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 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
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
1 – #279-420 Modular surface mount jack (4 or 6 conductor)
1 – #271-1720 Potentiometer. This is a 5k audio taper variable
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
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
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
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
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.
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.