BM786 Switch Issue

[EEVblog]

[J-R]

WHAT, you didn't fix either of them?

FYI for people reassembling a BM78x, the top corners of the PCB must first go underneath two plastic tabs molded into the bottom case.

[Fungus]

What's this?



(also note dirty black line on the PCB above it)

Edit: Another view...


(note that it's in line with the Ohms range)

[tautech]

Good spotting Fungus. 

[EEVblog]

[DenCraw]

What happen if you switch with the tester tips on hi voltage in that model?
can you burn a trace?
just because is some carbon there... a question for Joe Smith... hehe

[station240]

Best guess that hole in the switch contact is the result of:
A. A bubble in the source PCB material that popped and was removed during etching.
OR
B. A hole in the PCB that got bigger during etching.

It's curious that part of the gold plating has also been ripped off.
Makes me suspect that part of the copper wasn't clean, and the gold didn't bond properly.
I'm sure one of the PCB companies have seen this sort of thing before.

[joeqsmith]

Fungus finds the fungus...

"... and it's in ohms range so you would never get high enough current passing through the uh input protection resistors here and the PTCs to cause some sort of ridiculous you know arching of like a across that contact. Its just its just not possible"

At the rated voltage (1KV) it certainly is possible to draw an arc large enough to damage the contacts.  I had made a video to demonstrate this after anther member had damaged a 121GW.    While you should never rotate the function switch with the meter connected to an active circuit,  a few of us have bad habits.  Mix that with a little HV..... 

Brymen had sent me one of the early prototype meters.   I wonder if I could replicated this hole....

[EEVblog]

At the rated voltage (1KV) it certainly is possible to draw an arc large enough to damage the contacts.  I had made a video to demonstrate this after anther member had damaged a 121GW.    While you should never rotate the function switch with the meter connected to an active circuit,  a few of us have bad habits.  Mix that with a little HV..... 

Except that's not what happened here.
The fault was there out of the box, this is not a used meter, and there is no charring.
It's a physical damage issue, not electrical.

[Fungus]

Fungus finds the fungus...

I saw it right away, didn't even pause the video.   

I wonder if you could fill the hole with solder paste and warm it up to fill the hole. It wouldn't last forever but it could make the meter work again.

Google says there's such a thing as gold solder paste: https://www.google.com/search?q=gold+solder+paste

I see 14ct and 9ct variations. Not as cheap as lead but cheaper than a new BM786.

(I volunteer to try it if you send me the meter.  )

[joeqsmith]

At the rated voltage (1KV) it certainly is possible to draw an arc large enough to damage the contacts.  I had made a video to demonstrate this after anther member had damaged a 121GW.    While you should never rotate the function switch with the meter connected to an active circuit,  a few of us have bad habits.  Mix that with a little HV..... 

Except that's not what happened here.
The fault was there out of the box, this is not a used meter, and there is no charring.
It's a physical damage issue, not electrical.

I would have no idea how the meter had been treated or what happened in this particular case.  When the 121GW was damaged,  the owner said the meter had not been exposed to high voltages.   You posted a similar comment back then about the input which was the catalyst for the video.  Of course, we did eventually get the real story in that case. 

The energy to do that couldn't have come via the V jack, there is at least a PTC + 1K in series with every path. Even if the contacts shorted the VA range contacts and the ohms contacts, worst case path is V jack to GND via a PTC + 1K.

https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1598122/#msg1598122

It may be difficult to trace it down as the evidence was destroyed.  With that area missing, it went somewhere.   If the wiper welded to the pad, I could see it tearing off a small bit.   As the switch is rotated, any damage to the wiper may have been swept away.   

[floobydust]

The PC board has such bad scuffs and scratches on it, around the rotary switch area. Was there a gorilla skateboarding on it?

Aha - on the switch pads, there's several other dimples (pothole in the making) as I can see pogo-pins have poked these switch pads too hard. Look at the innermost ring's pads pt. 2 at around 07:46 -  all indented by test pins.  I'd say the wrong pogo-style is being used - sharp point? NO. Second part of my hunch, also explains why the damage might be in the same location(s) on the other problem PC boards - the test fixture is trashing the boards. They might have a crown pogo there. Oops.
Dave, look at the PCB on the other boards with the Ohms cutting out problem, see if the same location pothole is there.

I am not a fan of this style of rotary switch, basically a copy of 1989 Actron patent in ANENG, Mastech, Uni-T too although they use copper wipers (no gold). The design ends up with too much force from the wipers and you can see the PCB is worn at the south-east outer rings, so the rotor is not sitting parallel to the PCB, it's crooked for another reason.
When will Asia improve and use phosphor-coated brass dual cantilevers? Quit copying 32 year old tech already.

edit: I notice the new UT61E+ at least copied Fluke's detent spring arrangement, four cams. Those usually feel really good vs ball bearings w/hard springs. Progress.

[Algoma]

The meter did work, provided it was left alone when taking measurements.

Out of the box, on the Ohms range, very light tapping, bumping, squeezing or flexing the meter would cause the reset behaviors as noted in the video. It was a behavior that improved with use over a few days (Likely the initial wiper contamination wore off?).

There was no issue switching between modes, only other issue was the two beep-lit modes could Alarm intermittently as if measuring open/short faults with nothing connected, at first very random at first, but it became more functionally reliable over a few days of use. (again, likely switch wiper contamination on those two modes).

There was minimal use of the meter, besides basic low voltage testing a few 19v laptop power supplies, and out of circuit diodes and resistors.

Initially pressing any of the buttons or putting general pressure anywhere on the case would cause the reset in the Ohms mode. It improved with use over a few days, until the reset could only be caused by nearly directly manipulating the dial as seen in the video, or putting direct pressure on the body of the meter such as firmly pressing, squeezing or twisting on the body of the meter.

[raftronik]

Why power switch has to be on rotary switch not separate switch. My Metex has separate power ON OFF switch, it is very usefull.

[Kartika]

Hi, I'm just receiving it and I couldn't resist opening it ... this stinks of production abuse. why? how is that possible ,,, such a good multimeter with a large and inadmissible omission ...

[floobydust]

Um, what is the omission you are talking about?

I just see a gorilla used 180 grit sandpaper on the rotary switch portion of the PCB. They should use an angle grinder instead. WTF are they doing. Those scratches are terrible on thin copper.
The rotary switch tracks don't look gold plated? Maybe they are sanding off the ENIG and plating something else there. edit: nope, the via's still are gold. So they have some selective plating going on here.
I don't see pogo-pin indentations anymore.

P.S. - If you clean the PCB with IPA, don't forget to lubricate the switch portion.

[Kartika]

Why power switch has to be on rotary switch not separate switch. My Metex has separate power ON OFF switch, it is very usefull.

yes, very useful. increases switch cycles

[Kartika]

Um, what is the omission you are talking about?

I just see a gorilla used 180 grit sandpaper on the rotary switch portion of the PCB. They should use an angle grinder instead. WTF are they doing. Those scratches are terrible on thin copper.
The rotary switch tracks don't look gold plated? Maybe they are sanding off the ENIG and plating something else there. edit: nope, the via's still are gold. So they have some selective plating going on here.
I don't see pogo-pin indentations anymore.

P.S. - If you clean the PCB with IPA, don't forget to lubricate the switch portion.

which lubricant do you recommend

this is not burned but the layer is separated ... i belive bad procesing

[floobydust]

The lubricant to use... that discussion will start a holy war here. Best to ask Brymen what they suggest.
What do the wipers look like on the BM786 rotary switch? as far as metal and plating.

I found mineral oil is commonly used on the PCB, Fluke I believe uses that. A contact cleaner and lubricant with polyphenylene ether (PPE) is the only thing stopping oxidation from bad connections, when a switch has poor metallurgy or p[oor mechanical design (low force).

Is that PCB pothole on your multimeter? I don't see it in your first pics so I'm a bit confused.

[tautech]

Um, what is the omission you are talking about?

I just see a gorilla used 180 grit sandpaper on the rotary switch portion of the PCB. They should use an angle grinder instead. WTF are they doing. Those scratches are terrible on thin copper.
The rotary switch tracks don't look gold plated? Maybe they are sanding off the ENIG and plating something else there. edit: nope, the via's still are gold. So they have some selective plating going on here.
I don't see pogo-pin indentations anymore.

P.S. - If you clean the PCB with IPA, don't forget to lubricate the switch portion.

which lubricant do you recommend

this is not burned but the layer is separated ... i belive bad procesing

Yep it's another dud one like this one Fungus spotted earlier.
Ask Dave to replace it.

[bd139]

I bet at the factory they had a couple of QC failures and identified the cause as contact plating issues. So they just sand the top of the board now because it made one work. Never trust a QC engineer. They’re almost as bad as software engineers.

Either way screw that - they’re added to my vendor shitlist now. Nothing should be shipping like that.

[AVGresponding]

I believe the picture Kartika linked in their last post is simply an enlargement of the original one that Dave linked, with their (Kartika's) opinion as to the cause.

It is not good to see those scrubbing marks though, very disappointing from a manufacturer of Brymen's reputation.

[Kartika]

Is that PCB pothole on your multimeter? I don't see it in your first pics so I'm a bit confused.

is not just my comment from the post before, sorry

[Kartika]

I believe the picture Kartika linked in their last post is simply an enlargement of the original one that Dave linked, with their (Kartika's) opinion as to the cause.

It is not good to see those scrubbing marks though, very disappointing from a manufacturer of Brymen's reputation.

this is like cramp processing ... it worries me, and I only hope this is a mistake ...

[joeqsmith]

Looking at the board, this pad is indeed the one used to select the high speed clamp when in ohms.   Looks like a 1.3K PTC and a 1K, typical.   I took some decent before pictures of this area and will attempt to reproduce the picture shown.