Hi.
I liked @Protegimus' suggestion of adding polyester capacitors to the primary and secondary SMPS of the MSO5000.
The effect of drastically reducing the NOISE FLOOR was excellent, from 202.93uVrms to 67.715uVrms, on the 1mV scale, remember that this scale has the BW locked at 20MHz, see the “B” on the 1.00mV side.
The NOISE FLOOR reduction was 66.63%. This, in terms of NOISE FLOOR, elevates the MSO5000 to a higher level of quality.
One detail I noticed is that in the Excel BOM, the 0.1uF/200V and 0.01/200V capacitors, in parallel with C1, have insufficient insulation, the voltage there can reach Vp>340V. I suggest, in those positions, 630V isolation.
I also verified that a 10nF/J/400V polyester capacitor was added, in parallel with CS5, which is the +5VDC voltage output.
My suggestion is to add a 10nF/J/400V capacitor in parallel with the CS1 capacitor... You can keep the 10nF/J/400V capacitor, in parallel with CS5, without any problems. (see the photo).
Maybe the NOISE FLOOR will decrease a little more.
In my opinion, the topology of this MSO5000 SMPS is Fly-back. With the IC-U1 controlling the PWM and the TR POWER MOSFET, which does not have silkscreen on the board.
I also verified that the soldering of PTH components are not excellent. The suggestion is to carry out a welding review of these components, with a soldering station at 480ºC, and good quality solder.
Request: if anyone has already reverse engineered this MSO5000 SMPS, please post the circuit here in this topic. Identifying the name of main components such as IC-U1 and POWER MOSFET.
As soon as possible I intend to do this update on my MSO5074.