hi again Mark
I have owned a scope for many yrs and to be honest i have never though about sticking it across the mains to do any measuring
I do notice on my Tektronix scope tho that direct in it can only handle 200V AC or DC max, But using the x10 switch on the probe would extend that.
I just dont know if a normal probe could handle that sort of voltage, I do know there are hi voltage probes available for scopes and if I was you I would be looking into going that way
Presumably you are referring to the mains filter in that PSU you and I were talking about in the other thread. ?
IF and ONLY IF you find a safe way to scope the mains.... what you are looking for is hi freq ~ 20 - 100 KHz noise on the mains circuitry.
I dont know what your knowledge of SMPS Switch Mode Power Supply(s) is like, but basically, the incoming AC mains is rectified and smoothed producing ~ 320VDC. That DC voltage is applied to a transistor/coil arrangement oscillating/switching at hi freq ~ 20 - 100kHz. That coil(s) is the primary side of the step down transformer to the secondary side from whence the low voltage DC rails are sourced.
Why do they use high freq you ask ? ... the higher the freq the more efficient the transformer becomes, it needs less windings and a smaller iron/ferrite core and this is huge advantage of a SMPS over a linear PSU with a huge heavy transformer that is running at 50/60 Hz.
cheers
Dave