BM257s faulty readings - Page 1
Forum section: electronics / repair

author: laban93
author_rank: Contributor
timestamp: February 28, 2022, 08:35:10 pm
content: Hey,I just picked up 2 faulty bm257s multimeters from the trash bin, in the hope of beeing able to combine nr1 wich had a cracked lcd with the other one. Now the LCD is fine, but the resistance readings are way of. 1,5Mohm when shorted and OL when open. The VAC and VDC range is all fine. Any ideas of whan could be the culprit?

author: rsjsouza
author_rank: Super Contributor
timestamp: February 28, 2022, 08:44:31 pm
content: Suggestions about such high ohms readings with open probes are usually related to PCB contamination or a bad protection component. Some threads around here talk about it:https://www.eevblog.com/forum/repair/fluke-179-off-readings-(vdc-ohms)/

author: laban93
author_rank: Contributor
timestamp: February 28, 2022, 09:34:09 pm
content: It should be high ohms when probes are open, the issue is that shorted probes gives a 1,5Mohm reading instead of a low resistance. I have also cleaned the board and visualy inspected components and traces for damage. Did a simple diode and resistor check also.

author: giosif
author_rank: Frequent Contributor
timestamp: March 01, 2022, 12:08:32 pm
content: Can you switch to manual ranging in Ohms mode, then cycle through the different ranges and take measurements of the current being generated by the meter?

author: rsjsouza
author_rank: Super Contributor
timestamp: March 01, 2022, 02:04:10 pm
content: Quote from: laban93 on February 28, 2022, 09:34:09 pmOh, sorry, I got the other way around.One thing that might influence ohms readings (although voltages would be affected as well) is if a protection device (a PTC) has become very high impedance due to a fault event.

author: laban93
author_rank: Contributor
timestamp: March 01, 2022, 07:10:38 pm
content: Quote from: rsjsouza on March 01, 2022, 02:04:10 pmI have checked, and could not find any PTC's that where faulty. Also the voltage function is spot on!

author: laban93
author_rank: Contributor
timestamp: March 01, 2022, 07:11:18 pm
content: Quote from: giosif on March 01, 2022, 12:08:32 pm150uA @ 1k, 40uA @ 10k, 5uA @100k, 0.5uA @ 1M, 0.1uA @ 10M and 100M.

author: bdunham7
author_rank: Super Contributor
timestamp: March 01, 2022, 07:49:50 pm
content: What does the display of the defective meter read in each of those ranges, either while shorted or while measuring current as you did there?

author: giosif
author_rank: Frequent Contributor
timestamp: March 01, 2022, 08:25:35 pm
content: Quote from: laban93 on March 01, 2022, 07:11:18 pmI don't know the actual current values produced by the BM257s meter, but the readings look legit to me (i.e. fairly round numbers).If these are indeed the correct currents for the different ranges, and since VDC readings are good (which would indicate the input section of the meter is good), I would suspect someone messed with the calibration of the Ohms function.I would first look to confirm those current values, though (someone here on the forum with the same meter who can quickly check?).Also, would you happen to have resistors you can use in the different Ohms ranges to confirm if any of the ranges gives accurate readings?

author: rsjsouza
author_rank: Super Contributor
timestamp: March 02, 2022, 01:35:28 pm
content: Quote from: laban93 on March 01, 2022, 07:11:18 pmI get similar readings in my Greenlee DM-200A (Brymen BM251S), so indicating the ohms path seems ok.157,37µA @ 1kΩ41,18µA @ 10kΩ4,89µA @ 100kΩ0,48µA @ 1MΩ0,03µA @ 10MΩ0,02µA @ 100MΩAt this point, my best guess would be indeed an out of cal on ohms alone.