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Reply-To: julian@bongo.info.com

To: telecom@eecs.nwu.edu

Subject: Phone Patches

Message-Id: <9108050620.AA04638@bongo.info.com>

Date: 5 Aug 91 06:20:06 PDT (Mon)

From: Julian Macassey

Resent-Date:  Tue, 6 Aug 91 23:50:11 CDT

Resent-From: telecom@eecs.nwu.edu

Resent-To: ptownson@gaak.LCS.MIT.EDU

Status: RO

Dear Patrick,

Here is an article I wrote about phone patches. If you think it is

------cut, slash, deforest ----------------------

                BUILDING AND USING PHONE PATCHES

     From simple to elegant, patches help make the connection

By

                     Julian Macassey, N6ARE

              First Published in Ham Radio Magazine

                          October 1985.

     In  telephone  company parlance, a patch is  any  connection

between  a phone line and another communications device,  whether

t be a radio, a tape recorder, a data device (such as a  modem),

or even another phone line.

     Radio Amateurs, on the other hand, tend to limit the meaning

of "patch" to the connection of transmitters or receivers to  the

Amateurs  can  and do use phone patches for purposes  other  than

telephone conversations.  One particularly effective  application

s   for  checking  TVI  and  RFI  complaints;  simply  set   the

transmitter on VOX, go to the site of the interference complaint,

and  then  key your transmitter via the phone line.   Doing  this

the  problem.   If  it is, you can use this method  to  test  the

measures you've taken to correct the problem.

     A  phone line is, simply speaking, a 600-ohm  balanced  feed

unbalanced  inputs;  most  cassette  recorders  have  a   600-Ohm

unbalanced  input;  the "tape" outputs on home stereos  are  also

While there are various degrees of sophistication and  complexity

n patching, in an emergency, patches can be easily put  together

using  readily available components.  Before starting to build  a

on understanding phone lines.

The Simple Patch

     The  simplest way to patch a phone line to another piece  of

equipment  is  to use a couple of capacitors to block  the  phone

line  DC.   While this simple approach will work in a  pinch,  it

ntroduced.  The capacitors used should be nonpolar, at least  2-

ohm F, and rated at 250 volts or better (see fig.1).

     To  hold  the line, the patch should provide a  DC  load  by

means  of  a resistor (R6) or by simply leaving a phone  off  the

output stage from "motorboating."  Use two capacitors to maintain

the balance.

     With all patches hum can be lessened by reversing the  phone

The Basic Phone Patch

     Because  a phone line is balanced and carries DC as well  as

an  AC signal, a patch should include a DC block, a balun, and  a

DC load to hold the line.  The best component for doing this is a

and for coupling modem signals to the phone line, available  from

most electronics supply houses.  Old telephone answering machines

are   also   a  good  source  of  600-ohm   transformers.    Some

transformers are rated at 600-900 ohms or 900-900 ohms; these are

also  acceptable.   Make sure that the transformer  has  a  large

enough  core,  because  DC current will be  flowing  through  it.

(Some  small-core transformers become saturated and  distort  the

     In section 68.304 of the FCC Part 68 regulations, it  states

that  a coupling transformer should withstand a 60 Hz 1kV  signal

for one minute with less than 10 mA leakage.  For casual use this

may seem unimportant, but it provides good protection against any

nto  the  Amateur's equipment.  A 130 to 250  volt  Metal  Oxide

Varistor  (MOV)  across  the  phone  line  will  provide  further

     The  DC resistance of the transformer winding may be so  low

that  it hogs most of the phone line current.   Therefore,  while

using  a phone in parallel for monitoring and dialing - which  is

low.   Resistors  R1A  and R1B (see fig.2) will  act  as  current

limiters  and allow the DC to flow through the phone  where  it's

needed.    If   possible,  these  resistors  should   be   carbon

composition types.

     To  keep  the line balanced, use two resistors of  the  same

value  and adjust the values by listening to the dial tone  on  a

telephone  handset.  There should be little or no drop in  volume

     One  of these transformers, or even two capacitors,  can  be

used to patch two phone lines together, should there be a need to

allow  two  distant parties to converse.  There  will  be  losses

through the transformer so the audio level will degrade, but with

two good connections this will not be a problem.

     On the other side of the transformer - which could be called

the  secondary winding - choose one pin as the ground and  attach

the shields of the microphone and headphone cables to it.  Attach

the inner conductors to the other pin.  The receiver output  will

or  just  putting  receiver audio on the line there  will  be  no

crosstalk  or  feedback  problems.   In  some  cases,  the  audio

amplifier  in a receiver does not have enough output to feed  the

transformer with two secondaries (see the "improved" patch below)

or by coupling a 8:1 kilohm transformer between the audio  output

and  600-ohm transformer.  If RF is getting into the  transmitter

nput, a capacitor (C1) across the secondary should help.  A good

value  for the lower bands and AM broadcast interference  is  0.1

uF.   For  higher frequencies, 0.01 uF usually gets  rid  of  the

building  any patch into a steel box will help alleviate  hum  as

The Improved Phone Patch

     Several enhancements can be made to the basic phone patch to

mprove  operation.  The first is the addition of  a  double-pole

location  with the same equipment, but if it is, experiment  with

the  polarity of the transformer connections and adjust  for  the

least  hum.   Most of the time the balance will be so  good  that

throw switch to disconnect from the line.  The two secondaries on

the  "improved"  patch (fig.3) should be checked for  balance  by

connecting  the  receiver and transmitter and  checking  for  hum

conductors of the secondaries for minimum hum.

     Many transmitters do not offer easy access to the microphone

make  adjustment of the transmit level easy.  Placing R10  across

the  transformer allows easy adjustment of the level.  It can  be

adjusted.   This  will  also  work  on  the  basic  600-ohm   1:1

transformer.   Most  of  the  time a  1  kilohm  potentiometer  -

logarithmic  if  possible  - will work well.  If  not,  a  linear

better control.

Deluxe Operation and VOX

     Using  VOX  with  a phone patch may  cause  a  problem  with

triggering the VOX.  There may not be enough Anti-VOX  adjustment

to  compensate for this.  The usual solution for this problem  is

to use a hybrid transformer, a special telephone transformer with

a phasing network to null out the transmit audio and keep it  off

the  receive line.  Most telephones employ a similar  transformer

and circuit so that callers will not deafen themselves with their

own voices.  These devices are called "networks" (see figs. 4 and

     A network can be removed from an old phone and modified into

a  deluxe patch, or the phone can be left intact and  connections

made  to  the line and handset cords.  The line  cord  should  be

coupled to a 600-ohm 1:1 transformer  to keep the ground off  the

line.   Note,  in the network schematics, that the  receiver  and

transmitter  have a common connection; when coupling into  radios

or other unbalanced devices, make this the ground connection.

     There may be confusion about terms used in the network.  The

telephone  receiver  is receiving the phone line audio,  and  the

transmitter is transmitting the caller's voice.  For phone  patch

use,  a telephone receive line is coupled to the transmitter  and

the  transmit line is coupled to the radio receiver.  This  is  a

fast  way to put together  a phone patch and may be adequate  for

VOX use.

     A better patch can be built by using a network removed  from

a  phone or purchased from a local telephone supply house.   This

approach  offers the added advantage of being able to  adjust  or

null the sidetone.  The circled letters in figs. 4 and 6 refer to

the  markings on the network terminal block.  These  letters  are

common to all United States networks made by Western Electric (AT

&  T), ITT, Automatic Electric, Comdial, Stromberg  Carlson,  and

ATC.

     To  make  sidetone  adjustable, remove R4  (R5  in  European

networks)  and  replace it with R11 (for  European  networks  use

R12).   The Western Electric Network comes  point-to-point  wired

and sealed in a can;  the other networks are mounted on PCBs.  To

opened  by bending the holding tabs.  Don't be surprised to  find

that  the network has been potted in a very sticky, odious  paste

that has the texture of hot chewing gum and the odor of  unwashed

a  secret formula - will not wash off with soap and  water.   The

ngredients is said to be beeswax, ordinary beeswax solvents such

as  gum  turpentine, mineral turpentine (paint thinner  or  white

or heatgun.  You can also put the can out in the hot sun under  a

terminal strip may melt.  Even with a film of compound  remaining

on it, the network can be worked on.

Using a Patch

     For efficient use, a patch should have a telephone connected

n parallel with it.  This enables the operator to dial,  answer,

and  monitor  calls  to and from the patch, as well  as  use  the

     One useful modification to the control telephone is adding a

mute  switch to the handset transmitter.  This allows  monitoring

calls without letting room noise intrude on the line.  It's  also

a  good modification for high noise environments,  where  ambient

noise enters through the handset transmitter and is heard in  the

makes calls surprisingly easy to hear.  The mute switch can be  a

momentary switch used as a "Push-To-Talk" (PTT) or a Single  Pole

Single  Throw (SPST) mounted on the body of the phone  for  long-

term monitoring.  The switch should be wired as Normally  Closed,

(see  fig.4).   This makes the mute "clickless." If  the  monitor

     Transmit  and receive levels on the phone line are a  source

of  confusion  that  even  telephone  companies  and   regulatory

agencies  tend  to  be vague about.  The  levels,  which  can  be

measured  in  various ways, vary.  But all  phone  companies  and

ntelligibility,  but  not enough to cause crosstalk.   The  most

trouble-free  way  to set the outgoing level on the patch  is  to

adjust  the  feed onto the phone line until  it  sounds  slightly

louder  than the voice from the distant party on the phone  line.

for  a "bad line."  In this case, adjust the level to  the  patch

until  the other party lowers his or her voice.  The best way  to

another  Amateur  who can give meaningful signal  reports.   It's

because  the  signal  would  simply be  too  loud  to  listen  to

comfortably.  The major problem is sending too little signal down

the line.

     Coupling  the phone line into the radio transmitter  is  not

much  more difficult than adjusting a microphone to work  with  a

ndication  on  a wattmeter, the ALC on the transmitter  or  even

listening  to the transmitted signal on a monitor  receiver  will

lines  can  be  noisy,  and  running  too  much  level  into  the

transmitter  and  relying on the ALC to set  the  modulation  can

cause  a fair amount of white noise to be transmitted.   Watching

the RF output while there are no voice or control signals on  the

line  will  help  in  adjusting  for  this.   VOX  operation  can

alleviate  the problem of noise being transmitted  during  speech

     A  hybrid patch used for VOX operation needs to be  adjusted

carefully  for  good performance.  If it has a  null  adjustment,

this  should be set before adjusting the VOX controls.   Using  a

termination:  the  easiest way to do this is to dial  part  of  a

number; another way to do it is call a cooperative friend.   Tune

the  shack receiver to a "talk" broadcast station or use the  BFO

as  a heterodyne.  With the transmitter keyed into a dummy  load,

transceiver, place an oscilloscope or audio voltmeter across  the

microphone input terminals and, while receiving a signal,  adjust

for  the  lowest voltage.  For proper operation,  it's  important

that the phone be connected to the patch during these adjustments

terminated.

Reference

September 1985, page  38

Bibliography

Rogers, Tom, You and Your Telephone, Howard W. Sams & Co.,  Inc.,

Bell System Technical Reference 48005; Telephones, January, 1980.

British  Standard  Specification  for  General  Requirements  for

Apparatus for Connection to the British Telecommunications Public

Switched Telephone Network.  BS 6305.

Certification  Standard  for  Voice-Type Terminal  Equipment  and

Connectors, No.CS-01 and No.CS-03, Department of  Communications,

Government of Canada.

FCC  Rules  and Regulations: Part 68 -  connection  of  Terminal

Equipment  to  the Telephone Network,  United  States  Government

                           End of Text

                   ----------------------------

                    Fig 1. Simple Phone Patch

     Tip  \                   C5

     O----.\---o---------o----||----------O

          .    |         |

          .    |         /

        S1.    |       R6\       Shielded

          .    |         /       Wire

     Ring \    |         |    C5 To Transmitter

     O---- \--------o----o----||---o------O

               |    |              |

               |    |              |

               |    |            -----

               |    |             ---

               |    |              -

               |    |         C5

               |    |---------||---o------O

               |                   |

               |                   \ Shileded

               |                 R7/ Wire

               |                   \ To Receiver

               |              C5   |

               ---------------||---o------O

                                 -----

---

                 -------------------------------

                    Fig 2. Basic Phone Patch

      Tip \     R1A

     O---o.\o--/\/\/-----o-----  -------o----o----O

          .              |    |  |      |    | To Tx

          .              |    |  |      |    |

          .              |    )||(      |    |

          .             ---   )||(   C1---   -----O

        S1.         MOV ^ ^ T1)||(     ---     To Rx

          .             ---   )||(      |

          .              |    )||(      | Shielded

          .              |    |  |      | Cable

      Ring\     R1B      |    |  |      |    Common

     O---o \o--/\/\/-----o-----  -------o----o----O

                                           -----

---

                             ------------------

                   Fig. 3 Improved Phone Patch

             ----

     Tip   \ |  |    R1A

     o----o.\o  o---/\/\/--o-------||(-----------------o

          |.    |          |      |||(

          |.    |          |      |||( 8 Ohms   To RX

          |.    |          |      |||(   Shielded cable

        --|.    |          |      |||(------------o----o

        |  .    |          |   T2 )||             |

        | |-----|     MOV ---     )||(----o----o  |

        | |.              ^ ^     )||(    |    |  |

        | |. S2 Hksw      ---     )||(600 | C1 \  | R10

        --------           | 600  )||(   ---   /<------o

          |.    |          | Ohms |||(   ---   \  | To TX

          |.    |          |      |||(Ohms|    /  | Shielded

      Ring|\    |   R1B    |      |||(    |    |  | Cable

     o----o \o  o--/\/\/---o-------  (----o----o--o----o

             |  |                                 |

             ----                               -----

---

     NOTE: S2 Hook Switch is also a polarity reversal switch.

                    -------------------------

Fig  4.  Typical U.S. Network (425B). Note: Circled  letters  are

marked  on  Network Interconnection  block  terminals.  Component

values may vary slightly between manufacturers.

                         |-------------------|

                       ..|...................|

                       . |                  .|

     Sidetone balancing. |    C3            .|

     impedance & loop  . |    | |           .|

     compensation. >>> . o----| |-------o   .|

                       . |    | |       |   .|

                       . |              |   .|

                       . |    |<| VR2   |   .|

                       . o----| |-------o---.|

                       . |    |>|          |.|

                       . |                 |.|

                       . |   R4            |.|

                       . o---\/\/\/-----|  |.|

                       ..|..............|..|.|

                         |              |  | |

                         |        . (GN)|  | |

                     (R) -----)||(------|-------o-----|

                         TA1 1)||(5 TC  |  | |  |     |

               Loop           )||(      |  | |  |     |

 TIP    \      Compensation  2)||(6     |  | | ---    |

 o-----o.\----------o---------)||(------o  | | ^ ^ RX O

        .           |   (RR) . ||       |  | | ---    |

        .           |          ||       |  | |  |VR60 |

        .           \ 180      ||   C2 --- | |  |     |

        .           / Ohms     ||      --- | |--o-----o

        .  (F) C4   \          ||       |  |    |     |

   S1   .   o--||---|          ||       |  |    |     |

  HKSW  .          ---       . || .     |  |    o     |

        .          ^ ^   -----)||(------o---   \   TX O

        .      VR1 ---   |   3)||(7           S3|     |

        .           |    |TA2 )||(  TB          |     |

  RING \.           |    |   4)||(8       R3    |     |

 o----o \-----------o---------)||(---o----/\/\/-o------

      (L2)               | (C)       |         (B)

            ^            |           |

        Hookswitch        ------------

                       -------------------------

     Fig. 5. Typical European Network

      A  \

     o--o.\---------o----o----o-------|

         .          |    |    |       |

         .          | C4 |    |       |

         .          |   ---   \       |

         .          |   ---   / R5    |

         .          |    |    \       |

         .          |    |    |       |

         .          |    -----o----)|||

         .          |              )|||

     S1  .          |              )||o------o-----

    HKSW .          |         200  )|||   VR |    |

         .       TX O          Ohms)|||   60 |    |

         .          |              )||(    -----  |

         .          |              )||(     ^ ^   O RX

         .          ---------------|||(    -----  |

         .                     50  )||(60    |    |

         .                     Ohms)||(Ohms  |    |

      B \.                         )||(------o-----

     o--o\-------------------------)||

                        ----------------------------

     Fig. 6. Deluxe Phone Patch

                         |-------------------|

                         |                   |

                         |                   |

                         |    C3             |

                         |    | |            |

                         o----| |-------o    |

                         |    | |       |    |

                         |              |    |

                         |    |<| VR2   |    |

                         o----| |-------o--- |

                         |    |>|          | |

                         |                 | |

                         |   R4            | |

                         o---\/\/\/-----|  | |

                         |      ^ or R11|  | |

                         |      |-------|  | |

                         |        . (GN)|  | |

                     (R) -----)||(------|-------------

                         TA1 1)||(5 TC  |  | |       |

                              )||(      |  | |       |

 TIP    \    R1A             2)||(6     |  | |    R12/   To TX

 o-----o.\--/\/\/---o---------)||(------o  | |       \<---------

        .           |   (RR) . ||       |  | |       /

        .           |          ||       |  | |       |

        .           \ R2       ||   C2 --- | |--o----|---|------

        .           /          ||      --- |    |    |  ---

        .           \          ||       |  |    | R12\   -

   S1   .           |          ||       |  |    |    /<---------

  HKSW  .          ---       . || .     |  |   ---   \   To RX

        .          ^ ^   -----)||(------o---   ---   |

        .      VR1 ---   |   3)||(7          C1 |    |

        .           |    |TA2 )||(  TB          |    |

  RING \.   R1B     |    |   4)||(8       R3    |    |

 o----o \---/\/\----o---------)||(---o----/\/\/-o------

      (L2)               | (C)       |         (B)

                         |           |

                          ------------

Note:  T1  600 Ohm 1:1 Transformer would be between  R1  and  the

line.

                    -------------------------

     Parts List

     Item      Description

     C1        0.1 uF (see text)

     C2        1.5 to 2.0uF (Depending on manufacturer)

     C3        0.47 uF Not used in all networks

     C4        0.1 uF

     C5        2.0 uF 250 Volt Mylar Film (see text)

     MOV       130 to 250 Volt MOV (see text)

     R1A,B     100 to 270 Ohms (see text)

     R2        180 to 220 Ohms (depending on manufacturer)

     R3        22 Ohms

     R4        47 to 110 Ohms (depending on manufacturer)

     R5        1 Kilo Ohm

     R6        1 Kilo Ohm (see text)

     R7        10 Ohm (see text)

     R10       1 Kilo Ohm potentiometer (see text)

     R11       200 Ohm potentiometer (see text)

     R12       2 Kilo Ohm potentiometer (see text)

     S1        DPST or Hookswitch

     S3        NC Momentary switch (see text)

     T1        600 Ohm 1:1 transformer

     T2        600 Ohm primary. 600 Ohm and 8 Ohm secondary (see text)

     T3        Network Transformer

     VR1       Silicon Carbide Varistor or Back-to-back Zener

     VR2       Silicon Carbide Varistor or Back-to-back Zener

     VR60      Silicon Carbide Varistor or Back-to-back Zener

END

--

Julian Macassey, julian@bongo.info.com  N6ARE@K6VE.#SOCAL.CA.USA.NA

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