On Sun, 24 Jun 2007 20:28:56 -0700 (PDT), Michael Germino wrote:
>Good thing to try. Unless it's the DVD that's the problem.
Plugging and unplugging a cable does THREE very important (and
very different) things. First, it removes one device from a
circuit. Second, it removes a current path for noise current that
may be flowing from one ground to another ground. Third, it
removes an ANTENNA from the two devices that it connects. It is
VERY likely that both of those devices have a pin 1 problem (that
is, shield current enters the box and can cause hum, buzz, and
RFI).
The pin 1 problem makes troubleshooting more complicated than it
otherwise might be. For example, you remove a cable connecting a
DVD player from a stereo system and the RFI goes away. At least
three possible causes of the RFI.
1) The cable connecting the two boxes is acting as an antenna and
the TV is detecting the RF thanks to a pin 1 problem.
2) The cable connecting the two boxes is acting as an antenna and
the DVD is detecting the RF thanks to a pin 1 problem.
3) The cable connecting the two boxes increases RF current in a
third box that has RFI susceptibility (that is, the longer antenna
may be more efficient, closer to resonance, makes the total
receiving antenna have directional properties that favors your
station.
4) The DVD player is detecting the RFI due to lousy shielding, and
passing it along, but it has nothing to do with the cable between
them.
In other words, analysis is complicated by the behavior of the
cable as antenna. It is also complicated by the pin 1 problem in
either or both boxes. Ferrite chokes reduce RFI by reducing common
mode RF current (if its a coaxial cable, thats all flowing on
the outside of the shield). Thus, if a ferrite choke reduces RFI,
it gives us an important clue -- it tells us that the cable IS
acting as an antenna and coupling the RFI.
To learn more, we can use the dummy concept introduced by Bill
Whitlock. It uses a males to female barrel connector or cable with
the shields connected at both ends, but the signal conductor NOT
connected, and dummy loads at both ends of the cable. This
provides a path for the antenna current between boxes, but not the
audio signal. See www.jensentransformers.com, or my own RFI
Tutorial.
73,
Jim Brown K9YC
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