UNI-T UT139B Bad Measurements

[EJC_LeBron]

Hi people!!

Since two years I've been working with my UNI-T UT139B multimeter without problems. Since monday, it started to work wrong.
The first thing I noticed was when I was measuring my house supply voltage (it should be around 220v), but my device showed 190v. Later I've tried to measure a 9v battery and it showed 7.5v.

I've compared my device with other and I've realized mine was broken. I've opened it, all apears to be in excellent conditions. I've cleaned it but the problem continued.

Anyone had a similar problem? Anyone have an idea about which could be the problem?

Thanks in advance!

[lapot2]

Is the power supply OK? Digital multimeters start to lie when the battery is low. Usually the old ones overestimated the readings, but more modern ones can underestimate them.
This is worth checking first, including the internal voltage stabilizers.

[petehall347]

first try a new battery

[lapot2]

I screwed calibration in my UNI-T UT139C DMM - Page 1

Try doing this, and if it doesn't help, check everything else you asked.
But if you can't or don't understand how to measure all this, buy a new multimeter. It will be easier for everyone.

The readings changed by 1.2 times in all modes. I suspect that some reference source has drifted away, and with a very small probability that the calibration is damaged. But it is worth trying to calibrate.

[EJC_LeBron]

Is the power supply OK? Digital multimeters start to lie when the battery is low. Usually the old ones overestimated the readings, but more modern ones can underestimate them.
This is worth checking first, including the internal voltage stabilizers.

I've changed the battery, but it didn't fix the problem. I'm going to check the stabilizers as you said.

[EJC_LeBron]

I screwed calibration in my UNI-T UT139C DMM - Page 1

Try doing this, and if it doesn't help, check everything else you asked.
But if you can't or don't understand how to measure all this, buy a new multimeter. It will be easier for everyone.

The readings changed by 1.2 times in all modes. I suspect that some reference source has drifted away, and with a very small probability that the calibration is damaged. But it is worth trying to calibrate.

I'm cheking that, it seems really interesting. At this time, I have a new multimeter.

Anyway, I'm going to try fixing this problem just as a personal challenge.

I will post any bad or positive updates about this.

[lapot2]

Yes, it will be a useful experience for both you and others.

[petehall347]

I've changed the battery, but it didn't fix the problem. .

now clean the rotary switch .

[EJC_LeBron]

now clean the rotary switch .

That's the only thing I've haven't done yet because it has a certain kind of oil on it, it seems to be something added by manufacturers to facilitate the switch's rotation.

And I'm not sure if that "oil" is really necessary or I can clean it with isopropyl alcohol.

[petehall347]

That's the only thing I've haven't done yet because it has a certain kind of oil on it, it seems to be something added by manufacturers to facilitate the switch's rotation.

And I'm not sure if that "oil" is really necessary or I can clean it with isopropyl alcohol.

post pictures of it