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TUCoPS :: Phreaking Technical System Info :: telin.txt

Overcoming line interference

Below are some extracts from an article I wrote a couple of years ago
for Popular Communications Magazine. The Issue was Feb 1988, if you
want to see the whole thing.

                      TELEPHONE INTERFERENCE

                        (CAUSES AND CURES)

There is some hope when it comes to interference. The FCC has issued a
field bulletin on RFI.  The document is called "Bulletin FO-10,
Telephone Interference". Copies of it can be obtained from any FCC
field office.  Find your nearest FCC office by looking up FCC in the
phone book.  The Bell System has considered the problem and approach
it from the viewpoint of interference to regular 500 and 2500 type
phone sets. The Bell document concerning RFI is known as a Bell
Systems Practice (BSP). The document is BSP Section 500-150-100.

Before you tear things apart and spend money, there are a few things
to check. These checks can be done with an adjustable wrench and a
screwdriver. What is checked is anything on the line itself that could
be acting as an antenna or detector.  A dirty connection can work as a
diode to detect RF signals. Go over the internal wiring looking for
the following:

 1.  Corroded connections. Clean and tighten.

 2.  Loose wire terminations, including set wiring and all jacks and
junction boxes. Tighten any loose screws.

 3.  Abandoned wire still connected to the line. Remove any wire not
connected to a working phone.

 4.  Old unused devices still connected to the line.  Remove abandoned
phone answering machines, old telephones and bells etc.

If any of the above is what is causing the RFI, until you fix them,
there is little hope that anything else you do will cure the problem.

The other legal "do it yourself" fix is attachment of toroid cores.
These cores look like small black doughnuts, by wrapping wire round a
ferrite core a simple effective RF filter or choke can be made.
Ferrite cores are frequency selective, by the choice of the right
material, interference can be effectively hit on the head.

With modern phones, the most RF sensitive part of the phone is the
electret microphone and its preamplifier circuit. By application of
ferrite cores to the handset cord, there is a fair chance of easily
and cheaply fixing the problem.  If you are hearing radio signals on
the phone, there is a way of checking if the microphone/handset cord
is to blame. Dial a partial number to give you silence, listen for the
interfering signals and grab the handset cord.  If the signal changes
in volume - gets better or worse - try a ferrite core.

The best source of ferrite cores in small quantities is Amidon
Associates, 12033 Otsego street, North Hollywood, California 91607.
PHONE: (818) 760-4429. Amidon Associates have several ferrite "mixes"
available. For interference from 500 Khz to 10 Mhz, i.e.  AM broadcast
RFI, they recommend their 75 material. For interference from 1 to 30
Mhz they recommend their 73 material. The 73 material should take care
of all short wave Ham and CB interference. For low VHF and channel 2-7
RFI you can try a ferrite core made with the 43 material which should
take care of RFI between 1 and 70MHz. For best results use the
material that has the lowest cutoff point for your problem.  If the
local AM transmitter at 1070 KHz is your problem, use the 75 material,
it will give much better attenuation at that frequency than the 73

For a handset cord, a half inch core is ideal.  Wrap four or five
turns of the handset cord through the core and plug it back into the
handset. The cord can be held in place with black vinyl tape or glue,
hot melt glue works well.  Experiment with the positioning of the
core. Often having the core by the handset works best, other times
plugging in the cord with the core by the body of the phone is better.
Sometimes a core at each end of the cord is needed to do the trick.
The cores may look kinda clunky, but if they provide relief easily and
cheaply, who are you to complain.

The numbers for the half inch cores are: FT-50A-75, FT-50A-73 and
FT-50A-43. Yes you guessed it - the last two digits tell you the
material being used. For one and a half inch cores used with line
cords explained below, the numbers are: FT-140-75.  The last two
digits being the same as for the half inch cores.

For RFI that you suspect to be entering via the phone line, wrapping
the line cord round a large core can help. Usually it is best to place
the core at the telephone end of the line cord. Though like all RFI
cures, experimentation, otherwise known as "suck it and see", does a
better job than hard and fast rules.  With the large core on the line
cord, between six and twenty turns on the cord should do the trick. A
core on each end of the line cord may help in stubborn cases.

For authorized phone repair stations, telephone personnel and those
willing to risk "Open circuit surgery", there are several solutions.
Using ferrite cores, twenty turns or so of scrap 24 Gauge telephone
wire can be wrapped round a half inch ferrite core.  Use two cores,
one for Tip and one for Ring and place them inside the phone.  The
same kind of cores and windings can also be used, inside the phone,
on the transmitter (microphone) leads.

For those really handy with a smoking soldering iron there are some
more fixes to try. For phones using electret microphones, some well
placed capacitors may do the trick. Try a 0.01 uF (10 NF) across the
electret element. If that doesn't work try the same value of cap
across the hot side of the element to the "ground" of the pc board.
Regular phones with carbon transmitters can be helped with a 0.01 or
0.1 uF capacitor across the element. Solder the capacitor across the
contact fingers in the handset, not across the element, so if the
transmitter is changed, the RF proofing will stay with the phone.
Also, inside the phone, a 0.1 uF (100 NF) 250V capacitor across Tip
and Ring can be helpful. The type of capacitor to use is a Ceramic or

For those with access to AT&T parts or wishing to help the local phone
company, there are a couple of bits of helpful hardware mentioned in
Bell Systems Practice 500-150-100.  First there is a coil that should
be spliced into the phone line. It is called a 1542A inductor. It
should be spliced into the line as near as the offending telephone set
as possible. This means put it right before the modular jack. It has
six terminals, two for Tip, two for Ring and two for a ground, should
the phone still need a ground (yellow wire) for the ringer or party
line.  The ground terminals are not in any way connected to the coil,
so bringing a ground to the inductor, unless needed in the phone, will
not help cure any RFI.

The Bell document also mentions a capacitor, designated a 40BA
capacitor. It is actually four capacitors (see Fig 1) and the intent
is to place a capacitor between each leg of the phone line and ground.
The 40BA is usually installed at the telephone protector.  There is
always a good ground available at the protector, often a heavy gauge
solid, solid gray jacketed wire.  Those telephone personnel who do not
have access to a 40BA capacitor should find that a couple of 0.1 uF
250V Mylar capacitors will work just as well (see Fig 2). To install
the 40BA or 0.1uF capacitors, find the protector. The protector is
usually outside the building in a wall mounted small box, in the
basement or in a closet for businesses and apartment buildings.  If
the phone line comes in on overhead cable, the protector will be in
the first box the cable goes to after entering the premises.

That should be some help in beating the problem.  Don't forget that
some types of phones are more sensitive than others.  Some cases may
be so severe that nothing helps. AT&T no longer have RFI proofed
phones available, although an old style desk phone with some
capacitors added will be pretty immune to RFI.  Alas AT&T no longer
makes old style 500 and 2500 desk sets, although they sell
reconditioned ones. Several manufacturers such as ITT, Comdial, and
Northern Telecom still make old style phones.


This article appeared on Page 56 of the Feb 1988 edition of Popular
Communications Magazine.

Fig 1>
            250nF 250nF     250nF 250nF
 TIP O---| |--| |----O----| |--| |---O RING

AT&T 40BA capacitor schematic and connection diagram

Fig 2>

                100nF            100nF
 TIP O----| |--------O-----| |--------O Ring
Schematic for 100 nF capacitors on telephone protector.

Hope this helps.

Julian Macassey, n6are  {ucla-an!denwa!bongo!julian
N6ARE@K6IYK (Packet Radio) [] voice (213) 653-4495

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