I have news about how to remove the Nickel Shield.
After, no success with blue tuboflo, I come back with the mix of HCL+H2O2. I make a fresh 37% HCL and a fresh 35% H2O2.
From the HCL(37%) I make a 14% solution (this done with warm water).
I left the front plastic case of osciloscope into this solution for 4-5 hours.
After that there is no clean complete but after drying I see that there is no any continuity of surface (I measure it with multimeter). Seems like to away the Nickel layer and still a paint lacquer light gray colour.
Photos I''ll upload later.
I tested it complete open (only with the front case) and noise decreased by 10mV.
The same period with the test, I had took some flat ferrites from farnell.
Before I had test the area of tft flat cable that joined with adapter board by the probe tip and I have seen some major noise over there. It was a mysterious combination noise of tft and psu TR1 transformer.
For that reason I decide to order two flat ferrites from Farnel, a
FPC-31-12K 26mm KE Kitagawa flat ferrite and a
SSC-33.5-20M 26mm KE Kitagawa flat ferrite (datasheet:
http://www.farnell.com/datasheets/31877.pdf.
The
FPC has better result to noise. With complete open oscilloscope (only front case) I came to 34-44mV (normal or peak) and 20-24mV (Average 16/100-200usec)
Finaly, it wasn't accident that Owon puts this ferrite to this area at the later oscilloscope versions.