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The following article has been taken from issue 13 of TRSLINK Magazine, edited
by Luis M. Garcia-Barrio and Peter Besenbruch. The article is being re-
distributed with permission and may continue to be distributed freely provided
that no fee is charged for its distribution (excepting connect time fees for
telecommunications services, or a maximum fee of $1.50 to cover the cost of
computer media).
Linefeeds have been added to the file, as well as a left margin (eight spaces).
Modem Noise Killer (alpha version)
In this project, with some basic tools and a few
components from Radio Shack, you should be able to cut the
noise/garbage that appears on your computer's screen. I started
this project out of frustration at using a US Robotics 2400
baud modem and getting a fair amount of junk when connecting at
that speed. Knowing that capacitors make good noise filters, I
threw this together.
It 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), etc. 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. This is what you'll need from Radio Shack:
a) 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
b) 1 #279-420 Modular surface mount jack (4 or 6 conductor)
$4.49
c) 1 #271-1720 Potentiometer. This is a 5k audio taper variable
resistor. $1.09
d) 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
e) 1 100 ohm resistor - quarter or half watt. $.19
f) 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, pry open the modular block 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.
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. It 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.
Now that you've got it built you'll need to test it. 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:
1) If your modem has two modular plugs in the 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.
2) 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.
3) 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. 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 ORIGINAL 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 towel. 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 connecion! 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