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

 made little difference in the performance of the circuit.



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



Mike McCaule