Products > Test Equipment
Brymen BM789
floobydust:
At the bottom it's shunt R49 plate resistor MPC 72 or 73 I can't see all the text.
At the top box, the film cap looks small and one end connects directly to the DMM IC analog switches, so I think it's a filter that gets switched in/out maybe for VFD's. It's near the low voltage business on the board.
You can discern a bit from the switch video too https://www.eevblog.com/forum/testgear/bm786-switch-issue/msg3515584/#msg3515584
I'll follow traces on C1, for DC-block it needs to connect to the rotary switch which is miles away it does at S1A1, S2A1 but not shorted out for the majority of functions like DCV, ohms etc. so I'm still skeptical.
The user manual mentions for 600mVAC "Signal peak absolute values, including DC bias, less than 1000mV peak" which I take as a 1V max. so no cap.
bdunham7:
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--- Quote from: floobydust on September 21, 2021, 04:37:08 am ---I'll follow traces on C1, for DC-block it needs to connect to the rotary switch which is miles away it does as S1A1, S2A1 but not shorted out for the majority of functions like DCV, ohms etc.
--- End quote ---
Actually it looks to me that C1 is within the borders of the switch cover, above the + jack and to the left of the PTCs. I tried looking at Dave's videos regarding the switch problem where he has it torn apart, but they were mostly too closeup. I'm betting C1 is it.
Edit: C1 is right under where the switch contacts for the ACV range would be. R49 is in fact a 1R resistor. I'm guessing the reason for the combined mV system is that they were just out of switch positions due to the design not being 360-degree capable
joeqsmith:
I believe that's it but again, have not looked. I gave a little demonstration on using a 100 year old blow torch to do some surface mount soldering and had pulled this capacitor from the board. It ended up stuck on the one pad. It would be easy enough to measure the value and trace it out.
10000pF at a kV+ seems doable in this package.
https://product.tdk.com/en/system/files?file=dam/doc/product/capacitor/ceramic/mlcc/catalog/mlcc_commercial_highvoltage_en.pdf
Neutrion:
OK so we now know that there is at least a positive aspect of the DC coupling, so if Brymen could manage to adress the "overrange problem", it could be safer on the mV scale that some other meters.
I was trying to characterize the meter to know above which level I can not trust it anymore on the AC mV scale. In the worst case scenario when the width of the squarewave is just above the declared minimum of 350us this safety limit can be as low as 400-430mV. Above which the meter starts to show BS values. So it is not necessarily has to reach the 650mv displayed value to have something way higher real value. Even with 50% duty cycle It starts to show invalid values above 500mV.
Unfortunately we still don't know what happens, if we just raise the voltage to lets say 50V. Maybe it will overrange. What is this lowest safety marging with the 869?
The other question is, what could be the lowest safety margin with some kind of completely different waveform? Could it go even further down? Would be really nice to know.
Anyone has a newer firmware than the 08?
--- Quote from: joeqsmith on September 14, 2021, 11:48:24 pm ---So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested. You bet. Do I care, other than for a small bit of entertainment, not at all. :-DD I could run others but will wait and see what bdunham7 comes up with.
--- End quote ---
What was the exact waveform here?
And regarding the AC coupled mode:
Similar to the ipact wrench, or cordless drill tests where they are getting attached to eachother to see which is the stronger, we might see multimeter fights as well in the future! :)
It is really interesting that this issue surprises even experienced engineers. Might there are tonns of equipment which are getting damaged all the time because of this regulary, just people are not avare of the cause. Wonder how long they keep the charge.
2N3055:
--- Quote from: Neutrion on September 23, 2021, 03:36:24 pm ---
It is really interesting that this issue surprises even experienced engineers. Might there are tonns of equipment which are getting damaged all the time because of this regulary, just people are not avare of the cause. Wonder how long they keep the charge.
--- End quote ---
I don't find it interesting if experienced engineers would be surprised by this.
Thing that would come to my mind would be more like: they should know better.....
Don't get me wrong, I'm not talking about making mistakes. But if experienced engineer doesn't understand this, maybe he/she is not as experienced as they think. At least not in measurement and measurement instruments basics..
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