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EEVblog 1663 - BM786 Multimeter REPAIR + InEr Error Investigation

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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

Kleinstein:
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).

ballsystemlord:
I find it rather odd and humorous that you don't have a schematic for your own branded meter which you do warranty on.

floobydust:
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.

EEVblog:

--- Quote from: ballsystemlord 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.
--- End quote ---

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.

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