Brymen BM235 incorrect DCV readings

[Alex Kingston]

I picked up the EEVBlog BM235 a few years back and it has worked great up until a month ago. I started having an issue with the DCV measure function where the resolution has decreased significantly(on the order of a few volts). I recorded a video(link below) comparing the reading of the BM235 to another working meter as I swept the DCV range on a power supply. Its important to note that the readings increment does change with what range setting is selected. A very suspect example is on one of the ranges it only displays increments of 3.3V. I have confirmed that all other functions of the meter work flawlessly and the highest voltage it has ever been exposed to is 120VAC. I have also inspected the board for any visual cues and don't see any blown chips. Any insight is welcome, thanks!

[joeqsmith]

That's an odd one.  Just to rule out any problems with the power supply causing the meter to hunt, if you charge a cap and let it slowly discharge to the meter do you see the same effect?  Just removing the supply from the equation.   I doubt it would be the cause but may as well rule it out.   Didn't appear you had a scope...

[shakalnokturn]

Unless it reacts in a similar manner in DCA the only cause I can think of is a corrupt serial EEPROM.
Unfortunately if that's the case you'll need a good dump, a programmer and to readjust your meter.

[shakalnokturn]

Although your case seems unique, a search on the forum does return a significant number of BM235 with various strange problems.

A couple of them:
https://www.eevblog.com/forum/testgear/very-weird-problem-with-the-bm235-select-button/msg2368155/

https://www.eevblog.com/forum/testgear/(help)-how-to-switch-bm235-out-of-hexadecimal-display-mode/msg4100923/

So either a defective ASIC (but in your case I fail to see why the ADC would behave normally for other functions) or corrupt configuration data which would be worth giving a try.

I've been nice and gone to get you the only dumps I found hanging around although I haven't tested them myself, now if anyone with a wonky BM235 wants to send it over I'd be happy to experiment...
The dumps are from here:
https://remont-aud.net/dump/prochie/izmeritelnye_pribory_i_programmatory/brymen_bm235_shassi_main_board_bm235/483-1-0-119291