>------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
>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 | | | --- |
[31/200] (?-AQZKSRE):
32/200 [1] Networked alt.2600
Title: haq faq post
Date: Thu, 27 Oct 1994 19:22:26 UTC (6 days old)
Origin: anon.penet.fi (Unknown) [?]
From: an116797 @anon.penet.fi Internet
> 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
>
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