[morzh]

Gvel

Thanks.

This radio does not use line filtering, and, to be honest, I am not sure why it would need it.
I can try it, just to see if it does anything, but line filtering does not remove AC buzz.
(As I just wrote the buzz subsided after I found a disconnect).
It will filter out very high frequency noise, which you won't even hear.

What you refer to, I mean the article, is "Line filtering" caps, or (in case of switcher-supply) Y-cap. In the Switching Supplies (I designed few for very bad line conditions, like in India, where 270VAC is not uncommon where it should be 220VAC) they are used, and for a good reason.
Y caps are used to bridge the supplies from the "Hot" side to "Cold", to short-circuit common mode hi-freq. currents (going through chopping pulse XFMR), to keep the supply from radiating EMI (the one detected by an Antenna and a spectrum analyzer), and those are Y-rated, as failure will connect a customer to the AC line directly.
The line caps are used, together with a Common-mode chokes, also in the Switchers, to get rid of the conducted emissions, (detected by using LISNs and the spectrum analyzer).
That is all this is usually to make a device pass FCC/ETSI/CISPR22 and such.