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 fair 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 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

***END OF ORIGINAL FILE***

ADDITION TO ORIGINAL FILE - 12/5/88 - Paul Eckert, Fort Worth, Texas

	Okay, I'll admit it.  When I first saw this file about fixing
telephone noise problems I was at the end of my rope.  I couldn't get
my 1200-baud modem to work reliably at all!  I could get 30-40
characters of good data, then line after line of garbage.  I
subscribe to Ma Bell's indoor repair service where you pay
$1.50/month to have Ma Bell come and fix interior line and phone
problems--I'm calling up tomorrow to cancel it.

	I called in early September and Ma Bell told me they could only
run a check of the lines from the main office up to my house.  They
couldn't check any further unless I had "OnLine" interior service at
$1.50/month.  But they did send someone over to "check the lines".
He said it all looked okay.  Right!  He wasn't watching the garbage
come in on my modem.

	I then called up Ma Bell and subscribed to their monthly
"OnLine" service and after waiting a week, called Ma back.  This
time, they sent another guy who verified that (1) Ma Bell's lines
into my house were great;  (2) Ma Bell's lines from the entry box to
my modem were great.  Right!  I showed the service tech the modem
problems and he blamed my modem.  I swapped out the suspect modem
with a spare, and he blamed my computer.  I had a friend bring over
his computer and we tried it again and got as much garbage as the
City of Fort Worth Municipal Dump.  He was stumped, but the tech told
us that maybe our line was too good.

	"Too good?"  I repeated, dumbfounded.  "What does that mean?"

	He told me that if the line wasn't "lossy enough" (really!) that
the modem wouldn't work well.  He therefore attached a standard, Ma
Bell-issued black box to my line to "filter" it.  (These are his
words, really!)  It didn't do a bit of good.  I was as disgusted as
could be.  At 300 baud I could dial out and dial in, but life is too
short for that speed!

	I got the original file above from the Tarrant County Junior
College Dept.  of Engineering Technology Bulletin board (817)
571-4361.  I didn't hold out much hope because it just looked to me
like a little RC filter that wouldn't do diddly.

	But because it was cheap and I drove by a Radio Shack every day
on my way to work, I figured I'd slap it together.  I used Mike's
suggestion of two 1.0 mf caps in parallel and got an old 5K audio
taper potentiometer for the variable resistor.  Doggone it!  It
worked like a charm!  Mike was right when he said that the R value of
the pot didn't make much difference.  There was only a slight
degradation between 100 ohms (at the low end) and 5100 ohms (at the
high end), but the noise was mostly gone!

	I'm a believer, now!

	If you've got modem noise problems, don't call Ma Bell!  Just
whip this together as shown below:

                                100 ohm 1/2W                 ___________
   ------------- Red __________/\  /\  ______/\  /\  _______|Phone     |
                                 \/  \/        \/  \/       |Jack in   |
   ------------- Blk ----- N/C                 5K Pot       |Parallel  |
                                                            |with      |
   ------------- Yel ----- N/C          1.0 mf (200V)  _____|modem jack|
                                  ____||____          /     |__________|
   ------------- Grn ____________/    ||    \________/        | | | | 
                                 \_______||_/                 | | | |
                                         ||                   | | | |
                                   1.0 mf (200V)        To modem phone jack

- Paul Eckert


-------------------------------------------------------------------------------
Addition to above to clarify circuit diagram.
The diagram should look like this:
All this is is a Lo pass filter to reduce the spikes.

________________________________   ________________________________
     red                        \ /     2 ea 1uF 200V caps
                                 |   |----||-----|
                                 |---|           |---|
 PHONE LINE IN ->                    |----||-----|   |      TO MODEM ->
                                                     |
                                 |---\/\/\/\/\-------|
      green                      |    2.7 Kohm resistor
_______________________________ / \__________________________________
