Digital multimeter is malfunctioning, need support.

[Harish Achar]

I have a digital multimeter MECO 81-USB. It's started to malfunction.
In DC voltage setup the display shows fluctuating voltage from 25mV to 150mV. I did check the same without connecting the probe and result was same.
I measured a 9V battery using this multimeter and it's started to show 12.48V.
And I put the dial to AC measurement and display shows 900mV AC without connecting anything (it also fluctuates).

Any suggestions?

Note: I replaced the battery too. And have the same problem.

Can anybody share a schematic or basic block diagram of this multimeter / any generic DMM?

 

[BradtheRad]

My brandiess (inexpensive generic) DMM started displaying wild readings. It happens whether a device is connected or not connected. Usually it settles down after I turn the dial a few times. Then I get sensible readings from it.

Even so a negative 4 is added in the last place so it reads -4 on the millivolt range. -.4 on the volt range, -4 on the millAmp range. Etc.

I suspect its age and cheap construction. Maybe static charge. I opened it and saw the back has a metal film. A tiny spring touches it. The spring comes out of the pcb. I think it assists in draining static charge, until tarnish built up inside. It occurs to me it might work better if I clean the inside of the meter, and scour tarnish from metal contacts.

Same for your meter?

 

[andre_luis]

Once you are apparently interested in fixing the equipment, I would start with the classic approach in repairing equipment that interfaces with the outside world, by checking the input stage, if there are any fuses or protection components that may have been blown due to a measurement exceeding the scale, or even of a measurement other than that for which it should have been used (
resistance vs. voltage, voltage vs. current, etc.).

 

[danadakk]

Definitely the rotary function selector switch in these low end meters, usually a wiper
contacting segments on a PCB, get dirty and oily from environment and need cleaning
periodically.

Generally speaking you can use contact cleaner or pharmacy alcohol to do this.


Regards, Dana.