March 15, 2014

FCC Technician Exam Question Of The Day (T7B04)

Q) What is the most likely cause of interference to a non-cordless telephone from a nearby transmitter?

A) The telephone is inadvertently acting as a radio receiver

NJ2X Notes:
The FCC published an interesting bulletin (FO-10) in 1986 regarding telephone interference.  Technology has come a long way since then.  However, the article is still interesting and relevant.  The installed base of simple hard-wired telephones is still very large.

Federal Communications Commission
Field Operations Bureau
Telephone Interference

This  document has been prepared to assist you  in  understanding why  interference  to your telephone system  occurs.  Recommended solutions   for  you  and  your  authorized   telephone   service technician are also provided.

                                        Bulletin FO-10
                                        September 1986


Telephone   technology  today  uses  circuitry  which,   if  left unprotected,  will  respond to the radio frequency  (rf)  signals from nearby radio transmitters such as Amateur, Citizens Band and
AM/FM  broadcast stations.  When the telephone circuitry responds to the rf signal you hear the interference on your telephone. The rf  signal can be entering at the telephone instrument or on  the
inside or outside wiring.

Cordless telephones are also susceptible to rf signals.  Cordless phones  are low-power transmitters using  radio  frequencies.  As with  any radio transmitter,  they can receive interference  from other  nearby transmitters.  Interference can also occur if  your neighbor's  cordless  phone is using the same radio frequency  as yours.  Since  the FCC does not offer interference protection  to cordless   telephones,   you  should  contact  your   dealer   or manufacturer for assistance when interference occurs.

                      RECOMMENDED SOLUTIONS

The  installation  of  a modular filter is suggested as  a  first step.  Modular filters are available from most telephone  stores. It can be easily installed by you if your telephone has a modular jack.  You may wish to verify if the filter can be returned for a refund if it does not eliminate the interference.

The  effectiveness of filtering may vary according to the type of telephone  you  are  using.  Also,  modifying  certain  types  of telephones,  as suggested below, may be impractical or expensive.
When  it  is,  consider changing to another brand or  model  with better interference protection.

If you have taken the above steps,  and the interference is still present, the telephone instrument or wiring will probably have to be  filtered.  YOU  SHOULD NOT INTERNALLY MODIFY  THE  INSTRUMENT YOURSELF  NOR  ATTEMPT  FILTERING  OUTSIDE  PHONE   LINES.   ONLY AUTHORIZED  SERVICE  TECHNICIANS MAY INTERNALLY MODIFY TELEPHONE INSTRUMENTS.  ONLY TELEPHONE COMPANY PERSONNEL MAY FILTER OUTSIDE TELEPHONE  LINES.  This  rule is designed to ensure  that  phones continue to comply with FCC registration standards.

Interference  picked up by the inside wiring can be corrected  by installing  SHIELDED  inside wiring.  If you have  subscribed  to inside line maintenance,  the local telephone company can install
shielded  inside wiring for you.  If you have NOT  subscribed  to inside line maintenance,  they can still perform this service for a fee.


If  your customer has tried the modular filter and shielded their inside wiring and the interference is still present, we recommend you filter the inside of the telephone instrument.  Interference   in  older  rotary  dial  phones  (without  special features)  can  usually  be  resolved  by  bypassing  the  carbon
microphone.  Install  a 0.001 mfd ceramic disc capacitor  in  the back of the mouthpiece in the handset. Where possible, solder the capacitor  directly  to the microphone contact fingers  with  the
shortest possible leads.

Phone instruments with special features such as memory, automatic redial,  speakers, push-button dialling, and sound amplification, contain  components which are sensitive to rf signals.  Shielding
and  bypassing of these components are necessary to  isolate  the affected  circuit(s).  Refer to the design  specifications.  Bell System  technicians may refer to the Bell Systems Practices Plant
Series Manual Section 500-150-100 for necessary modifications.

Another  possible  solution would be to  install  ferrite  cores. These  are donut-shaped devices through which the phone cord  can be wound.  Ferrite cores are available in various sizes.  Use one
with a hole large enough to permit passing the phone cord through it two or three times.

To  install  a  ferrite core,  first disconnect  the  phone  cord between  handset and telephone base.  Loop the phone cord through the core two or three times.  Tape or fasten the core as close as possible to the handset.  Reconnect the phone cord.  Two  ferrite cores  on the handset might be necessary.  If so,  place one near the handset and another near the telephone instrument.

If  the interference continues after filtering the phones  and/or installing inside shielded wiring,  the interference is  probably entering  the  system  through  the  outside  wiring.  The  local telephone  company  service  department should be  contacted  for assistance.  Only telephone company personnel may filter  outside telephone  lines.  Two devices which may be used for  eliminating outside interference are:

-a  40 BA capacitor installed at the service entrance  protector,
-a 1542A inductor installed at the connector block.

Your  next  step  would  be  to  have  the  telephone  instrument filtered.   If  you  own  your  phone,   contact  the  dealer  or manufacturer for assistance. If you lease your phone, contact the local phone company's service department.  You may wish to  share the  Service Technician Information section of this document with the technician assigned to assist you.

Even  though you may be experiencing interference to  other  home electronic entertainment equipment, such as televisions, stereos, or  VCRs,  the telephone should be filtered.  If the RF signal is entering  the  telephone system,  the interference  can  only  be eliminated  at the point of entry.  Filtering or shielding of the phone instrument also offers future protection to your  equipment from other rf signals.

You  may  provide  to  the FCC the name  and/or  address  of  the owner/operator   of  the  radio  equipment  involved  when  other equipment  is receiving interference.  The FCC  will  communicate
with  that  person  to  determine  if  they  can  assist  you  in eliminating the interference.  FCC office addresses are listed at the end of this document.

CAUTION:   Only    authorized   service   technicians    may internally  modify  telephone  instruments.  Only telephone  company  personnel may filter  outside telephone lines.


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