My new oscilloscope seems to be reading DC voltages very wrong. Connecting it to a rasperry pi's voltage rail (which is usually spot on 3.3v) gives a reading of about 4.5v: The probes are adjusted correctly (the 1kHz square wave has flat tops and bottoms, but is 5.6v rather than the rated 5v). Testing a battery pack, measured with a multimeter to be 5.1v, gives 6.9v: I have no idea what's going on!
The self calibration makes little to no difference, and there doesn't seem to be an option for manual calibration.
Here is the user manual (I have the 1022i version): http://files.owon.com.cn/probook/VDS_Series_User_Manual.pdf