Black Box MC144A, MC145A, MC146A, MC147A, MC140A Specifications

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Blocks out radio­frequency interference
from your telephone
lines.

One- and two-line
models are available.

Choose from four
different frequency
ranges: LF, HF, VHF,
and UHF.

To eliminate severe
interference, install
one filter on a line and
another filter right
behind the first one.

T

elephone interference happens

when your telephone picks up
external radio frequencies and
mistakenly decodes them along
with the real telephone signal.
Your telephone’s own cord can be
its worst enemy, acting as an
antenna to pick up unwanted
radio signals. It’s worse than ever
in modern telephones, whose
solid-state components make
them susceptible to even the
slightest levels of radio-frequency
interference (RFI).

The interference can come
from any radio source—broad­cast radio stations, cellular
phones, even copiers or fluor­escent lights. Not only does it
make conversations noisy, but it
can also play havoc with other
devices that use the phone
lines—scrambling modem data,

You don’t have to put up with noise on your phone lines.

It’s surprisingly easy to get rid of almost any interference.

Key Features

RFLINE FILTERS

accidentally triggering an
answering machine, or corrupting
a fax.

There might be more inter­ference on one phone than on
another in the same building—
specific radio waves reach differ­ent places, and various phone
designs are affected differently by
the interference.

Can you get rid of RFI? Almost
always. All it takes is a little work
diagnosing the problem and the
right RFI filter combination.

Diagnosing the Problem

If you’re having trouble with
interference on your phone, be
observant. Is the problem con­stant? Is it the same on all exten­sions on the same line? Does it
change when you change any­thing on the line—when you add
or remove a phone, for example?

Looking for Bad Phones

The first thing to check is
whether it’s your phone that’s
causing the problem. Modern
telephones, with their solid-state
components, are especially sus­ceptible to RFI. And a telephone
might easily keep on working
even with a malfunction that
causes interference to other
phones on the line.

If you have two or more tele­phones, try this test to find out
whether telephones connected to
your line are adding interference:

1. Make sure all the phones
are “on hook” (hung up).

2. Pick one phone where you
can hear the interference.
We’ll call this the “control
point.”

3. Unplug all the other phones
and any equipment con-

Black Box Corporation • 1000 Park Drive • Lawrence, PA 15055-1018 • Tech Support: 724-746-5500 • www.blackbox.com • e-mail: [e-mail]

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Black Box Corporation.

8/5/2005 #17099

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nected to the phone line—
answering machines,
modems, surge protectors,
and so on.

4. Listen for interference at
the control point.

5. Now reconnect one device
and listen for interference
at the control point again.
If the interference has
increased noticeably,
the device you just plugged
in is a source of, or at least
a contributor to, the
interference.

6. Unplug that device and
plug in another, then listen
again. Use the same meth­od to check all the
equipment.

7. To complete the test, move
the original control-point
telephone to another phone
jack. Plug another phone
into the jack at the control
point. Listen at the new
phone for any change in
the interference.

If one or more phones or other
devices are causing most of the
interference, you’ve found your
problem.

Mapping Out Your System

Home telephone systems start
at the protector box, which is
installed on the outside of the
house, where the line comes in
from the telephone company.
(This box protects the home
telephone wiring from lightning.)
Next, the wiring goes inside the
house to the “service entry,”
sometimes called the “interface”
or “connector block.”

From here, the wiring can go
to the phone jacks in one of two
ways:

1. In “loop series” wiring, one
continuous cable feeds all
the telephone jacks in the
house. If you see only one
cable leaving the service
entry, you have loop-series
cabling.

2. “Parallel distribution” uses
a different cable for each
telephone jack. If you see

several cables leaving the
service entry—one for
each telephone jack in the
house—your phone system
is wired with parallel
distribution.

It’s also possible to have a
combination of the two—say, a
loop that serves the main house,
with separate wires for an
addition.

What Condition Is Your System In?

Telephone wiring is different
from one house to another, but
every wiring system should meet
your phone company’s specifica­tions. If it doesn’t, it will be too
sensitive to RFI. That’s why inter­ference can be worse at one
house than at another, even when
both houses are exposed to the
same source of interference.

So start by examining your
telephone wiring. Even exper­ienced telephone personnel
sometimes overlook these prob­lems that contribute to RFI: poor
system ground, system deterior­ation, and installation errors.

Important Safety Note:

If you’re not familiar with tele­phone wiring, don’t try to fix the
wiring yourself. Dangerous cur­rents can flow through the wires,
especially when a ring signal
comes in.

1. System Ground

To keep your lines as free as
possible from interference, make
sure you’re using the highest­quality system ground. Proper
installations establish system
ground either at the protector box
or at the service entry. You should
see a heavy-gauge ground wire
that goes either to the power­company ground or to a cold­water pipe that is known to be
conductive. Be suspicious of
connections to any other ground.
Your telephone company will
probably correct an improper
ground at its own expense.

Good grounding goes a long
way toward shielding your
telephone wiring from inter­ference. Sometimes upgrading

the ground can eliminate the
interference altogether; other
times, upgrading the ground will
reduce the interference notice­ably, so that you need fewer filters
to eliminate it.

2. System Deterioration

Your telephone system might
have many problems and still
keep working. When system
deterioration is slow, you might
not notice anything to be con­cerned about until a nearby radio­frequency field is present. Poor
connections in telephone wiring
make your phone system much
more susceptible to RFI.

If you’ve already determined
that the system ground is good,
look for signs of deterioration.
Check for corroded wires, poor
connections, moisture, decaying
insulation on wires and cables,
and stretched or damaged cables.
Examine as much of your system
as you can—from the protector
box to the service entry to every
visible cable run to every phone
jack. Fix whatever problems you
notice.

3. Installation Errors

When your house was wired
for telephones, did the installer
take shortcuts? You might not
notice—remember that many
problems won’t show up until a
nearby interference source
appears. Here are some install­ation errors to look for:

• In loop-series installations,
a common oversight is inter­ruption of continuous
ground. Telephone cable
leaving the service entry
includes the “pair” and the
ground wire. Correct wiring
brings the cable to the first
phone jack in the system,
connects the pair to the jack,
and then continues the pair
and ground to the following
jacks. Sometimes an install­er will continue the pair but
not the ground. As you ex­amine each telephone jack,
make sure the ground con­ductor continues to the next
jack. Wiring will stop at the
last jack in the system.

• Beware of speaker wire,
thermostat wire, or any other
kind of wire that isn’t actual
telephone wire. Unqualified
installers often add new
telephone jacks without re­gard for the proper type of
wire. Even if the wire is tele­phone wire, the installer
might not have chosen a
matched pair of wires in the
cable.

• Staples driven through tele­phone cables can create
real havoc. They can cause
wires to cross, resulting in
an imbalance that upsets the
interference rejection that’s
supposed to be designed
into the system. If an install­er’s badly aimed staple had
severed one wire in the pair,
the telephone fed by that
cable wouldn’t work, and the
installer would have fixed
the error. But if it severed
only the ground wire, or if it
caused crossed wires, it
could easily go unnoticed.
Look carefully for staple
damage like that.

Unused Conductors and Cables

To upgrade your system’s RF
shielding, ground all unused wires
inside active cables. Unused
wires are commonly left hanging
at the service entry. Simply con­nect them to system ground.

If your house is wired for
parallel distribution and there are
extra phone jacks you’re not
using, disconnect the cables
feeding them at the service entry.
The cables you disconnect don’t
need to be grounded, since
they’re no longer part of the
system.

Installing Filters

When you’ve checked out
your wiring system and you’re still
having RFI problems, use our fil­ters to stop the interference.

Install an RF Line Filter at the
line-telephone’s cord input. It’s
important to locate the filter near
the telephone, because long line
cords can intercept radio fre­quencies in the 14-to-30-MHz
range. Check the results by listen­ing for interference.