Products > Test Equipment
BM786 Switch Issue
Algoma:
Looks like about 5 quick scrapes with fairly coarse abrasive.
I would think A simple scrub with a fine steel wool would put texture and shine on the pads without scraping off vast gouges across the solder mask.
The scratch pattern looks like they're using an edge of a heavy grit sanding block that barely fits into the given space of the already populated board.
joeqsmith:
Like I said, I never asked them about the scratches and without taking that first step, I would have no way to who, why or when it was done.
Getting back to my comments about HV and the ohms function.
https://www.eevblog.com/forum/testgear/bm786-switch-issue/msg3516068/#msg3516068
https://www.eevblog.com/forum/testgear/bm786-switch-issue/msg3516184/#msg3516184
First, Brymen supplied four different meters. One meter was a non-functional, incomplete prototype. This is the meter I used for this test. As you can see from the pictures, the layout and scratches are similar to what was released. The meter was supplied with no wiper contacts. Sorting through the junk pile, I found a wide contact that came close to fitting. Of course, this is different than the contacts supplied with the meter. I used my kill-u-dead power supply which you may have seen blow one of my HV 400mA fuses.
Looking at the attached pictures, you can see how the wiper contact did not ride down the center of the pad and is off to one side. We can see that the heat was enough to delaminate the board. What is interesting is the size of the area of damage. Had the contact been centered, would we even see the delamination? Maybe not. I would even suggest that the plating could fail some time later. By chance, it looks like it made better contact on the other pad and there appears to be no damage. I tested the two transistors that make up the high speed clamp (Q4&Q5) prior to running the test and after. The both appear fine. I would have expected that but it's good to know.
Will the PTC and surge rated resistor limit the current enough to save the switch contacts under all conditions? I would say no.
Could the input signal be within the range of the meter and damage it? I would say yes.
Could the PCB be damaged without there being any damage to the front end and clamping circuits? I would say yes.
Could the damage be such that it creates enough heat to delaminate the PCB? I would say yes.
Are there sources of HV/HC that could do this level of damage that are common? Seems like the MOT is a common source..
Again, I have no idea what happened in this particular case. I do find it odd that the failure just happened to be on one of the modes using the high speed clamp. That the delamination is only on one spot of the entire board. That spot just happens to be where the wiper contact would sit when the ohms mode is engaged.
Algoma:
I can assure you, this meter only encountered light usage in a computer repair shop before it was set aside for exchange from Dave due to the noted issues as before. The replacement meter has worked flawlessly for weeks now.
the original meter in question was only subjected to the following list over the few days it was out of the box:
1) DHL's shipping process, they certainly did their best effort to indent the lower right corner of the the packing box.
2) Dell Laptop 19.5VDC 5A power supply, all tested good.
3) RH05020R00FE02 20Ohm 1% Vishay Dale Chassis Mount Load Resistor
4) CMF601K0000BHEB 1K 1W 0.1% Vishay Dale Axial Resistor
5) CMF60100R00BHEB 100Ohm 1W 0.1% Vishay Dale Axial Resistor
6) HEATHKIT IN-3117 DECADE RESISTANCE KIT
Highest voltage nearby would be 120v Mains power bar +- Static electricity .. at best.
I'm neither qualified or experienced enough go anywhere near the limits of a brand new meter I paid good money for, especially poking conductive things towards dangerous energy sources, on any range. Your original testing videos assisted in the decision to select this meter as a safe and well qualified meter for my usages. I'm simply a Network Engineer with curiosity about the foundations of the technology I work with. Not an electrician
The minimal usage it encountered is very unlikely to have caused any such damage to the trace, unless it was some form of high voltage qualification testing done at the factory.
Fungus:
--- Quote from: bdunham7 on March 27, 2021, 01:15:28 am ---I don't know if it has been mentioned before, but it looks like your picture is missing the same chunk off the end of the trace as the other meter that failed?
--- End quote ---
Who says it's missing a chunk? In Dave's video all the traces have square ends, even though there seems to be a round end on the PCB.
The trace at the top looks like a boolean union between the exposed gold trace (with square end) and the non-exposed trace under the solder mask:
joeqsmith:
--- Quote from: Algoma on March 27, 2021, 05:54:16 pm ---I can assure you, this meter only encountered light usage in a computer repair shop before it was set aside for exchange from Dave due to the noted issues as before. The replacement meter has worked flawlessly for weeks now.
the original meter in question was only subjected to the following list over the few days it was out of the box:
...
Highest voltage nearby would be 120v Mains power bar +- Static electricity .. at best.
I'm neither qualified or experienced enough go anywhere near the limits of a brand new meter I paid good money for, especially poking conductive things towards dangerous energy sources, on any range. Your original testing videos assisted in the decision to select this meter as a safe and well qualified meter for my usages. I'm simply a Network Engineer with curiosity about the foundations of the technology I work with. Not an electrician
The minimal usage it encountered is very unlikely to have caused any such damage to the trace, unless it was some form of high voltage qualification testing done at the factory.
--- End quote ---
As I have said, I would have no way of knowing what happened. For all I know, the meter was left sitting out and someone borrowed it for a test run. I wasn't there, I didn't receive your meter to evaluate and looking at it now would be rather pointless as any evidence left would have been taken care of by the end of a metal blade. All I can do is present data that shows that it is possible to delaminate the PCB while in the ohms mode while injecting a signal to the inputs of the meter as well as point out other details we can see from the video and pictures.
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