Broken UNI-T UT61c or am I missing something?

[mrkva]

Hello,

I just bought a UNI-T UT61C multimeter [ http://www.uni-trend.com/UT61C.html ] and it behaves quite weird. When I try to measure in mV range, it starts to beep and it is not measuring anything.

I made a video and uploaded it here:


Also, when I try to measure DC current, it doesn't work, and when I switch to AC current measurement, it shows very strange values, depending on the distance of my hand to the meter (!!??)

Any idea what can be the culprit? I already changed the battery - that wasn't the issue.
I am desperate, I was expecting a new multimeter to work... and now this.

Thank you,
Jonas

[alanb]

Have you tried connecting test leads?

[mrkva]

Of course - no help. [V] range works fine, mV simply doesn't. It helps when I switch to mV ac measurement and back sometimes, but most of the times not.

[alanb]

On the mV range what did you try to measure?

[mrkva]

I just tried shorting it (to get 0) or potential difference accros the skin (touching the probes by hand).. No results - just annoying beeping as in video.

[retiredcaps]

I don't have a UT61C, but from watching the video, it looks defective to me.  It shouldn't beep on mV.  For the A mode, it should show something close to 0.0 (not 0L).

I would exchange it/get refund or if you want to try and fix it yourself, take it apart and post some pictures here.  Maybe we can spot something?

[mrkva]

I wrote an e-mail to UNI-T and filed a complaint at Dealextereme (site where I bought it), will see what happens.
Actually I would prefer fixing it myself rather than spending additional money on sending it back to china, so this might become a teardown as well
I tried opening it today but didn't spot anything unusual - but I check the back only. Fuses are OK, no blown resistors.caps...

Thanks for kind words though

[retiredcaps]

I just bought a UNI-T UT61C multimeter [ http://www.uni-trend.com/UT61C.html ] and it behaves quite weird.

The stock photo used in the link above even shows the reading is 0.000 when set to A mode.

[retiredcaps]

Does the UT61C use FS9922-DMM4 for the main IC?

[mrkva]

Yes, it does. I saw it in this teardown: https://www.eevblog.com/forum/testgear/uni-t-ut61c-multimeter-teardown-and-review/

[retiredcaps]

Okay, I'm learning as I go, but see

http://www.ic-fortune.com/upload/Download/FS9922-DMM4-DS-11_EN.pdf

If you look at page 23 it says mV is input by SA (pin 14). It says current measurement enters the IC through R23 (also connected to SA).  Since these two functions are failing on your UT61C, see if R23 is 100k ohm.  This is assuming the UT61C follows the sample FS9922 datasheet.

Your R23 might not be soldered on properly or failed open circuit?

[mrkva]

Whoa, you're right. I got out of bed (its 1am here) and checked it out - R23 is giving me 1.12MOhm instead of 100kOhm!
So you think I should replace it?

[mrkva]

I was wrong, R23 from datasheet is different one than on multimeter. I tracked down the SA pin on the actual board, and it seems to be connected to 000 resistor (see pic). Do you think this might be the issue? Very strange

[retiredcaps]

I tracked down the SA pin on the actual board, and it seems to be connected to 000 resistor (see pic).

000 (0 ohms) is a SMD jumper. What does this SMD jumper connect to?  That is, follow the other side of 000 and see where it leads to?

[mrkva]

I traced it and discovered something missing (see pic). Could it be the 100k resistor? Would be greate if someone could take a look at their UT61C 
Thanks a lot for advices

[mrkva]

Interesting developments! I soldered in a 100k 1/4W resistor in the place and current measurement works!!!
But mV range still doesn't - in DC mode. It just beeps. AC mode seems fine.

[retiredcaps]

I traced it and discovered something missing (see pic). Could it be the 100k resistor?

It looks like there was something there, but maybe the solder paste didn't adhere to it and it fell off during the assembly or someone forgot to solder that on?  Maybe the meter was dropped in shipping and the resistor fell off?

If you look at electronupdate's video in 720p (high res) at the 15:12 mark, you can clearly see that something is occupying that pad (just left of C10).  Unfortunately, I can't quite make out the exact value as it is cut off at the bottom of the screen.

[retiredcaps]

Interesting developments! I soldered in a 100k 1/4W resistor in the place and current measurement works!!!

When you say it works, does the A range now show 0.000?  Is the current measurement fairly accurate compared to another multimeter?

But mV range still doesn't - in DC mode. It just beeps. AC mode seems fine.

For fun, can you try measuing an AA battery in normal DCV mode to see if that is working and accurate compared to another multimeter?

When you say AC mV mode seems fine, did you try measuring something or is it just showing something close to 0.0?

[retiredcaps]

What is the resistance measurement on PTC1 (just above the V input jack)?

[mrkva]

Good morning. Author of the teardown video was actually kind enough to send me a detailed photo of the board, so I am attaching it to this post. Strangely, it shows that the 'blown' resistor is 30k, not 100k as I expected. So I will try to fix that first and see what happens.
Voltage measurement in DCV mode is working OK, I compared the results with other meter. Current - I cannot compare (my other meter has blown fuse in mA range), but it matches my theoretical calculation of the circuit (LED + 1k resistor on 12V - ~12mA).
Will keep you updated on developments!

[mrkva]

Don't have 30k resistor on hand unfortunately :/ Will have to get one first

[mrkva]

OK, soldered proper 30K resistor in place, current measurement (still) works, but mV range STILL beeps. Any ideas what to measure?

[mrkva]

I measured the PTC and it works properly (1.2KOhm). Could it be the transistors under-neath?

[retiredcaps]

OK, soldered proper 30K resistor in place, current measurement (still) works, but mV range STILL beeps. Any ideas what to measure?

According to page 14 of the datasheet, the beeping is due to "the measured DC/ACmV mode is higher than 600.0mV".

The sample application circuit looks like the following.  I suggest you find the corresponding Q1 Q2 and the resistor hanging off SGND and verify all 3 components are good.

I also notice in both videos that when the UT61C is on plain DCV, there is roughly 350mV displayed.

[mrkva]

Hey,

I am trying to measure the transistors by measuring the base to emitter saturation voltage with diode meter. Q12 and Q11 appear to be short between base and emitter, Q6 and Q10 are reporting 0.620V which is within spec, as far as I understand.
So, do you think I should replace Q12 and Q11? They should have the same results since they are the same transistor (2n3904), right?

Thank you!