I want to share my experience.
Today I received DSO203 and channel A was not working (no trace on screen (1) and was the suspicious calibration results (2)). First I’ve compared voltages on inputs AIN, /AIN, BIN and /BIN of ADC. AIN was much lower than Uref (1.22) and constand. BIN changed when I change vertical position of nonexisting trace on the screen using menu. After studying of schematic and PCB I’ve found the PCB etching defect in series with R23. (3)
After soldering of a wire (4) all works (5) and (6).