Uni-T UT61E shows negative DC voltage w/o probes

[yearmix]

Hi folks, I had an issue with my UT61E, it shows negative DC voltage around -0.28 V, I don't know how it happened but when I measuring DC I have a big difference between positive and negative values. I don't have an equipment for check right values but the resistance show correctly. When short probes together nothing change and continue shows -0.28. Please provide some help with it. ThankX

[retiredcaps]

Some suggestions.

1. Try a fresh battery.
2. Take it apart and look for debris, dirt in the meter and around the range switch area.
3. Look at the input terminals for dirt, debris, etc.

[wasyoungonce]

replace pot VR1 on the PCB.   The OEM are god awful and known to cause issues.   Schematics and images on this are in the ut61e teardown thread (below)

https://www.eevblog.com/forum/testgear/uni-t-ut61e-multimeter-teardown-photos/msg1580215/#msg1580215
 
https://www.eevblog.com/forum/testgear/ut61e-drift-and-recalibration/msg335037/#msg335037

[yearmix]

thanks a lot folks for your suggestion. I cleanup PCB in ultrasonic cleaner it doesn't help, do not change VR1 yet,but after opening found some strange things(see screenshot):
first one it's a "R" on PCB does it mean refurbished?
the second it's a "tower of capacitance" - 3 capacitors on top of each other
and third it's a lost capacitors
 

[yearmix]

in continuing of previous message  - fourth it's tiny clicking each half a second

[wasyoungonce]

The R is probably "Rework" meaning they had to rework some components for "CAL".  Many UT61E's have component stacks like that.

No idea what the tick is.  You can probably Zero the reading by shorting the probes and pressing the "REL" (Relative) button.  The Pot I mentioned is a known issue and it put mater DC readings out all over the place.  Replacing it....fixed!

[sky666]

Hi,
My UT61E cannot be zeroed in ACV or ACmV ranges with shorten test leads (auto or manual range).
The last digits always show a value that drops but never reaches zero and never responds to shortening leads.The value appears to be transmitted to any next range.
For example (ACV):
0.0052V
0.052V
0.52V
5.2V
Is this normal for True RMS multimeter?

[MosherIV]

Hi
Welcome to the forum.
It would be better to start a new topic (thread) but that is ok for a beginner.

No, it looks like there is something wrong. I would expect a very small random error voltage in AC, the true rms converter cannot do very small voltages.
It should not show 5.2V when the leads are shorted!
0.52V is also bad.
The others may be valid BUT the fact that it is alway '52' is suspicious!
Do you see 52 on top of actual measured values, eg 10V AC reads as 15.2V AC ?

(I have no idea what is wrong or how to fix it by the way)

[sky666]

Thank you for the answer.
It's kind of weird. As if it's not a true value, but the same change is bound to the last 2 numbers on the display in any range.
For example, when the multimeter is set to AC range, it starts with a value of 15 (0.0015V; 0.015V; 0.15V depending on the range) and progressively decreases (like discharging). In 1 minute it can drop to 0.5 or 1 (in 750v range is 0.5V or 1V, in other range is 0.005V or 0.01V). Shortening the test leads has absolutely no effect on the value at any time.
The measurement values ​​are identical if the measurement starts at a value of 15 or 2 for example.
However if REL. button is pressed the measured value drops (231VAC without relative button, 213VAC with relative button).
Too bad I can't find a stabilized source of lower AC voltage for testing. I'll try to open new topic ...

[yearmix]

So folks bad news - change of VR1 doesn't helps with the issue, any other thought?

[walker1234]

HI,i have the same problem with my UT61E. If somebody have the same problem here is how i "resolved":
I resolved the problem removing the resistor vr1, I applied bigger and bigger resistor and then I removed definitely.
I think that the problem is somethig with the microcontroller.
I removed the resistance VR1 but the resistance between point VR and ground is aproximatly 100 kohm.

So without the resistance VR1 the lecture of the multimeter is passed fom -0.420V to -0.04V.
I'm satisfy with this result but know it's not a long term fix.
Now i think that UT61E is a shitty multimeter.

[vladant]

I had clicking sound in different multimeter - it is either bad battery, bad connection to battery or bad/broken filtering capacitor in power supply line.

[wasyoungonce]

Pin 38 (Vr) is supposed to be around -100mVvolyage reference, to compare against.  This is set up by pin 37 the resistors etc.   If you look at the cct its not precise and the IC data sheet identifies this.  I did see a mod where an external reference "LT1790ACS6-1.25" was introduced which fixed this issue...and some cal drift.

Indeed some later UT61E multimeters incorporated this Mod IC or has pads to install your own.

http://forum.eepw.com.cn/thread/239953/1

This will (should) fix the issue.  Even if you have to mod the PCB with a sub daughter PCB for the LT1970 IC.  ALso use a good high quality pot with low PPM drift.