|Build an RFI/EMI powerline filter for AM radios|
Another angle in getting better reception of weak AM radio stations on vintage solid state or tube AM table radios. This one is
to restrict RFI riding on the powerline from getting into the radio. Took
an EMI filter network from an old computer power supply or monitor, like this below left (Part values are not at all critical):
and placed it between the radio's power cord and the powerline input of the radio circuits. I cut out the section of circuit board from the power supply or monitor that has these parts and used it directly here. Use everything from the the old power supply's powerline input to its bridge rectifier. I'd also leave its ground not connected. We want to keep RFI from riding in via any ground or power wire. Added an extra coil too. Position the filter as far away from the AM loop or ferrite rod antenna as practical, to avoid magnetic coupling into the antenna.
You can tell if this RFI filter mod is a reasonable thing to do by this test: While listening to a weak
station with powerline RFI buzzes, disconnect the power plug and quickly listen to see if
the buzz goes away before the station and the radio dies. We're talking about 1/4 second,
before the main filter caps in the radio's power supply looses charge. You can repeat this
test after installing this filter and see if any buzz left over still stays on the station
after pulling the plug. Idea is that you effectively pulled the plug on the path the RFI was
taking. That your plug in radio acts like a portable on batteries. This mod almost makes it seem
like the early 70's on the AM band, before switching power supplies existed.
Here I cascaded two such filters inside an old PC power supply case (I kept the fuse in circuit to protect the coils from excessive current):