BM257s faulty readings

[laban93]

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?

[rsjsouza]

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)/

[laban93]

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.

[giosif]

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?

[rsjsouza]

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.

Oh, 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.

[laban93]

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.

Oh, 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.

I have checked, and could not find any PTC's that where faulty.  Also the voltage function is spot on! 

[laban93]

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?

150uA @ 1k, 40uA @ 10k, 5uA @100k, 0.5uA @ 1M, 0.1uA @ 10M and 100M.

[bdunham7]

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?

[giosif]

150uA @ 1k, 40uA @ 10k, 5uA @100k, 0.5uA @ 1M, 0.1uA @ 10M and 100M.

I 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?

[rsjsouza]

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?

150uA @ 1k, 40uA @ 10k, 5uA @100k, 0.5uA @ 1M, 0.1uA @ 10M and 100M.

I get similar readings in my Greenlee DM-200A (Brymen BM251S), so indicating the ohms path seems ok.
157,37µA @ 1kΩ
41,18µA @ 10kΩ
4,89µA @ 100kΩ
0,48µA @ 1MΩ
0,03µA @ 10MΩ
0,02µA @ 100MΩ

At this point, my best guess would be indeed an out of cal on ohms alone.