149 lines
8.0 KiB
Plaintext
149 lines
8.0 KiB
Plaintext
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The following article has been taken from issue 13 of TRSLINK Magazine, edited
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by Luis M. Garcia-Barrio and Peter Besenbruch. The article is being re-
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distributed with permission and may continue to be distributed freely provided
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that no fee is charged for its distribution (excepting connect time fees for
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telecommunications services, or a maximum fee of $1.50 to cover the cost of
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computer media).
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Linefeeds have been added to the file, as well as a left margin (eight spaces).
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Modem Noise Killer (alpha version)
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In this project, with some basic tools and a few
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components from Radio Shack, you should be able to cut the
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noise/garbage that appears on your computer's screen. I started
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this project out of frustration at using a US Robotics 2400
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baud modem and getting a fair amount of junk when connecting at
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that speed. Knowing that capacitors make good noise filters, I
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threw this together.
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It is very easy to build, however conditions may be
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different due to modem type, amount of line noise, old or new
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switching equipment (Bell's equipment), etc. So it may not work
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as well for you in every case. If it does work, or if you've
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managed to tweek it to your computer/modem setup I'd like to
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hear from you. I'd also appreciate any of you electronic
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wizzards out there wanting to offer any improvements. Let's
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make this work for everyone!
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Please read this entire message and see if you understand
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it before you begin.
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OK. This is what you'll need from Radio Shack:
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a) 1 #279-374 Modular line cord if you don't already have one.
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You won't need one if your phone has a modular plug in its
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base. $4.95
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b) 1 #279-420 Modular surface mount jack (4 or 6 conductor)
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$4.49
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c) 1 #271-1720 Potentiometer. This is a 5k audio taper variable
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resistor. $1.09
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d) 1 #272-1055 Capacitor. Any non-polarized 1.0 to 1.5 uf cap
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should do. Paper, Mylar, or metal film caps should be used,
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although #272-996 may work as well. (272-996 is a non-
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polarized electrolytic cap) $.79
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e) 1 100 ohm resistor - quarter or half watt. $.19
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f) 1 #279-357 Y-type or duplex modular connector. Don't buy
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this until you've read the section on connecting the Noise
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Killer below. (A, B,or C) $4.95
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First, pry open the modular block with a screwdriver.
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Inside you'll find up to 6 wires. Very carefully cut out all
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but the green and red wires. The ones you'll be removing should
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be black, yellow, white, and blue. These wires won't be needed
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and may be in the way. So cut them as close to where they enter
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the plug as possible. The other end of these wires have a spade
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lug connector that is screwed into the plastic. Unscrew and
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remove that end of the wires as well.
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You should have two wires left. Green and red. Solder one
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end of the capacitor to the green wire. Solder the other end of
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the capacitor to the center lug of the potentiometer (there are
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three lugs on this critter). Solder one end of the resistor to
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the red wire. You may want to shorten the leads of the resistor
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first. Solder the other end of the resistor to either one of
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the remaining outside lugs of the potentiometer. It doesn't
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matter which.
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Now to wrap it up, make a hole in the lid of the mod block
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to stick the shaft of the potentiometer through. Don't make
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this hole dead center as the other parts may not fit into the
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body of the mod block if you do. See how things will fit in
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order to find where the hole will go.
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Now that you've got it built you'll need to test it. Twist
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the shaft on the potentiometer until it stops. You won't know
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which way to turn it until later. It doesn't matter which way
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now. You also need to determine where to plug the Noise Killer
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onto the telephone line. It can be done by one of several ways:
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1) If your modem has two modular plugs in the back, connect the
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Noise Killer into one of them using a line cord. A line cord
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is a straight cord that connects a phone to the wall outlet.
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2) If your phone is modular, you can unplug the cord from the
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back of it after you're on-line and plug the cord into the
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Noise Killer.
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3) You may have to buy a Y-type modular adaptor. Plug the
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adaptor into a wall outlet, plug the modem into one side and
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the Noise Killer into the other. Call a BBS that has known
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noise problems. After you've connected and garbage begins to
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appear, plug the Noise Killer into the phone line as
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described above. If you have turned the shaft on the
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potentiometer the wrong way you'll find out now. You may get
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a lot of garbage or even disconnected. If this happens, turn
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the shaft the other way until it stops and try again. If you
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don't notice much difference when you plug the Noise Killer
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in, that may be a good sign. Type in a few commands and look
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for garbage characters on the screen. If there still is,
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turn the shaft slowly until most of it is gone. If nothing
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seems to happen at all, turn the shaft slowly from one side
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to the other. You should get plenty of garbage or
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disconnected at some point. If you don't, reread this
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message to make sure you've connected it right.
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*** END OF ORIGINAL FILE ***
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ADDITION TO ORIGINAL FILE - 2/29/88
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Mike McCauley - CIS 71505,1173
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First, a personal recomendation. _THIS WORKS!!!_ I have
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been plagued with noise at 2400 for some time. I went round and
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round with Ma Bell on it, and after they sent out several
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"repair persons" who were, to be kind, of limited help in the
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matter, I threw in the towel. I saw this file on a board up
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east a few days ago, and thought I'd bite. Threw the gismo
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together in about 10 minutes, took another five to adjust the
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pot for best results on my worst conection, and guess what? No
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more worst connecion! A few pointers:
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1) The pot need not be either 5K or audio taper. I used a 10K
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15 turn trim pot. Suggest you use what is handy.
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2) I used 2MFD's of capacitance (two 1MFD's in parallel) Two
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R.S. p/n 272-1055 work fine. Remember that about 90 Volts
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will appear across red & green at ring, so the caps should
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be rated at 100VDC+.
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3) I ended up with a final series resistance value (100 ohm +
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pot) of 2.75K. I speculate that one could probably use 2MFD
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and a fixed 2.7K resistor and do the job 90% of the time.
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The adjustment of the pot is not very critical. Changes of
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+/- 1K made little difference in the performance of the
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circuit.
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Hope it works as well for you as it did for me.
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Mike McCauley
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