low Ohms mode.07:19 - A clue?08:05 - A need a new desoldering gun. Removing the input jacks10:30 - Bodge testing of the input detection threshold12:38 - Not one but TWO failed Megger Insulation meters!13:50 - Megger testing the connectors at 500V15:19 - Testing a 2nd failed InEr error meter17:00 - Cleaning the input jacks19:11 - Heating up the input jacks20:18 - Getting desperate, removing the input jacks21:49 - Getting really desparate, removing the input resistors22:38 - Reassembling the original repaired unit24:26 - Repaired BM786 GIVEAWAY

author: Kleinstein
author_rank: Super Contributor
timestamp: January 25, 2025, 05:05:39 pm
content: The treshold for detecting a connection is rather high. This is made to case issues from dirt and humidity.At least the amps terminal could use a little lower threshold, as there a few µA of extra current would not matter. For the µA range one would really want a very low current, so a bit tricky to check with a low threshold - one could still turn off the check when actually in the µA range.It may be worth checking the threshold on a working meter, that did not had the problem. Part of the problem would be to high of a threshold, e.g. from some of the resistors drifting up (not to rare for high value carbon resistors).

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: January 25, 2025, 05:42:37 pm
content: I find it rather odd and humorous that you don't have a schematic for your own branded meter which you do warranty on.

author: floobydust
author_rank: Super Contributor
timestamp: January 26, 2025, 10:51:02 pm
content: I agree with Kleinstein, the whole thing needs to be sensitive but is way too sensitive lol.I would first suspect flux contamination in/under the jack and don't forget the other PCB traces nearby. The factory PCB wash can have troubles with long parts or ones missing a vent hole, and left some residue. I find all fluxes are hygroscopic.You can just pull off some of the cotton at the end of the q-tip to fit it inside a jack to clean it. Most I find are really dirty inside.I would've followed the PCB trace to whatever the pullup resistor/protection clamp/logic gate is, to know what the circuit is.It would be interesting to test the firmware's behaviour when there is something detected in BOTH jacks at the same time, that could cause a chuckle.As I understand, the 5MEG resistors are huge 2W parts (3.2kV) because HV can only ever appear right at the jack when the fuse is blown. The UL fuse clear test is min. 3kV as I remember, they add inductance to make an arc happen.For the Soldapullt, technique I was taught was to never let the tip touch the soldering iron, it will melt it.You heat up the solder joint, fast like lightning pull the soldering iron out of the way and then put the Soldapullt nozzle on the liquid solder, square on the pad and hit the button.You get better sealing and suction this way. Also, the piston really likes some light grease Lubriplate on it.I know all this because I was poor as a tech, no fancy-schmancy desoldering stations. Used that desoldering tool constantly.Lately the old Hakko I have is shit, lead-free clogs it up easily. I spend more time cleaning it.

author: EEVblog
author_rank: Unknown
timestamp: January 26, 2025, 11:19:28 pm
content: Quote from: ballsystemlord on January 25, 2025, 05:42:37 pmBoth I and other dealers are not going to do component level repair on these meters and they know it. So I guess the figure they don't want to risk having the schematic leak out.I only do it in the odd case for the funnsies of a video.

author: mwb1100
author_rank: Frequent Contributor
timestamp: January 27, 2025, 12:31:18 am
content: This may be prying a bit much, but I’ve been curious: do manufacturers like Brymen provide some sort of credit for warranty returns or does the dealer have to eat the cost?

author: EEVblog
author_rank: Unknown
timestamp: January 27, 2025, 01:11:24 am
content: Quote from: mwb1100 on January 27, 2025, 12:31:18 amThe dealer can get replacement units from Brymen for warranty. But the dealer eats the shipping costs etc.

author: thm_w
author_rank: Super Contributor
timestamp: January 27, 2025, 09:39:05 pm
content: Quote from: floobydust on January 26, 2025, 10:51:02 pmYeah thats a good question, is there a ~50Meg pullup somewhere? It seems incredibly high.Maybe it was optimized for power consumption so they just went as high as they could.

author: floobydust
author_rank: Super Contributor
timestamp: January 27, 2025, 10:06:37 pm
content: Any pullup resistor for jack detect adds uA error.The 121GW has 30MEG pullup to +3.6V for 0.12uA I believe. 1.2V comparator trip so that's around <15MEG jack detect threshold unless I missed something.Fluke 87V is 1MEG to -0.7V and I would have to think more about it, there is a 10MEG to -2.5V as well for the two-diode source.Newer Flukes adding an LED to the jack to flash and remind you, we'll see how that works...

author: EEVblog
author_rank: Unknown
timestamp: January 27, 2025, 10:10:42 pm
content: Quote from: thm_w on January 27, 2025, 09:39:05 pm5M is 0.2uA @1V, which is 2LSB on the 600.0uA range. So you'd want to be well outside that range, which means 10's of megs at least.

author: tszaboo
author_rank: Super Contributor
timestamp: January 27, 2025, 10:24:46 pm
content: Quote from: EEVblog on January 27, 2025, 10:10:42 pmI've found that some flux was initially non-conductive, and became conductive after it was exposed to humidity/moisture.I don't have too much data as it wasn't reproducible, and cleaning it solved the issue. Or rather dissolved it...But I thought to mention it.

author: thm_w
author_rank: Super Contributor
timestamp: January 27, 2025, 10:24:51 pm
content: 121GW 30Meg is right, I should have checked that schematic first:https://github.com/tpwrules/121gw-re/blob/master/pdfs/121GW%20EEVBlog%20Circuit%20diagram.pdf2x 4.7M in series and then three 10M pullup resistors.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 27, 2025, 11:03:13 pm
content: At least the amps range could use a smaller resitance with no problem.The µA range would be a bit tricky with the offset. In theory an offset could be subtracted - depending on the DMM chip there may be an offset already to compensate the amplifier offset.Flux can become conductive when humid. This is especially the case when the flux was not heated all the way to activate.The problem here is however more like dirt collecting in the terminals, as they are open and no easy way to clean.

author: mwb1100
author_rank: Frequent Contributor
timestamp: January 27, 2025, 11:14:46 pm
content: Fluke 28II solves these problems by using optics to detect jack insertion. Some other meters must do the same.The drawback would be increased BOM costs (I wonder if making part of the jacks transparent is maybe the biggest cost increase?). The list price of of the 28II is $700. Interestingly, as far as I can tell the 87V Max is identical to the 28II other than the name. Its list price is $100 less - still pretty whopping.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: January 28, 2025, 02:07:17 am
content: Quote from: mwb1100 on January 27, 2025, 11:14:46 pmAnd yet a photo interrupter sensor w/IR diode costs less $0.16 USD shipped...Maybe we'll all soon be designing our own meters because we can't afford even the name brand's base models.

author: EEVblog
author_rank: Unknown
timestamp: January 28, 2025, 02:24:47 am
content: Quote from: mwb1100 on January 27, 2025, 11:14:46 pmYes, same meter. Just marketing having a wank.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 28, 2025, 08:42:08 am
content: A photo-interrupter would need quite a bit more power. It would be extra effort to turn on / off to save power.A relatively easy way around would be using a little more current and turn off the test current when the meter is actually in the µA range. The test current would than not interfere with the sensitive measurement ad when in the amps more it is not longer important to detect the connector.Another solution is a mechanical lock: block the terminals when not in amps mode or maybe a 2nd off position at the other end.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: January 28, 2025, 05:37:24 pm
content: Quote from: Kleinstein on January 28, 2025, 08:42:08 amYou mean, gasp, we might actually get bigger batteries in our DMMs than AAA cells?

author: floobydust
author_rank: Super Contributor
timestamp: January 28, 2025, 06:28:25 pm
content: Optical jack-sense is used on the Fluke 287, 189/289.Surprisingly it's not super reliable, people break the IR LED or phototransistor somehow.They areoddball,right angle-looking Osram parts.The input jack can break one of the two pins as well.https://www.eevblog.com/forum/repair/fluke-289-shows-message-warning-leads-connected-incorrectly-i-have-detected/https://www.eevblog.com/forum/repair/fluke-289-lead-detection

author: EEVblog
author_rank: Unknown
timestamp: January 29, 2025, 12:16:28 am
content: Quote from: ballsystemlord on January 28, 2025, 05:37:24 pmThe 121GW originally had AAA cells, but I decided to make the case thicker to accomodate AA instead.

author: TnF
author_rank: Contributor
timestamp: April 29, 2025, 07:09:29 am
content: BIG STORY short, mine, a BM319s (which is similar in construction to BM235) i had from 2016, just randomly developed "InEr" in a ranges a few days ago. I disassembled, didn't find anything, cleaned the jacks multiple times, followed the jack detection trace and didn't find any issues.Reading the comments from the recent Dave's video regarding little residual flux from manufacturing getting conductive with time, i put it inside my resin 3d print washer full of Isopropyl alcohol, 15min wash each side, then used a hairdryer on hot, problem FIXED!Just leaving this here for others to find if they get the same problem.