edlee@chinet.chi.il.us (Edward Lee) (10/15/89)
[Moderator's Note: This was on a local group on Chinet, and I thought it
might be of interest to readers here. PT]
I obtained the following file last year from CBBS #1.
My thanks go to the uncredited author.
==================================
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 screen.
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)
$4.49
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
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. 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 ORIGNAL 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
==============================
Second addition:
26 July 1989
The Noise Killer also works well for me. I own a Product R&D 2400 bps
modem and have been irritated by noisy modem connections to Chinet and
CBBS #1 for the past few years. Today, I went to a Radio Shack store
and bought the parts for the Noise Killer for less than $4 dollars and
built it in less than an hour. (I am a klutz with a soldering iron.)
I used all the parts listed in the original message except for the
modular surface mount jack. The Radio Shack store I visited did not
have the jack, so I instead bought a #279-1261 Flexible Telephone
Extension Cord for $1.20 (it was on sale). I already had the modular
line cord and the Y-type modular phone adaptor, but I did not need to
use the latter. Turning the stick of the 5k audio taper potentiometer
counter-clockwise almost all the way made the connections to Chinet
and CBBS #1 nearly noise-free. I continue to tune the noise filter.
Parts list (not including modular phone cable) Cost
#271-012 Two 100 ohm resistors (use only 1) 1/2 watt $0.19
#279-1261 Flexible Telephone Extension Cord 30 ft. $4.99 ($1.20)
#271-1720 Audio taper potentiometer 5k ohms $1.09
#272-1055 1.0 MFD @200 volt metalized film capacitor $0.99
-Ed L
edlee@chinet.chi.il.us