Topband: plasma TV troubleshooting

[Jim Brown]

On Tue, 5 Dec 2006 09:29:58 EST, Telegrapher9 at aol.com wrote:

>You must also take care 
>to properly bond the filter case to earth ground 

HUGE MISCONCEPTION!  Current flows in loops, and the source of the noise current 
is the TV, not "earth ground." "EARTH GROUND" will not solve a noise problem -- 
the purpose of the bond of the filter case is to provide that current loop back to 
the chassis of the source of the interference, BY THE SHORTEST POSSIBLE PATH. That 
path carries noise current, and that current will create magnetic and 
electromagnetic fields. Your antenna will hear those fields, just like any other 
signal. Now, that chassis, MUST, by code, be connected via the green wire of the 
power cord to a proper "ground" within the building. But a connection to EARTH has 
NOTHING to do with eliminating the noise. It has ONLY to do with SAFETY. 

That "shortest possible path" is usually most easily obtained by breaking the 
power cord and adding the filter in series, VERY close to the set. And we all 
agree that we don't want to open the box (the neighbor's TV). ") Also, the green 
wire from the power outlet MUST be carried through from the wall plug to the set. 

>such that the connection cannot be removed from outside the box 

This is a limitation placed upon manufacturers in the design of product, so that 
users cannot defeat safety or filtering. 

>Let's say the AC line is not the point of egress. Connect the video input 
>back. Let's say the RFI becomes strong now. You will need a few hundred ohms of 
>common-mode attentuation at 1.8 MHz. A ferrite core and several turns of the 
>coax will do it, given the correct core. 

My experience is that FAR more than a few hundred ohms will be needed. Rather, I 
would aim for a few kOhms. See the measured data for ferrite chokes on my website. 

http://audiosystemsgroup.com/publish

73,

Jim Brown K9YC