Repurpose DSL Filter As Common Mode Telephone RFI Filter

[HighMountainRadio]

Greetings To All !

     The purpose of this Common Mode Telephone RFI filter is to prevent stray RF from your transmitter from being rectified internally inside the telephone's components and causing whatever you are broadcasting to be heard on your telephone.



     Here I shall explain how to repurpose a DSL filter provided by internet service providers for DSL internet service for reuse as a Common Mode Telephone RFI Filter. If you are not already familiar with these DSL filters just ask around, chances are someone who has or had DSL internet service has a few of these extra DSL filters laying around, squirreled away in a junk drawer.

     Once you obtain one or more of these DSL filters, carefully separate the two clam shell halves of the plastic enclosure by carefully separating the two halves with a good solid knife edge or something similar, taking caution not to dismember yourself in the process ! Once you have successfully separated the two halves, remove the small circuit board that is inside.

     Proceed my removing ALL of the components that are on the circuit board. Keep them for spare building parts if you like or toss them into the trash as we will not be using any of them for this small project. 

     Scavenge from old computer power supplies or scrap circuit boards, especially switch mode power supplies as you very often will find your source for the toroid core we will use to build the common mode filter. Carefully examine scrap circuit boards for ANY small toroid (donut shaped) ferrite core. The size is important only because once we remount it to the stripped DSL circuit board, don't forget, it will have to fit inside the plastic clamshell case for proper completion ! Any 'mix' of ferrite core will suffice, only the physical size is all that is important, any size that will fit onto the circuit board and inside the clamshell case will be fine.

     Now, next, you must scavenge some small gauge 'magnet wire' which is an enamel coated solid copper wire very often and almost always used in old scrap switching power supplies in one form or another, usually already looped around a toroid ferrite core for use as a transformer, it is very easy to scavenge !

     Now, next, cut two equal lengths of the magnet wire about 1.5 feet long to allow for extra wire to work with, it can be trimmed to size later. Temporarily, twist the ends of the two equal length wires together to hold them together while you will wind it onto the ferrite toroid core, bifilar wound, which means two strands of solid copper magnet wire of equal length looping the two strands through the ferrite toroid core. Be sure to leave ample extra length at the beginning end of the first loop for later connection to the stripped circuit board ! continue to loop the two strands of wire through the toroid core until you completely cover the toroid in side by side loops of wire. Leave about 5 inches of length at beginning and end of looped wire for later connection to the circuit board.

     Now, next, you must determine where the RED and GREEN center two wire terminals in a RJ-11 telephone plug and jack. We will not be connecting anything to the two outermost terminals of the RJ-11 connectors, so forget about them as we are only concerned with the two center most terminals of the RJ-11 connectors. now, take the two strands of wire that started the looping process and ring out the connector or carefully examine the circuit board to see where the tracks go on the circuit board for the RED and GREEN wires from either the female or male RJ-11 connector. solder the lead wires to these two terminals using ANY pre-existing, appropriate solder point that leads back to the RED and GREEN terminals. Solder these two wires into place. 

     Next, do the exact same thing with the remaining two leads sticking out from the toroid core to the same RED and GREEN, centermost two terminals for the opposite end RJ-11 connector. You may place some silicon adhesive to the toroid to hold the windings in place and firmly adhere the toroid core to the circuit board. Re-Install the circuit board back into the two clamshell halves and cement them back together, that's it, sounds like a lot but really it is very basic and a quick project once you have scavenged the necessary components. See attached pictures for visual aids.

     
     This is a very effective and cheap way of making them yourself and learning something new in the process !

Have fun ! It's what it's all about !

73  Spooky...  "Broadcasting From Somewhere High In The Remote Appalachian Mountains"

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