Building My Own Oscilloscope

[boiss]

Your displayed waveforms have quite a few components present. What does the waveform represent, was the input S/C ? I think that you can you can use your pc as a scope to fault find at least to 20KHz (free software available on the net). I presume that the ferrite cored transformer is the EHT source. This must be screened with magnetic material.
It is necessary to find out where the noise!!!!! is getting into the deflection cct.
can you swap X and Y amps? is the result the same?
Great project !
Mike