Uni-T UT81B "Scopemeter"

[shakalnokturn]

I recently bought one out of curiosity, well it was under another name, if I'd known at the time it was Uni-T I would have been less curious...

The idea behind the thing isn't that bad and it feels as with a lot of Chinese equipment they got the job half done.
It's a 4000 count DMM built around the FS9721 on front PCB. The back PCB takes care of power supplies, scope (ADC, gain/attenuation), the display and scope functions are mainly FPGA based, a NXP MCU reads the FS9721 serial data before display on LCD. It also reads the keyboard and links to the scope functions.

First problem found:
Negative offset on scope from -150 to -500mV depending on range with ramp-up, also displayed on multimeter DCV reading and measurable at the input sockets.
I expected some form of contamination (usually battery leakage). All I noticed was that both PCB's were covered in white flux residue. They both got a thorough wash leaving them with a clean look.
Offset problem was unchanged so I had to start real troubleshooting. After the function switch the V input branches off in parallel (not switched as the scope uses the DMM,s voltage reading) one side to the FS9721 10MΩ input the other to the scope's 10MΩ input (so yes, that's a 5MΩ input).
Ended lifting both 10M resistors, the voltage was coming neither from FS9721 nor from scope input yet still measurable on input sockets.
The problem turned out being conductive solder flux (not rosin based) accumulation under a header interconnecting both PCB's. There was leakage between 3.3V supply and input signal.

Now this is solved I have a question to other owners:
Can anyone confirm the scope is DC coupled only on this meter?
Changing to AC in DMM mode will make the scope display AC voltage values but trace still displays any DC voltage at input. I can't see anything obvious on PCB that would be used for switching a DC blocking capacitor.

Now I've found another problem to tackle:
It came with Ni-MH batteries and a 6V adapter.
The manual doesn't mention anything about in-meter charging only that power is batteries or adapter.
Adapter is working but won't supply the meter... I'll look into that soon. I'd like to know if it is meant to charge the batteries though.

[electr_peter]

Can anyone confirm the scope is DC coupled only on this meter?

IIRC AC mode should work, but AC/DC mode setting is not straightforward between scope/DMM view. I can check this week.

It came with Ni-MH batteries and a 6V adapter. The manual doesn't mention anything about in-meter charging only that power is batteries or adapter. Adapter is working but won't supply the meter... I'll look into that soon. I'd like to know if it is meant to charge the batteries though.

Do you talk about big mains adapter with long DC plug? UNI-T UT81A/B/C adapter is intended for powering DMM only, not for charging NiMH batteries (default batteries are alkaline). See review UNI-T UT81C scopemeter teardown/review for picture of DC socket.

[shakalnokturn]

Thanks for the reply Peter.

Switching the display in scope mode to AC is done in DMM mode by using the AC/DC toggle key.
All the same I'm under the impression this only changes voltage readout in scope mode, the trace seems to be DC coupled however. The manual does mention something that seems to confirm, another owner's input would be welcome though, given the effort that was put into the construction of the meter I hardly trust the manual.

Yes exactly that adapter. So it isn't meant to charge batteries...
I'll still have to look into why it isn't powering the meter at all.

[electr_peter]

Bandwith:   16 MHz, 1 channel, 80 MS/s, no AC coupling (only DC coupling), 50ns-5s (1-2-5 steps)

[electr_peter]

Scope mode does not have AC coupling. AC/DC setting refers only to DMM mode. Values (AC/DC V) in scope mode come from DMM and are not calculated from sampled waveform (thus 0 AC V wil be shown for high frequencies).

UT81C rated BW is 16MHz, but it kinda works till ~40MHz (half sample rate of 80 MSa/s). Sinc interpolation seems to be used at high sample rates. I would suggest to add steeper low pass filter to not include >40 MHz signals.

[shakalnokturn]

That's a shame for the lack of AC coupling, it's a big miss for a very few extra components.
I pondered on modding it to add a DC blocking capacitor and SSR or reed relay either reading FS9721 unused LCD output or serial data output.
My other big grudge against it is the jittery trigger, I doubt I'll be holding on to it for long. (Anyone interested?)

The power adapter problem turned out to be a melted input jack, ordered a few on Aliexpress should have them in a month or so...

[coppice]

The UT81B has been around a long time. It used to be a low cost way to get a basic waveform display on the move, but the price went up. Its time has passed.