Problems with this device. At this point, I think my unit has some kind of hardware failure. I'll try to summarize what I've found.
I bought it in March on AE, not from the official store, but at another store that has sold over 1,000 units. I didn't need it, but I saw a good deal and bought it. When it arrived, I did some quick tests and it seemed to work fine. I didn't notice anything strange about the packaging, everything looks as new, so I stored it away until now, which is when I discovered the problems.
It comes with FW version 3.0.6_III, and 2.0.1_III HW. I needed to check the amplitude of two related signals, and before doing so I checked the accuracy of this device using a DC voltage of 2.5 volts. The first measurements gave me approximately 2.55v on channel 1. It wasn't great, but not so bad either. After all, you can't expect great accuracy from a device with these characteristics. However, there were inconsistencies with channel 2, which gave more amplitude for the same signal. I used the calibration menu a couple of times with the same results. At some point, I restored the factory settings as indicated in the manual, and from then my measurements on both channels were 2.63 volts. After recalibrating several times I continued to get the same results.
What happened? After several attempts at recalibration I installed several firmwares: 3.0.5_III (the latest official version), 3.0.6_III, and 3.0.6_III_mod from GitHub. Everything remained the same after recalibrations, 2.63v, but I noticed that restoring the factory settings does not affect the calibration internal data, you have to do a recalibration after updating the FW to a different version, otherwise the zero reference is not correct. After recalibration this reference is always perfect on both channels.
I also found what may be a FW bug, or perhaps it only happens with my faulty unit, I hope other users will comment if they also have the following problem. In version 3.0.5, the signal amplitude depends a lot on where it is located on the screen, see the attached photos (measurement labels in yellow on black). In version 3.0.6, it seems to have been fixed, but not completely, it still happens, although to a lesser extent, if the zero reference is moved to the upper and lower limits (without exceeding them), see attached photos with measurement labels in white on cyan. I think for this reason i was getting 2.55 volts, because i have the channel 1 signal located at the bottom of the screen, after restoring factory settings this signal is located in the center of screen and displays 2.63v.
In any case, I am getting 2.63 volts for 2.5v signals. This also happens with the built-in generator wave, I have verified with another oscilloscope that the wave it generates is 2.5 Vp-p, but it is displayed as 2.63 Vp-p, like the 2.5v DC signal.
I have seen in YouTube videos that other units are reading around 2.48-2.5Vp-p for this generated wave, so my unit must have some kind of fault. I decided to open it up, and surprise! see the attached photo. The low-quality 100uF electrolytic capacitor immediately caught my attention, and even more so when I checked that it had been soldered in parallel with a SMD capacitor on the board. I looked at photos from other people and saw that it doesn't appear in any of them. The battery doesn't seem to be the original either, as it doesn't have any markings like the ones on other units. It looks like a botched repair of a returned device, I don't know if it was done by the seller or the manufacturer. The PCB appears to be of good quality and seems to be original. At first glance, I haven't found any differences when comparing it to pictures of other units, but I will have to take a closer look.
This capacitor is associated with the IP2312 battery charging IC, there is other user in this thread having serious problems with this IC. I will take a look at it, although I don't have much hope that it's directly related to the bad amplitude readings. The reading error magnitude is not fixed but linear: for 1.25v it reads 1.3v, for 2.5 it reads 2.63, for 2.8 it reads about 3v... for 10v it reads approximately 10.5x, with the same readings on both channels, the zero volts reading is perfect on both channels, same results at x1 and x10. It could be more like something related to the ADC reference being wrong for some reason, perhaps a component damaged by the fault that led to the repair? So I'll look into that too. Thanks to @jebem and other members for the info about the HW that appears in this thread, this will help me. I won't spend too much time on it either, as equipment with these characteristics isn't worth it, so if I don't find anything quickly I'll give up.
Finally, I would also like to thank all people involved in modifying the FW, and @rssdev for the executable to update the FW, which works perfectly on Linux.
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