OK on your efforts at installing a filtered IEC fitting in the older power
It is not an original idea but I decided an 'easy' solution might be to
a filtered IEC socket into the power supply box. A quick look at the spec
suggested that a 1 Amp or 3 Amp rated unit would be best as the chokes
a higher inductance in the lower current rated sockets. I decided that
although a 1 Amp rated socket should be OK for a 250W supply I would play
safe and install a 3Amp unit (I also just conveniently happened to have
in the junk box!!).
The results are quite pleasing. The clutch of six or seven 50Hz spaced
lines, with the strongest at S9 have disapeared and I can just see two or
three weak lines (<S1) on the waterfall display when the background is
during the day. I suspect I may be able to get rid even of these by a bit
careful re-routing of the leads and aerial feeds.
Be wary of the range of inductance values used in IEC sockets. Some
filtered sockets have rather low rejection in the LF band, even if they are
fine for VHF/UHF broadcast bands.
Another problem with some computer power supplies is that there are pairs of
wire links "substituting" for a ferrite suppression choke in the mains input
section. Suppression chokes have standard pinout, so they can be
retrofitted if you can salvage them from some other PC junk.
An external filter can also be effective, and is not so constrained as to
size of inductors, for example, potcores can be used. At LF conducted noise
is the main problem and the extra metre or two of power cords to reach the
external filter is acceptable at the lower bands, as such a short lead has
negligible "radiation resistance". Having an external inline filter also
allows the computer peripheral devices to have their mains cords filtered as
well (almost any PC device has a ruddy switch mode power supply, I have yet
to find one that has no radio noise on the lower bands). Having an external
filter is also quite handy for testing as sometimes it is the power supply
in a monitor rather than the PC box that generates worst radio noises. For
some time I have used an external inline filter on the household PC and
another on the household TV, video, etc.
Keep an eye out for mains filters at junk sales ....