USB Meter KWS-V30 : It seems broken but it just needs a calibration

[Filippo52]

I purchased this cheap meter on Aliexpress to take quick measurements while charging cell phones.



As soon as it was opened, the reported voltage and current values were incorrect by an unacceptable percentage equal to 7-8% for the voltage and also by 10-15% for the current. All faulty errors.
I tried to do a few maneuvers and suddenly I found myself first with even more incorrect values: errors of up to 50% less and then finally with all zeros.
This is really a big problem for this tester as the button, indicated with a red arrow in the first photo, is in a position where a "finger" can occur while inserting the tester and therefore carrying out a calibration or a reset in a completely unconscious way. I have read reviews of this tester where it was declared faulty which suggest this problem.



I thought of a faulty example, but then continuing to tinker I realized that the function button intended to change some readings in reality, if pressed while inserting the tester into the USB port, caused either a "reset" or a "calibration" of the meter.
With a little patience I managed to find how to calibrate the voltage:
just finish a reading by extracting the tester with the button pressed to read the watts and reinserting the tester, still with the button pressed. At start the tester will indicate the voltage value equal to 5.00 volts and from that point it will follow the variations in applied voltages.
Simply insert the tester in this mode into a USB port that is supplying an "exact" voltage of 5 volts and once the button is released we will have our device perfectly calibrated.

Unfortunately I was unable to find the similar procedure for calibrating the current that I believe should exist. I couldn't even find the instructions for this model. Other models from the same manufacturer have a very easy but still very different on-screen wizard.

Now I have the tester which is quite precise on voltage but which gives current values as low as 10%.
Opening the object I identified the shunt resistor (R005) for reading the current; but intervening on it, having to increase it to increase the reading of the measured current, is, as you imagine, very difficult.

I ask you if any of you have had more luck than me and using this object managed to find the procedure to calibrate the current measurement

Thanks in advance
​Filippo