Path: bongo!julian
From: Julian Macassey
Newsgroups: comp.dcom.telecom
Subject: Re: Radio Interference to Phones
Message-Id: <357@bongo.UUCP>
Date: 2 May 91 07:43:43 GMT
Reply-To: Julian Macassey
Organization: The Hole in the Wall Hollywood California U.S.A.
Resent-Date: Fri, 3 May 91 0:08:59 CDT
Resent-To: ptownson@gaak.LCS.MIT.EDU



Sooner or later, you are going to become the victim, or cause of, interference to a telephone or phone system. You may be lucky and only know someone who is the victim of phone interference.

The sort of interference we are discussing here is Radio Frequency Interference (RFI). It is obvious that RFI can be caused by radio transmitters, and also TV transmitters. The radio transmitters can be AM broadcast, Two Way, CB, Ham, FM broadcast and studio links. Less obvious can be garage door openers as well as anything that generates electric sparks such as electric motors and transformers that are breaking down.

Any device that would also be registered with the FCC as a Part 15 Subpart J device is liable to cause RFI. A part 15 subpart J device is any device using a microprocessor and generating square waves. This means any digital device, including the onboard computer in the family chariot and the lawn sprinkler timer. Favorite generators of RFI of this type that are often overlooked are programmable scanners.

Bear in mind also that many phones today are "feature" phones which means they too have microprocessors on board and can interfere with themselves, other phones, or even nearby radios and TVs. Of all types of phones, the old style desk phones - rotary types known as the 500 set and the Touch-Tone version 2500 set - are much much less susceptible to interference than modern electronic phones. Modern electronic phones tend to have transistors and integrated circuits in them. These semiconductors act as detectors like the old crystal sets. The amplifiers in these modern phones then tend to amplify the detected signals. The result...your good buddy from channel 16 tends to get on the phone line with you. So a fast simple cure for interference can often be to drag the old black phone back out of the closet.

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". It is reproduced here (see box) and 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.

The FCC document is quite adamant that only authorized personnel can open phones to modify them against RFI and that only telephone company personnel can add coils and capacitors to the telephone protector. Modifications to protectors and internal modifications to phones are described here for those who are authorized to do such work.

There are many modifications that can be done by the consumer. Some of these modifications are very effective. One modification that can be done quickly and cheaply is the addition of ferrite cores to handset cords and line cords. Also easy to add are modular plug-in filters. Requiring more work is the re- routing of phone wire or the installation of shielded wire. Don't forget that although opening your phone or fooling with the protector may be a no-no, you can do whatever you want with the wire in your home.

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. All of the above fixes are legal for any subscriber to do without falling foul of the FCC or local Telephone Company. The exception is of course those with party lines. Party line subscribers should be wary of adjustments to wires and telephones and call in the phone company. There are many ways of wiring party lines and the telephone company will be familiar with their own equipment. If you mess up the wiring of a party line phone, it may cease to ring or refuse to dial.

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 material.

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.

The FCC in their document mention plug-in modular RFI filters. Despite their breezy assurance, these can be hard to find locally. Those lucky enough to have an AT&T phone store handy can buy a device called a "Radio Frequency Filter", it will cost about $6.00 and is modular. All you have to do is take it home and plug it in. Those far from an AT&T store can order one by phone. With credit card in hand, dial (800) 555-8111 and ask for a Radio Frequency Filter.

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 Mylar.

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.


Julian Macassey, N6ARE
742 1/2 North Hayworth Avenue
California 90046-7142
PHONE: (213) 653-4495

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

Julian Macassey, n6are ucla-an!denwa!bongo!julian
742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495

Path: bongo!julian
From: Julian Macassey
Newsgroups: comp.dcom.telecom
Subject: Re: Radio Interference to Phones
Date: 2 May 91 07:42:35 GMT
Reply-To: Julian Macassey
Organization: Tired Pedagogues Hollywood California U.S.A.
Resent-Date: Fri, 3 May 91 0:08:36 CDT
Resent-To: ptownson@gaak.LCS.MIT.EDU The folowing is a "Bell Systems Practice" on telephone RFI. The document is complete apart from two photographs of the 1542A inductor and the 40BA capacitor. Also missing is the schematic and and diagram of the 425J schematic. The values of the 40BA capacitor are shown, normally this capacitor would be a 250 Volt Mylar type. There is no value for the 1542A inductor.

AT&TCo Standard Issue 7, January 1974


1.01 This section contains information on the reduction of radio interference in telephone sets.

1.02 This section is reissued to:

o Add information on single slot coin telephones

o Show 241A amplifier replaced by 241B amplifier.

1.03 Radio interference frequently occurs where a radio station is located near telephone facilities. Generally the radio signal is picked up by the wire which acts as an antenna, and demodulated (changed to audio frequency) by nonlinear components, such as varistors, transistors and diodes in the telephone set. HOWEVER DEMODULATION CAN ALSO RESULT FROM CORRODED CONNECTIONS AND TERMINATIONS.

1.04 Before ordering a telephone set modified for radio signal suppression, check for the following:

o Corroded connections (inside and outside plant)

o Loose wire terminations (including set wiring)

o Abandoned drop wire still connected to line

o Inside wire connected but not used

o Foreign attachments

Installing a modified telephone set on the customers premises will not be effective if demodulation is caused by any of the above conditions.

1.05 Telephone set components that may act as demodulators are:

o Speech equalization varistors in networks

o Transistors in amplifiers of some networks and handsets

o Varistors and transistors in TOUCH-TONE dials

o Diodes in polarity guards

o Click suppression varistors across receiver units

o Carbon transmitters

1.06 With the introduction of the 425J and 4010E networks, the cause of radio signal demodulation in networks has effectively been minimized. In these networks the speech equalization varistors have been replaced by resistors and a strapping option that provides speech equalization.

1.07 All telephone sets, excluding coin, will be available from the local distributing house modified for radio signal suppression. Modification will include changing to new type network, where applicable, and placing bypass capacitors across all other components that may act as demodulators as listed in 1.05.

A    (See 1.04)

1.08 Single slot coin telephones are not modified at the service center, and where radio frequency is a problem, it is recommended that a 40BA capacitor and/or a 1542A inductor be added to the protector and connecting block.

1.09 Where demodulation is attributed to components in a telephone set, or associated voice coupler, in addition to changing to a modified set it is recommended that a 40BA capacitor be placed at the protector and a 1542A inductor at the connecting block. Refer to section in Division 463 for voice coupler modifications. The capacitor will bypass, to ground radio signals picked up by the drop wire while the inductor will tend to attenuate radio signals picked up by the inside wire.

1.10 Where an adjunct (TOUCH-TONE phone) dial forms a part of the customers equipment and a modified telephone set is installed for radio interference reasons, the adjunct dial should also be replaced by one modified for radio suppression.

1.11 MD telephone sets will not be modified for radio suppression. Where a telephone set rated MD is encountered, it should be replaced by an equivalent set in the current series, modified for radio suppression.

1.12 If possible arrange for operation of the radio station during the trouble visit in order that the effectiveness of corrective measures taken may be evaluated immediately.


2.01 Suppression Devices

(a) 425J Network (Fig. 1):

o Eliminates radio interference by having the speech equalization varistors replaced by resistors and a strapping option ("H" and "J" leads)

o Can be used for rotary or TOUCH-TONE dial applications

o Contains an equalization network for TOUCH-TONE dials ("X" Terminal)

(b) 4010E Network (Fig. 2):

o Similar to 425J

o Strapping option consists of "N" and "V" leads

(c) 1542A Inductor (Fig. 3):

o Attenuates RF line current

o Can be substituted for the 42A connecting block

o Removable link between terminals B and Y (current models only). Required when used as connecting block for telephone sets with dial lamps or with 30A voice coupler.

(d) 40BA Capacitor (Fig. 4):

o Located at protector to bypass RF signals to ground


(a) Telephone sets, except coin, currently using the 425- or 4010-type networks will be modified as follows:

o Existing network replaced by a 425J or 4010E network, whichever is applicable

o TOUCH-TONE dial will have bypass capacitors installed across the varistors and transistors and will also have an additional (red-slate) lead soldered to the "X" terminal which in turn connects to the "X" terminal on the network.

o Bypass capacitors installed across all other components of the telephone set that may act as demodulators, such as transmitter unit, receiver unit, transistors in amplifiers, polarity guards, etc.

(b) TRIMLINE telephones will be modified by having bypass capacitors placed across all the components in the set that may act as demodulators.

(c) Modified sets will not be recoded but will be identified in some manner by the local distributing house.

2.03 TOUCH-TONE dials and all G-type handsets will be available from the local distributing house modified for radio suppression. These will be used where an adjunct dial must be installed or where the dial or handset must be replaced for maintenance reasons.

2.04 Other modified telephone set components include 241B, 242B, and 277A amplifiers; D-180191 and D-180191 polarity guard assemblies; and 694A and B subsets. Modified versions of additional items will be made available in the future when they are needed.


o Capacitor, 40BA

o Inductor 1542A-

-49 Gray, -50 Ivory

o Set, Telephone--RF Modified

o Set, Telephone, Hand 220A--RF Modified

o Set, Telephone, Hand 2220B--Modified

o Dial--(TOUCH-TONE dial only) RF Modified

o Set, Hand G--RF Modified



(a) Installed in usual manner

(b) For connections see connection section of type set modified

(c) Sets are shipped with speech equalization option leads insulated and stored:

(1) For loops greater than 500 ohms leave insulated and stored.

(2) For loops 500 ohms or less connect "H" or "N" (Yellow) and "J" or "V" (Orange-Black) leads to terminals RR and R respectively, on the 425J or 4010E network.

4.02 Modified TRIMLINE Telephones

(a) Install in usual manner

(b) See Section 502-321-400 for connections

4.03 1542A Inductor (Fig. 5)

(a) Use as connecting block for telephone set cord.

(b) Locate as near as possible to wall-type sets.

4.04 40BA Capacitor (Fig. 6)

(a) Install near and connect to protector with as short as possible inside wire.

(b) Station wires must connect to the capacitor

(c) Mounted inside when outside protector is used



5.01 Normal maintenance can be performed on modified telephone sets. Modified TOUCH-TONE dials and G-type handsets are available from the local distributing house for maintenance.

Fig. 3--1542A Inductor schematic

RING               |R1              R |            RED
                   |                  |
TIP                |G1              G |          GREEN
                   |                  |
GROUND             | B              Y |         YELLOW
        LINE       |__________\_______|  SET CORD
                          REMOVABLE LINK

Fig. 4--40BA Capacitor Schematic

            0.25uF   0.25uF     0.25uF  o.25uF
      --------| (------| (----o---| (-----| (------|
      |                       |                    |
      |                       |                    |
      |                       |                    |
      |                       |                    |
      |                       |                    |
      |                       |                    |
      |                       |                    |
      |                       |                    |
      o                       o                    o
     TIP                   GROUND                RING

Julian Macassey, n6are  ucla-an!denwa!bongo!julian
742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495
From Fri May  3 05:38:53 1991
Path: bongo!julian
From: Julian Macassey 
Newsgroups: comp.dcom.telecom
Subject: Re: Radio Interference to Phones
Message-Id: <355@bongo.UUCP>
Date: 2 May 91 07:40:31 GMT
Reply-To: Julian Macassey 
Organization: Adult Children of Geriatrics California U.S.A.

                   INFORMATION BULLETIN
             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 
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.
Julian Macassey, n6are  ucla-an!denwa!bongo!julian
742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495