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WA2ISE > TECH 12.03.21 04:07z 47 Lines 1964 Bytes #999 (0) @ WW
BID : 19965_AL0Y
Read: GUEST
Subj: powerline RFI filter for MW SW AM radio sets
Path: HB9ON<IW2OHX<UA6ADV<LU4ECL<VK2RZ<VK2IO<W0ARP<KF5JRV<N3HYM<AL0Y
Sent: 210312/0406Z @:AL0Y.#NNJ.NJ.USA.NOAM #:19965 LinBPQ6.0.21
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):
-------------eeeeee---- ----eeeeee---- ----eeeeee---- ----
powerline | | |
mains = = R 680K to load
| | |
-------------eeeeee---- ----eeeeee---- ----eeeeee---- ----
2x7mH 0.2uF 2x56mH 0.2uF 2x56mH
These values not 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.
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