Products > Test Equipment
BM786 Switch Issue
joeqsmith:
--- Quote from: EEVblog on March 17, 2021, 12:30:17 pm ---
--- Quote from: joeqsmith on March 17, 2021, 12:19:52 pm ---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.....
--- End quote ---
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.
--- End quote ---
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.
--- Quote ---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.
--- End quote ---
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 ...
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