Modding The EEVblog BM786 Multimeter

[Astray]

I have a question concerning the differences between the BM789 and BM786 after playing around with each one. I wanted a BM789 because of the extra functionality but I really like the light blue color more than the red so I got both of them with the intention to swap the rubber and have a BM789 with the BM786 colors and then resell the BM786 with the red color. Yes I know I'm vain. Later found out that it's not really possible to swap the rubber since it's been fused to the plastic so then I had to look into swapping the electronics since the cases themselves are compatible.

However while playing around I found out the BM786 actually does have the LOZ mode! The only thing blocking access to it is the little plastic knob in the case that keeps you from turning the selector knob to the same position as the BM789 for LOZ mode. As far as I can tell, you can just cut/file the plastic piece blocking you from turning the knob counterclockwise from the off position one click and it should work fine. If you go to the other selector knob locations without functions it doesn't do anything and just stays off and doesn't appear to damage anything.

So my question basically amounts to are there any real differences between the two if the LOZ mode exists on both? Is the firmware slightly different? It seems like they might actually be identical in all but the plastic knob selector and colors but I haven't really delved deep into testing the other functions yet.

[J-R]

First, double posting is not allowed because it splits the conversation into multiple threads, causing confusion and frustration.  So probably edit your other post to keep it on topic for the ongoing clamp conversation.

Swapping cases really is a horrible idea on so many levels, don't do it!  You'll be creating two DMMs with incorrect labels, greatly damaging their proper usability and perhaps even someone's safety.

Hacking LoZ into the BM786 might be of interest to some people, I suppose.  Adding a small sticker or painting or etching/lettering into the top cover could probably be done.

The big test to see if that "hack" really works would be to measure the input impedance when in LoZ mode.  It should be roughly 2.1k Ohms.  Beyond that, verify that mode's full functionality and accuracy.

[EEVblog]

I split this post out, because if true it deserves it's own thread.
I have not thought about this, will investigate now.
In theory the only thing a LowZ mode adds is a PTC across the input for the low impedance. From the teardown photos I didn't see any PTC's missing. I don't have the schematic so would have to trace things out. But this can be measured at the terminals.
The blank PCB is the same for the 786/789, but they do have different firmware code base releases for some reason Brymen won't tell me. I think I did ask once why that's the case.
I'll try this mod now...

[Astray]

First, double posting is not allowed because it splits the conversation into multiple threads, causing confusion and frustration.  So probably edit your other post to keep it on topic for the ongoing clamp conversation.

Sorry about that, since my question concerned both models I figured it was okay to post twice in the respective threads. It seems the administrator has separated this into its own topic now and I've pointed others to come here from the other thread now.

Swapping cases really is a horrible idea on so many levels, don't do it!  You'll be creating two DMMs with incorrect labels, greatly damaging their proper usability and perhaps even someone's safety.

I definitely understand your concerns here and I would be very careful to disclose what I have done so anyone that ends up with my other DMM would know what's happened. The only reason I considered this to be okay is due to transparency and the DMMs being so close in functionality and performance that it would not be a concern. They use basically identical electronics and fuses, fit in the same cases perfectly, and the only differences are likely due to firmware only.

The big test to see if that "hack" really works would be to measure the input impedance when in LoZ mode.  It should be roughly 2.1k Ohms.  Beyond that, verify that mode's full functionality and accuracy.

I'll leave this testing to people that have better understanding of the electronics involved. I'm not sure how I would go about fully testing the LOZ function without doing some research first and I probably lack the proper equipment to fully test all its functionality to verify properly.

[J-R]

Yes, they are generally close, but there are still quiet a few differences which will lead to confusion later on due to the different/missing labels.  For example, when in the mV position and dBm mode, you have to press RANGE to change the "dBm-Ohm" setting.  Similarly for temperature, it seems like you would press Select, but you need to use Range to change between T1/T2 modes, plus the label for T2 on the input jacks will not be there.  Some performance aspects are also different, possibly due to some parts binning.

For checking LoZ, just connect the two DMMs together, common to common, V to V.  Set one DMM to Ohms, set the other to LoZ or DC V.  In the DC switch position it should measure about 10M Ohms, while in the LoZ it should be around 2.1K Ohms.

[Astray]

Yes but I'm keeping the one with BM789 board and missing labels so that's my burden to bear. The functions on the BM786 with the BM789 labels just won't work so it won't cause much confusion I think.

Just did a quick measurement on the BM786 in LOZ mode and got ~2.1k Ohms though.

[EEVblog]

I tried it.
It does go into "LowZ" mode and PTC2 is the load, measures 2kohm, and it does heat up as expected when voltage applied. But I get no reading even with 100V AC in the input.
So seems like it needs the BM789 firmware 

[Astray]

Dang, that's disappointing. Is it possible to flash the BM789 firmware on it assuming you have access to the firmware and hardware necessary to flash it?

[EEVblog]

Just confirmed with Brymen that the BM786 does not have the LowZ feature in firmware.
I do have 789 firmware from 2021, I'll try that tomorrow. But given that no one else has access to the firmware or programmer, it's moot.

[Astray]

If that does work maybe Brymen or someone releases the tools and firmware sometime down the line it could be useful though.

[PushUp]

I wanted a BM789 because of the extra functionality but I really like the light blue color more than the red so I got both of them with the intention to swap the rubber and have a BM789 with the BM786 colors and then resell the BM786 with the red color. Yes I know I'm vain.


In early times I was also that "mad" about design, that I ordered ELMA (a Brymen-Clone with a bright green holster) paying for a 829S the same amount of money like a 869S from Brymen itself in red! 

Therefore I understand your problematic situation... 

The german dealer "welectron" offers a FW update for the Brymen BM780 series for 20 Euros plus shipping. Maybe you can "persuade" them, not to look at the colour and/or model number...eventually he/she is even part of the EEVBlog-Forum...

bilder hochladen handy

https://www.welectron.com/Brymen-BM780-Firmware-Update_1


Cheers! 

[EEVblog]

If that does work maybe Brymen or someone releases the tools and firmware sometime down the line it could be useful though.

No way they'll ever do that.

[TomKatt]

If that does work maybe Brymen or someone releases the tools and firmware sometime down the line it could be useful though.

On top of then having to support meters that people accidentally bricked using those tools, there's probably a significant liability issue here as well.

Yeah - that ain't ever gonna happen.

[Fungus]

Not to mention the safety implications of people hacking meters and messing around with firmwares.

[EEVblog]

Not to mention the safety implications of people hacking meters and messing around with firmwares.

One of the reasons KaneTest didn't want to open source the 121GW firmware.

[Astray]

Very understandable if unfortunate. By any chance did the firmware update on the BM786 to make it a BM789 work? Curious if that's even something that's possible or if they maybe put safeguards to prevent that.