>------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 > ------------------------------------------------------------------------- To find out more about the anon service, send mail to help@anon.penet.fi. 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