Hi everybody,
finally I found you after some time searching the web for OWON users.
I bought a SDS 9302 a while ago and when testing it soon dicovered the noise problem.
At first couldn't believe it and wrote to OWON but they did not answer.
After opening the thing and further testing soon I located the problem. It's as you also know - I haven't yet read through the entire post here so forgive me if I repeat things already posted - the power supply, the main one but also the distributed little switchers as i found out later.
I couldn't stop my self and started to denoise the main PS. After that lots of spikes where still present and I contacted the seller of my OWON (Präzitronik Hennig / Messgeräte Chemnitz) and he send my mail again to OWON and this time they answered that they are already working on the problem and also send the service manual I had asked for.
So, meanwhile I was digging myself deeper and deeper into this Scope and finally realized what they did. IT'S UNBELIEVABLE!!!
I think I have to say now that I worked with Tektronix for about 10 years as a service engineer her in Germany. Now I'm retired since many years but still do electronics a lot .
Yeah, back to the OWON, Step by Step I removed the little Buck converters from the interface PCB and even one from the main board - AGAIN UNBELIEVABlE TO PUT AN UNSHIELDED HIGH FREQUENCY SWITCHER ON THE SAME PCB WHERE THE INPUT CIRCUIT IS!
Next thing was to give the input circuit a decent shielding soldered tight around the input circuit on the main PCB and with the covers screwed on for to have access still.
I'm not through with all the switchers yet. One is still left thats the one for the Display, not the backlight. I couldn't find any IC datasheet for this one and also the one that provides the power to the LED backlight. Perhaps one of you found that out and can post it here maybe again for me until I heve read through all pages.
Will also check the display maker, Maybe there is some app note with these switchers.
I have the schemes of what I did so far, could reduce the noise remarkably, and if you are interested can also post some photos of what I have done.
For the moment I will stop and wait for what you say.
Thanks for reading + best wishes.
Sarasir