Metrix MX51EX showing offset voltage on startup

[JIROB]

I've purchased a Metrix MX51EX but it's showing a DC voltage of about -2.8v and OL on mV range.

Running a fuse check for F1 (500mA Littlefuse) it appears to fail OL indicating a blown fuse.

My instinct is that the offset on voltage on startup without any connections isn't going to be fixed by a new fuse and this may show an underlying issue that's more than a cal to fix.

Does anyone have any experience of these meters or any insight into what might be up with this thing?

If it's not an easy fix it'll be a return job....

Thanks for the help,

Jim

[shakalnokturn]

Usually battery contamination.
Start by checking under the black foam on the PCB..
How does it behave on Ohms?
Once you're fixed on the pollution, it's worth disassembling the rotary switch for a check or preventive regluing of the contact springs.

[JIROB]

Ah, thats good info, thanks. Not sure how to open this up to check inside though.
The battery compartment looks clean with no leakage signs. New battery makes no difference.

The Ohms is showing something similar @4Mohms, slowly dropping.
Clean on all other settings which is what had be baffled.
I've left it on for most of the day and it's slowly drifted down to -0.627V across that period but still shows OL on mV. Doesn't return to -3.7V previous reading even with switch off and on again.
Readings match my other dmm (Fluke and Metrix) when I test voltage. Does this make sense?

[shakalnokturn]

Yes, to me it further confirms the PCB is contaminated.
The readings won't be off in most cases, I'd expect some error for high value resistance measurements.

Opening the case isn't hard once you know, easy to butcher when you don't.
The clip-locks at the ends need pushing inwards while slightly levering the 2 halves of the case appart.
It's hard to know how much leverage to apply, the water-proofing washer does offer some resistance adding doubt to whether you've actually released the locking clips.

[JIROB]

Thanks so much for your help on this. Not had time to get into the thing yet but stumbled across someone doing a teardown of a B&K Precision 5390 which is a Metrix in disguise and has the same case etc..

https://www.eevblog.com/forum/testgear/metrix-mx56c-bk-precision-5390-multimeter-teardown/