[RFI] Core for common-mode choke?

[Jim Brown]

I have several thoughts. First, you left out some very important information about your 
problems.  It is impossible to consider the design of any filtering without knowing the 
answers to both of those questions. Toroids come with a wide variety of frequency 
characteristics, so you need to tailor the design of the filter to the operating frequencies. 

    1) What frequencies do you operate when you have problems?  
    2) What TV channels experience the problems?  
    3) Does the TV "blank" as you describe it if you switch to the video input (if it has one)? 
    4) What kind of receive antenna system do you have?  Does the set get a lot of signal, 
or are you in a fringe area?  Is there a good high pass filter on it?  

Also, have you considered the possibility that the TV may be poorly shielded, and that that 
could be part (or all) of your problem?   

If you think the problem is related to common mode coupling to ground via the AC line, 
perhaps you could run the set from a UPS long enough to see if that eliminates or reduces 
the interference. Frankly, I would doubt that mechanism and suspect shielding instead, 
since you've disconnected everything else. If it's shielding, I'd be inclined to sell the set on 
EBay and start over. 

Jim Brown K9YC

On Mon, 13 Dec 2004 22:46:34 -0500, jeff_list at burnshome.org wrote:

>I am trying to eliminate RFI problems with my TV. Even with all the inputs
>disconnected and terminated I still get moderate blanking of the screen when
>I key my rig at 100w. Hopefully, adding a common-mode choke to the AC line
>will solve the problem. Since I will probably need chokes on several other
>lines once I start reconnecting the various cables I want to find the most
>cost effective source for the chokes. 
>
> 
>
>Amidon lists FT-240-77 toroids for $9 each. Is there a cheaper source for
>these? MFJ-701 RFI Choke Kits are only $15 for four. Are these effective
>enough on the AC line?
>
> 
>
>If I find that I need a choke on the component video cable from my DVD
>player what should I use? It may be too big to wrap on an FT-240-77 toroid. 
>
> 
>
>Jeff Burns
>
>AD9T
>
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