I have reviewed the mainboard of this oscilloscope, and I found some atrocities.
The A/D (ADC08D1010 on my oscilloscope) is powered by a TPS4022, entirely. OWON engineers are a little beast! But there is more.
The ADC08D1010 datasheet says (Pag.33): "The Va and Vdr supply pins should be isolated from each other to prevent any digital noise from being coupled into the analog portions of the ADC. A ferrite choke, such as the JW Miller FB20009-3B, is recommended between these supply lines when a common source is used for them"
Q: Where is the ferrite?
A: Very simply, does not exist. Why spend money on this? LOL...
Probably -7.6V at the PSU generated with a MC34063 switching at 100KHz, is used to power a portion of the input stage. And to do this before goes through a 79M05A (RR ~40dB @ 100KHz), ie in all probability the input stage is using +-5V. With this configuration -5V should have a good amount of noise, but surely +5V plane/line have more noise even.
I not know the new changes introduced by OWON, but unless OWON made a miracle I can not believe the results shown in the previous images. This problem has been corrected? Only with a new PSU and DC/DC board?
OWON engineers are as good as their customer service