Author Topic: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation  (Read 1687 times)

0 Members and 1 Guest are viewing this topic.

Offline EEVblogTopic starter

  • Administrator
  • *****
  • Posts: 39277
  • Country: au
    • EEVblog
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 message
03:23 - Split jacks and input alert circuit
05:19 - In-Circuit probing TIP and Low Ohms mode.
07:19 - A clue?
08:05 - A need a new desoldering gun. Removing the input jacks
10:30 - Bodge testing of the input detection threshold
12:38 - Not one but TWO failed Megger Insulation meters!
13:50 - Megger testing the connectors at 500V
15:19 - Testing a 2nd failed InEr error meter
17:00 - Cleaning the input jacks
19:11 - Heating up the input jacks
20:18 - Getting desperate, removing the input jacks
21:49 - Getting really desparate, removing the input resistors
22:38 - Reassembling the original repaired unit
24:26 - Repaired BM786 GIVEAWAY

 
The following users thanked this post: mwb1100, Avelino Sampaio

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15227
  • Country: de
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #1 on: January 25, 2025, 05:05:39 pm »
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).
 
The following users thanked this post: EEVblog

Offline ballsystemlord

  • Frequent Contributor
  • **
  • Posts: 257
  • Country: us
  • Student
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #2 on: January 25, 2025, 05:42:37 pm »
I find it rather odd and humorous that you don't have a schematic for your own branded meter which you do warranty on.
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7765
  • Country: ca
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #3 on: January 26, 2025, 10:51:02 pm »
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.
 
The following users thanked this post: thm_w

Offline EEVblogTopic starter

  • Administrator
  • *****
  • Posts: 39277
  • Country: au
    • EEVblog
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #4 on: January 26, 2025, 11:19:28 pm »
I find it rather odd and humorous that you don't have a schematic for your own branded meter which you do warranty on.

Both 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.
 
The following users thanked this post: ballsystemlord

Offline mwb1100

  • Frequent Contributor
  • **
  • Posts: 624
  • Country: us
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #5 on: January 27, 2025, 12:31:18 am »
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?
 

Offline EEVblogTopic starter

  • Administrator
  • *****
  • Posts: 39277
  • Country: au
    • EEVblog
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #6 on: January 27, 2025, 01:11:24 am »
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?

The dealer can get replacement units from Brymen for warranty. But the dealer eats the shipping costs etc.
 
The following users thanked this post: thm_w, mwb1100, Exosia, ballsystemlord

Offline thm_w

  • Super Contributor
  • ***
  • Posts: 7660
  • Country: ca
  • Non-expert
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #7 on: January 27, 2025, 09:39:05 pm »
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.

Yeah 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.
Profile -> Modify profile -> Look and Layout ->  Don't show users' signatures
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7765
  • Country: ca
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #8 on: January 27, 2025, 10:06:37 pm »
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...
 
The following users thanked this post: thm_w

Offline EEVblogTopic starter

  • Administrator
  • *****
  • Posts: 39277
  • Country: au
    • EEVblog
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #9 on: January 27, 2025, 10:10:42 pm »
Yeah 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.

5M 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.
 
The following users thanked this post: thm_w

Online tszaboo

  • Super Contributor
  • ***
  • Posts: 8295
  • Country: nl
  • Current job: ATEX product design
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #10 on: January 27, 2025, 10:24:46 pm »
Yeah 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.

5M 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.
I'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.
 

Offline thm_w

  • Super Contributor
  • ***
  • Posts: 7660
  • Country: ca
  • Non-expert
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #11 on: January 27, 2025, 10:24:51 pm »
121GW 30Meg is right, I should have checked that schematic first: https://github.com/tpwrules/121gw-re/blob/master/pdfs/121GW%20EEVBlog%20Circuit%20diagram.pdf

2x 4.7M in series and then three 10M pullup resistors.
Profile -> Modify profile -> Look and Layout ->  Don't show users' signatures
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15227
  • Country: de
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #12 on: January 27, 2025, 11:03:13 pm »
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.
 

Offline mwb1100

  • Frequent Contributor
  • **
  • Posts: 624
  • Country: us
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #13 on: January 27, 2025, 11:14:46 pm »
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.
 

Offline ballsystemlord

  • Frequent Contributor
  • **
  • Posts: 257
  • Country: us
  • Student
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #14 on: January 28, 2025, 02:07:17 am »
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.
And 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.
 

Offline EEVblogTopic starter

  • Administrator
  • *****
  • Posts: 39277
  • Country: au
    • EEVblog
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #15 on: January 28, 2025, 02:24:47 am »
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.

Yes, same meter. Just marketing having a wank.
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15227
  • Country: de
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #16 on: January 28, 2025, 08:42:08 am »
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.
 

Offline ballsystemlord

  • Frequent Contributor
  • **
  • Posts: 257
  • Country: us
  • Student
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #17 on: January 28, 2025, 05:37:24 pm »
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.

You mean, gasp, we might actually get bigger batteries in our DMMs than AAA cells? ;)
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7765
  • Country: ca
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #18 on: January 28, 2025, 06:28:25 pm »
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 are oddball, 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
 

Offline EEVblogTopic starter

  • Administrator
  • *****
  • Posts: 39277
  • Country: au
    • EEVblog
Re: EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation
« Reply #19 on: January 29, 2025, 12:16:28 am »
You mean, gasp, we might actually get bigger batteries in our DMMs than AAA cells? ;)

The 121GW originally had AAA cells, but I decided to make the case thicker to accomodate AA instead.
 
The following users thanked this post: ballsystemlord


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf