UNI-T UT81C scopemeter teardown/review

[electr_peter]

Hello, EEVblog forum users,

I acquired UNI-T UT81C scopemeter some time ago. It served me well so I decided to do a teardown.
Other forum members have done a teardown/review of a more popular version UT81B (lower bandwith and sample rate spec.) - search forum if you would like to compare.

Teardown

I know that picture quality is not the best. If forum members are interested in something specific, I could redo the pictures.

UT81C scopemeter before teardown.


Teardown was an easy process - unscrew few screws and open the bottom of the meter.




Meter is constructed from 3 PCBs (buttons PCB + DMM PCB + scope PCB) connected via 0.254 mm headers and display board.


Overall, there is lack of shielding - see huge holes on top/bottom. Case sides are high and they are not shielded at all. Shielding contact is too stiff/sharp metal piece, which made a hole in the shield.




Input jacks are soldered directly to the PCB at one point. They are split type, bent metal. Certainly not as great as solid jack construction with stress relief.


Note that UT81C has not the best input protection (glass fuses, overvoltage protection could be improved).


DMM board has five crappy adjustment pots, processing PCB has one adjustment cap. I do not know exact purpose of these, so left those unadjusted.


Soldering quality of SMD parts are OK, but though hole stuff is messy (unclean, but solder joints are good enough). Note that processing board has 2 ICs that were manually soldered (resoldered) - maybe these chips differ between UT81A/B/C models.





DC input jack is recessed very far from the edge of the PCB, presumably to limit a leverage of DC jack and increase robustness.


Scopemeter works as before after assembly.


Review

Scopemeter specifications

• Bandwith:   16 MHz, 1 channel, 80 MS/s, no AC coupling (only DC coupling), 50ns-5s (1-2-5 steps)
• Voltage:   20mV-500V, current:   200µA-1mA/20mA-100mA/2A-5A
• Trigger:   auto/normal/shot, raising/falling (auto only with no trigger from 200 ms and up)
• Screen:   160*160 monochrome LCD with backlight
• Probes:   Multimeter probes or 1x/10x probe with adapter
• Other:   10 screenshots memory, isolated USB serial via IR adapter, 4 AA batteries or DC adapter.

   
Notes:

• 4*AA batteries are very hard to insert - space for batteries seems to be 0.5-1.5 mm too short
• Different time settings seems to have different performance (in terms of frequency, level response - possible mismatch between ranges).
• Input impedance is all over the place - check table below. Almost impossible to cal. probe and make absolute measurements if signal is in a wide range and your switch between DMM/scope modes.
     
• Oscilloscope range impedance, equivalent DC/AC range on multimeter:
• 5.017MOhm 200V-500V -> 1000V/750V DC/AC
• 5.017MOhm 20V-100V -> 400V DC/AC
• 5.043MOhm 2V-10V -> 40V DC/AC
• 5.410MOhm 200mV-1V -> 4V DC/AC
• 20.525MOhm/7.215MOhm 20mV-100mV -> 400mV DC/AC
   
• Multimeter, frequency/duty mode impedance:
• 10.507MOhm frequency/duty -> 1V
• Double impedance (~10MOhm 200mV-500V, 20.525MOhm 20mV-100mV)(similar to DC range) on multimeter mode, but significant capacitance added
• Voltage mode has good rise and fall time on "Voltage mode", but "Frequency" mode has terrible rise and fall time.
• Little shielding - mains 50Hz noise are easily visible. See unshielded areas and hole in the back shield. Poor shielding overall - no shield on the sides or battery compartment. Mains hum easily couples in.
• UNI-T multimeter-BNC plastic holder has intermittent connection with UNI-T scope probe (ground doesn't connect properly to outer ground ring).
• DMM side of things is OK - accurate enough, but slow update on screen and slow continuity.

Advantages:

• USB interface works, basic functionality (streaming data only). Live screen viewing though USB communication, can save data, show current and saved waveforms.
• 80 MS/s is true spec. Can test high voltages on scope. Scopemeter is useful for a single channel low freq. measurements.
• Killer feature - you can measure current waveform directly with DMM probes only! On µA/mA/A ranges!

Disadvantages:

• Issues with input impedance and absolute measurements.
• Not enough shielding - mains hum shows up if you are not careful.
• Probe adapter can be better (try not to lose this small piece).

I could add additional comments or pictures if requested.

[electr_peter]

i have a model B. how did you calibrate the unit? i need some tips on "sharpening" this tool

There are two problems of calibration to consider - DMM calibration (to correctly measure voltage/current/resistance) and oscilloscope response calibration (for frequency response flatness).

As for DMM cal. see other forum post https://www.eevblog.com/forum/testgear/calibration-of-ut81b/ about digital calibration.
Additionally, there are 5 pots on the DMM PCB (see pics) - these are easy to access, but I did not try to adjust these, because I do not have exact schematic. Maybe other forum members have a schematics/calibration manual? It would be interesting to see.
MJLorton is brave enough to adjust any of pots on DMMs - see some of his DMM related videos to get the idea (that is, tweak the pots and check them against stable voltage source).

As for oscilloscope response flatness, I measured/estimated my cables and DMM resistance/capacitance for each measurement range. I would direct your attention to the fact that mode change form scope to DMM and back changes actual ranges  and can significantly change impedances. At first with my setup I noticed that my signals roll off too soon - I made some simulation and measurements and got adjustment ideas.
Also, do not expect that 10x probe calibration capacitor will make any change for the better - scope input cap. seems to be bigger than typical normal scope. Add on top 5/10/20MOhm input resistance issues. 10x probe is not 10x with this scopemeter.
This is general idea. I cannot say anything specific, as your setup is different and input impedance of UT81* series are all over the place.

One last thing - cleaning those awful hand soldered joints would be a good idea.

[Lightages]

Thanks for the tear down and review. I have been considering buying one of these but felt very unsure about it. Now I am sure, I will not buy one.

[electr_peter]

Thanks for the tear down and review. I have been considering buying one of these but felt very unsure about it. Now I am sure, I will not buy one.

You are welcome, good decision. UT81C has very poor price/performance ratio - you can get much better multimeter for less (much less) or much better handheld scope with 2 channels for a bit more. Unless you get it real cheap, I would not recommend it.

Few things UT81C is good at is high voltage measurements in scope mode, AUTO mode for any waveform, isolated measurements via PC IR interface and current measurement in scope mode.

Regarding the scope function, I forget to add that scope adjustments are very hard to do right because there is not enough buttons for vertical/horizontal/trigger settings. You can have AUTO(which works really good) if you have stable waveform, otherwise you will be jumping between vertical/horizontal/trigger screens all the time.

[soptea]

[/quote]
You are welcome, good decision. UT81C has very poor price/performance ratio - you can get much better multimeter for less (much less) or much better handheld scope with 2 channels for a bit more. Unless you get it real cheap, I would not recommend it.
[/quote]
Please name some much better handheld scope with 2 channels for a BIT more...maybe at a double price .In his price range UT81B is best choice ,I see just 128x128 scopemeters or crappy 200MS/s  scope with MP4 case..

[mcinque]

It seems that the board (EDIT: OF THE UT81B VERSION) has been updated (i.e. no more wire-wrap cable corrections); I've spotted these pics on http://radiokot.ru/lab/otk/kotodromyxlo/10/

[R_G_B_]

I have one of these in bits. I did not like the scope it had poor triggering, piss isolation, battery capacity crap. total waste of money.

The hps140i is a better hand held scope but it also has issues with the software and it also has  very basic triggering.
 You can't trust the measurement with this the readings are way off. At least on my version 1313.

if you can find a fluke scopemeter going cheap id say this is your best bet save you money and get something reliable like a fluke 123 20Mhz or 124 40Mhz.  may be old but still its better than the  hand-held scopes mentioned above.   

[electr_peter]

You are welcome, good decision. UT81C has very poor price/performance ratio - you can get much better multimeter for less (much less) or much better handheld scope with 2 channels for a bit more. Unless you get it real cheap, I would not recommend it.

Please name some much better handheld scope with 2 channels for a BIT more...maybe at a double price .In his price range UT81B is best choice ,I see just 128x128 scopemeters or crappy 200MS/s  scope with MP4 case..

Not recommended (because of many issues), but if you have to choose from UT8X series, choose UT81A/B instead of UT81C. 1 ch poor scope is still better than no scope at all.

As for 2 ch handhelds, have a look at a Hantek model range http://hantek.com/en/ProductList_1_1.html. Hantek handhelds are similar to Rigol DS1052E series bench scope in capabilities. Hantek also released new DSO1000E and DSO8000E series recently.

Also, there is Owon 2 ch handhelds. See http://www.owon.com.hk/products.asp?ParentID=57&SortID=62
Look at newer range with colored rubber buttons.

Price and availability with handhel scopes (exotic specialist items) may be an issue depending on distributors - you have to shop around.

There is new player in the market - MICSIG. Also, take a look at GW INSTEK - they have few models.
If you prefer brand names or have money for this, consider new/used Fluke or Agilent/Keysight. Problem with most Agilent/Keysight/Fluke handhelds (apart from the price) is that they are too big.

[SantaClaw]

Yeah, I can confirm the Mains hum issue, in the right environment, I'll get 50 Hz sometimes 25 hz even using the Uni-T passive probes, so it's deffo an issue with the shielding itself.

[Dominik]

Can test high voltages on scope. Scopemeter is useful for a single channel low freq. measurements.
Killer feature - you can measure current waveform directly with DMM probes only! On µA/mA/A ranges!

Is there any other, newer Scopemeter on market, that have these features? I tested a couple of cheap China-Scopemeter, but no one can do current waveform (without external shunt/circuit and self-calculating).

Onfortuneatly long time base (e.g. 0,5s/div - 10s/div) don't work with single-shot trigger. Display is a really bad lcd. Actually lcd should be better readable for outdoor in sun than tft/ips/oled displays. But this one is really hard to read.

[Aldo22]

I don't know about a scopemeter, but this dirt-cheap device can do something like that.

https://youtu.be/MUzz0Dgoojk?t=1058

[electr_peter]

Consider Owon HDS120 scopemeter owon-hds120hds100-1ch-dsodmm-1mhz5msps-(september-2024)