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Modding The EEVblog BM786 Multimeter

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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:

--- Quote from: J-R on July 02, 2024, 02:42:39 am ---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.

--- End quote ---
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.


--- Quote from: J-R on July 02, 2024, 02:42:39 am ---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.

--- End quote ---
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.


--- Quote from: J-R on July 02, 2024, 02:42:39 am ---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.

--- End quote ---
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.

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