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
it 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
phone line for phone conversations. But there's more to it -
Amateurs can and do use phone patches for purposes other than
telephone conversations. One particularly effective application
is 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
will indicate whether your transmitter is or is not the source of
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
device - which also happens to be how professional audio can be
described. Most modern Amateur transmitters have 600-ohm
unbalanced inputs; most cassette recorders have a 600-Ohm
unbalanced input; the "tape" outputs on home stereos are also
600-ohm unbalanced. All this makes patching relatively simple.
While there are various degrees of sophistication and complexity
in patching, in an emergency, patches can be easily put together
using readily available components. Before starting to build a
patch, however, it might be helpful to read last month's article
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
will tend to introduce hum to the line because of the unbalance
introduced. 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
hook. The receiver output may need a DC load (R7) to prevent the
output stage from "motorboating." Use two capacitors to maintain
the balance.
With all patches hum can be lessened by reversing the phone
wires. A well-made patch will have no discernible hum.
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
600-ohm 1:1 transformer such as those used in professional audio
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
signal.)
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
destructive high voltage that may come down the phone line, and
into the Amateur's equipment. A 130 to 250 volt Metal Oxide
Varistor (MOV) across the phone line will provide further
protection if needed.
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
recommended - the audio level on the incoming line may be too
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
when the patch transformer is switched across the phone line.
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
work well into the 600-ohm winding, and if transmitting simplex
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
phone line at an adequate level; this can be handled by using 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
input, 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
problem. Unshielded transformers are sensitive to hum fields and
building any patch into a steel box will help alleviate hum as
well as RFI.
The Improved Phone Patch
Several enhancements can be made to the basic phone patch to
improve operation. The first is the addition of a double-pole
double-throw switch to reverse the polarity of the phone line to
reduce hum. This may not be necessary with a patch at the same
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
switching line polarity makes no difference. The switch should
have a center "off" position or use a separate double-pole single
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
while transmitting and receiving. Switch the shield and inner
conductors of the secondaries for minimum hum.
Many transmitters do not offer easy access to the microphone
gain control. There may also be too much level from the patch to
make adjustment of the transmit level easy. Placing R10 across
the transformer allows easy adjustment of the level. It can be
set so that when switching from the station microphone to the
patch the transmitter microphone gain control does not need to 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
potentiometer will do. A 2.5kilohm potentiometer may provide
better control.
Deluxe Operation and VOX
Using VOX with a phone patch may cause a problem with
receive audio going down the line and into the transmit input,
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
5).
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
remove R4 from the Western Electric network, the can has to be
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
shirts. (This material - alleged to be manufactured according to
a secret formula - will not wash off with soap and water. The
phone company has a solvent for it, but because one of the secret
ingredients is said to be beeswax, ordinary beeswax solvents such
as gum turpentine, mineral turpentine (paint thinner or white
spirit) and kerosene will work.) To remove the bulk of the
potting compound, heat the opened can for 30 minutes in a 300
degree F (148 degree C) oven, or apply heat from a hot hairdryer
or heatgun. You can also put the can out in the hot sun under a
sheet of glass. Don't use too much heat because the plastic
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
in parallel with it. This enables the operator to dial, answer,
and monitor calls to and from the patch, as well as use the
handset for joining in conversations or giving IDs.
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
receiver, masking the incoming call. Muting the transmitter
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,
so that the transmitter element is muted by shorting across it
(see fig.4). This makes the mute "clickless." If the monitor
phone uses an electret or dynamic transmitter it should still be
wired as shown in fig.4.
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
regulatory agencies aim for the same goals; enough level for
intelligibility, 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.
If the level out from the patch is not high enough, the distant
party will ask for repeats and tend to speak louder to compensate
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
get a feel for the level needed is to practice monitoring on the
handset by feeding a broadcast station down the phone line to
another Amateur who can give meaningful signal reports. It's
difficult to send too much level down the phone while monitoring
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
radio transmitter. Depending on the setup, the RF output
indication on a wattmeter, the ALC on the transmitter or even
listening to the transmitted signal on a monitor receiver will
help in adjusting the audio into the radio transmitter. Phone
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
pauses.
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
separate receiver/transmitter setup is the easiest to adjust the
patch. The phone line should be attached to a silent
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,
set the null adjustment potentiometer R11 (R12 for European
phones) for a minimum RF output on the transmitter. Using a
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
since the hybrid relies on all inputs and outputs being
terminated.
Reference
1. Julian Macassey, N6ARE, "Understanding Telephones," ham radio,
September 1985, page 38
Bibliography
Rogers, Tom, You and Your Telephone, Howard W. Sams & Co., Inc.,
Indianapolis, Indiana 46206. ISBN No. 0-672-21744-9.
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
Printing Office, 1982.
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
742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495