Modem Noise Killer

      Modem Noise Killer (alpha version)

       With this circuit  diagram,  some  basic tools including a soldering
       iron, and four or five components  from  Radio  Shack, you should be
       able to cut  the  noise/garbage  that  appears  on  your  computer’s

       I started this  project  out  of  frustration at using a US Robotics
       2400 baud modem and getting a fare amount of junk when connecting at
       that speed. Knowing that capacitors make good noise filters, I threw
       this together.

       This is very easy to build, however  conditions may be different due
       to modem type, amount of line noise, old or new switching  equipment
       (Bell’s equipment), and  on  and  on. So it may not work as well for
       you in every case. If it does work, or if you’ve managed to tweek it
       to your computer/modem setup I’ d like to hear from you.

       I’d also appreciate any of you electronic wizzards out there wanting
       to offer any improvements. Let’s make this work for everyone!

       Please read this entire message and  see if you understand it before
       you begin.

       OK, what you’ ll need from Radio Shack:

           1 –  #279-374 Modular line cord if you don’t already  have  one.
                You won’t  need one if your phone has a modular plug in its
                base.  $4.95

           1 –  #279-420 Modular surface  mount  jack  (4  or  6 conductor)

           1 –  #271-1720 Potentiometer. This is a 5k audio  taper variable
                resistor. $1.09

           1 –   #272-1055  Capacitor.  Any non-polarized 1.0 to 1.5 uf cap
                should do. Paper, Mylar, or metal film caps should be used,
                although #272-996 may work as well.
                (272-996 is a non-polarized electrolytic cap) $.79

           1 –  100 ohm resistor – quarter or half watt. $.19

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           1 –  #279-357 Y-type or duplex modular connector. Don’t buy this
                until you’ve  read  the  section  on  connecting  the Noise
                Killer below. (A, B,or C) $4.95

       First off, open the modular block.  You  normally just pry them open
       with a screwdriver. Inside you’ll find up to 6 wires.

       Very carefully cut  out all but the green and red  wires.  The  ones
       you’ll be removing  should  be black, yellow, white, and blue. These
       wires won’t be needed and may be in  the  way.  So cut them as close
       to where they  enter the plug as possible. The other  end  of  these
       wires have a spade lug connector that is screwed into the plastic.

       Unscrew and remove  that  end  of the wires as well. Now, you should
       have two wires left.  Green and red. Solder one end of the capacitor
       to the green wire. Solder the other  end  of  the  capacitor  to the
       center lug of  the  potentiometer  (there  are three  lugs  on  this
       critter). Solder one end of the resistor to the red wire.

       You may want  to shorten the leads of the resistor first. Solder the
       other end of the resistor to either  one  of  the  remaining outside
       lugs of the potentiometer.  Doesn’t matter which. Now to wrap it up,
       make a hole in the lid of the mod block to stick  the  shaft  of the
       potentiometer through.

       Don’t make this hole dead center as the other parts may not fit into
       the body of  the  mod  block  if  you do. See how things will fit in
       order to find where the hole will go.

       Well, now that you’ve got it built  you’ll  need  to  test it. First
       twist the shaft on the potentiometer until it stops.  You won’t know
       which way to turn it until later.

       It doesn’t matter which way now. You also need to determine where to
       plug the Noise Killer onto the telephone line. It can be done by one
       of several ways:

          A.  If  your  modem  has  two  modular plugs in back, connect the
              Noise Killer into one of them using a line cord. (a line cord
              is a straight cord that connects  a phone to the wall outlet.
              Usually silver in color)

          B.  If your phone is modular, you can unplug the  cord  from  the
              back of  it  after  you’re on-line and plug the cord into the
              Noise Killer.

          C.  You  may  have  to buy a Y-type  modular  adaptor.  Plug  the
              adaptor into a wall outlet, plug the modem into  one side and
              the Noise  Killer  into  the other. Call a BBS that has known
              noise problems.

       After you’ve connected and garbage  begins to appear, plug the Noise
       Killer into the phone line as described above. If  you  have  turned
       the shaft on the potentiometer the wrong way you’ll find out now.

       You may get  a lot of garbage or even disconnected. If this happens,
       turn the shaft the other way until  it  stops  and try again. If you
       don’t notice much difference when you plug the Noise Killer in, that
       may be a good sign.

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       Type in a  few  commands  and  look  for  garbage  characters on the
       screen. If there still is, turn the shaft slowly until most of it is
       gone. If nothing seems to happen at  all, turn the shaft slowly from
       one side to  the  other.  You  should  get  plenty   of  garbage  or
       disconnected at some point. If you don’t, reread this message to
       make sure you’ve connected it right.

       ***END OF ORIGNAL FILE***

       ADDITION TO ORIGINAL FILE – 2/29/88 – Mike McCauley – CIS 71505,1173

       First, a personal   recomendation.  _THIS  WORKS!!!_   I  have  been
       plagued with noise at 2400 for some  time.  I  went  round and round
       with Ma Bell on it, and after they sent out several “repair persons”
       who were, to be kind, of limited help in the matter,  I threw in the

       I saw this  file  on a board up east a few days ago, and thought I’d
       bite. Threw the gismo together in  about  10  minutes,  took another
       five to adjust the pot for best results on my worst  conection,  and
       guess what? No more worst connection! A few pointers:

           1)  The  pot  need not be either 5K or audio taper. I used a 10K
               15 turn trim pot.  Suggest you use what is handy.

           2)  I used 2MFD’s of capacitance  (two  1MFD’s  in parallel) Two
               R.S.  p/n 272-1055 work fine. Remember that  about  90 Volts
               will appear  across  red & green at ring, so the caps should
               be rated at 100VDC+.

           3)  I ended up with a final series  resistance  value (100 ohm +
               pot) of 2.75K.

       I speculate that  one  could  probably  use 2MFD and  a  fixed  2.7K
       resistor and do  the  job 90% of the time. The adjustment of the pot
       is not very critical.  Changes of  +/-  1K made little difference in
       the performance of the circuit.

       Hope it works as well for you as it did for me.

       Mike McCauley