617 lines
29 KiB
Plaintext
617 lines
29 KiB
Plaintext
From telecom@eecs.nwu.edu Wed Aug 7 00:47:09 1991
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Reply-To: julian@bongo.info.com
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X-Mailer: Mail User's Shell (6.4 2/14/89)
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To: telecom@eecs.nwu.edu
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Subject: Phone Patches
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Message-Id: <9108050620.AA04638@bongo.info.com>
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Date: 5 Aug 91 06:20:06 PDT (Mon)
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From: Julian Macassey <julian@bongo.info.com>
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Resent-Date: Tue, 6 Aug 91 23:50:11 CDT
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Resent-From: telecom@eecs.nwu.edu
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Resent-To: ptownson@gaak.LCS.MIT.EDU
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Status: RO
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Dear Patrick,
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Here is an article I wrote about phone patches. If you think it is
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worth it, stuff it in the archives.
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------cut, slash, deforest ----------------------
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BUILDING AND USING PHONE PATCHES
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From simple to elegant, patches help make the connection
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By
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Julian Macassey, N6ARE
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First Published in Ham Radio Magazine
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October 1985.
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In telephone company parlance, a patch is any connection
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between a phone line and another communications device, whether
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it be a radio, a tape recorder, a data device (such as a modem),
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or even another phone line.
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Radio Amateurs, on the other hand, tend to limit the meaning
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of "patch" to the connection of transmitters or receivers to the
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phone line for phone conversations. But there's more to it -
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Amateurs can and do use phone patches for purposes other than
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telephone conversations. One particularly effective application
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is for checking TVI and RFI complaints; simply set the
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transmitter on VOX, go to the site of the interference complaint,
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and then key your transmitter via the phone line. Doing this
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will indicate whether your transmitter is or is not the source of
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the problem. If it is, you can use this method to test the
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measures you've taken to correct the problem.
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A phone line is, simply speaking, a 600-ohm balanced feed
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device - which also happens to be how professional audio can be
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described. Most modern Amateur transmitters have 600-ohm
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unbalanced inputs; most cassette recorders have a 600-Ohm
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unbalanced input; the "tape" outputs on home stereos are also
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600-ohm unbalanced. All this makes patching relatively simple.
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While there are various degrees of sophistication and complexity
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in patching, in an emergency, patches can be easily put together
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using readily available components. Before starting to build a
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patch, however, it might be helpful to read last month's article
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on understanding phone lines.
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The Simple Patch
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The simplest way to patch a phone line to another piece of
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equipment is to use a couple of capacitors to block the phone
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line DC. While this simple approach will work in a pinch, it
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will tend to introduce hum to the line because of the unbalance
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introduced. The capacitors used should be nonpolar, at least 2-
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ohm F, and rated at 250 volts or better (see fig.1).
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To hold the line, the patch should provide a DC load by
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means of a resistor (R6) or by simply leaving a phone off the
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hook. The receiver output may need a DC load (R7) to prevent the
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output stage from "motorboating." Use two capacitors to maintain
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the balance.
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With all patches hum can be lessened by reversing the phone
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wires. A well-made patch will have no discernible hum.
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The Basic Phone Patch
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Because a phone line is balanced and carries DC as well as
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an AC signal, a patch should include a DC block, a balun, and a
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DC load to hold the line. The best component for doing this is a
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600-ohm 1:1 transformer such as those used in professional audio
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and for coupling modem signals to the phone line, available from
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most electronics supply houses. Old telephone answering machines
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are also a good source of 600-ohm transformers. Some
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transformers are rated at 600-900 ohms or 900-900 ohms; these are
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also acceptable. Make sure that the transformer has a large
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enough core, because DC current will be flowing through it.
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(Some small-core transformers become saturated and distort the
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signal.)
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In section 68.304 of the FCC Part 68 regulations, it states
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that a coupling transformer should withstand a 60 Hz 1kV signal
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for one minute with less than 10 mA leakage. For casual use this
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may seem unimportant, but it provides good protection against any
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destructive high voltage that may come down the phone line, and
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into the Amateur's equipment. A 130 to 250 volt Metal Oxide
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Varistor (MOV) across the phone line will provide further
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protection if needed.
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The DC resistance of the transformer winding may be so low
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that it hogs most of the phone line current. Therefore, while
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using a phone in parallel for monitoring and dialing - which is
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recommended - the audio level on the incoming line may be too
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low. Resistors R1A and R1B (see fig.2) will act as current
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limiters and allow the DC to flow through the phone where it's
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needed. If possible, these resistors should be carbon
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composition types.
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To keep the line balanced, use two resistors of the same
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value and adjust the values by listening to the dial tone on a
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telephone handset. There should be little or no drop in volume
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when the patch transformer is switched across the phone line.
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One of these transformers, or even two capacitors, can be
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used to patch two phone lines together, should there be a need to
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allow two distant parties to converse. There will be losses
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through the transformer so the audio level will degrade, but with
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two good connections this will not be a problem.
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On the other side of the transformer - which could be called
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the secondary winding - choose one pin as the ground and attach
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the shields of the microphone and headphone cables to it. Attach
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the inner conductors to the other pin. The receiver output will
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work well into the 600-ohm winding, and if transmitting simplex
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or just putting receiver audio on the line there will be no
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crosstalk or feedback problems. In some cases, the audio
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amplifier in a receiver does not have enough output to feed the
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phone line at an adequate level; this can be handled by using the
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transformer with two secondaries (see the "improved" patch below)
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or by coupling a 8:1 kilohm transformer between the audio output
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and 600-ohm transformer. If RF is getting into the transmitter
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input, a capacitor (C1) across the secondary should help. A good
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value for the lower bands and AM broadcast interference is 0.1
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uF. For higher frequencies, 0.01 uF usually gets rid of the
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problem. Unshielded transformers are sensitive to hum fields and
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building any patch into a steel box will help alleviate hum as
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well as RFI.
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The Improved Phone Patch
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Several enhancements can be made to the basic phone patch to
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improve operation. The first is the addition of a double-pole
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double-throw switch to reverse the polarity of the phone line to
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reduce hum. This may not be necessary with a patch at the same
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location with the same equipment, but if it is, experiment with
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the polarity of the transformer connections and adjust for the
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least hum. Most of the time the balance will be so good that
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switching line polarity makes no difference. The switch should
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have a center "off" position or use a separate double-pole single
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throw switch to disconnect from the line. The two secondaries on
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the "improved" patch (fig.3) should be checked for balance by
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connecting the receiver and transmitter and checking for hum
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while transmitting and receiving. Switch the shield and inner
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conductors of the secondaries for minimum hum.
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Many transmitters do not offer easy access to the microphone
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gain control. There may also be too much level from the patch to
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make adjustment of the transmit level easy. Placing R10 across
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the transformer allows easy adjustment of the level. It can be
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set so that when switching from the station microphone to the
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patch the transmitter microphone gain control does not need to be
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adjusted. This will also work on the basic 600-ohm 1:1
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transformer. Most of the time a 1 kilohm potentiometer -
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logarithmic if possible - will work well. If not, a linear
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potentiometer will do. A 2.5kilohm potentiometer may provide
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better control.
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Deluxe Operation and VOX
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Using VOX with a phone patch may cause a problem with
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receive audio going down the line and into the transmit input,
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triggering the VOX. There may not be enough Anti-VOX adjustment
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to compensate for this. The usual solution for this problem is
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to use a hybrid transformer, a special telephone transformer with
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a phasing network to null out the transmit audio and keep it off
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the receive line. Most telephones employ a similar transformer
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and circuit so that callers will not deafen themselves with their
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own voices. These devices are called "networks" (see figs. 4 and
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5).
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A network can be removed from an old phone and modified into
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a deluxe patch, or the phone can be left intact and connections
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made to the line and handset cords. The line cord should be
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coupled to a 600-ohm 1:1 transformer to keep the ground off the
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line. Note, in the network schematics, that the receiver and
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transmitter have a common connection; when coupling into radios
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or other unbalanced devices, make this the ground connection.
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There may be confusion about terms used in the network. The
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telephone receiver is receiving the phone line audio, and the
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transmitter is transmitting the caller's voice. For phone patch
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use, a telephone receive line is coupled to the transmitter and
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the transmit line is coupled to the radio receiver. This is a
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fast way to put together a phone patch and may be adequate for
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VOX use.
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A better patch can be built by using a network removed from
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a phone or purchased from a local telephone supply house. This
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approach offers the added advantage of being able to adjust or
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null the sidetone. The circled letters in figs. 4 and 6 refer to
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the markings on the network terminal block. These letters are
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common to all United States networks made by Western Electric (AT
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& T), ITT, Automatic Electric, Comdial, Stromberg Carlson, and
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ATC.
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To make sidetone adjustable, remove R4 (R5 in European
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networks) and replace it with R11 (for European networks use
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R12). The Western Electric Network comes point-to-point wired
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and sealed in a can; the other networks are mounted on PCBs. To
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remove R4 from the Western Electric network, the can has to be
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opened by bending the holding tabs. Don't be surprised to find
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that the network has been potted in a very sticky, odious paste
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that has the texture of hot chewing gum and the odor of unwashed
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shirts. (This material - alleged to be manufactured according to
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a secret formula - will not wash off with soap and water. The
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phone company has a solvent for it, but because one of the secret
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ingredients is said to be beeswax, ordinary beeswax solvents such
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as gum turpentine, mineral turpentine (paint thinner or white
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spirit) and kerosene will work.) To remove the bulk of the
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potting compound, heat the opened can for 30 minutes in a 300
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degree F (148 degree C) oven, or apply heat from a hot hairdryer
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or heatgun. You can also put the can out in the hot sun under a
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sheet of glass. Don't use too much heat because the plastic
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terminal strip may melt. Even with a film of compound remaining
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on it, the network can be worked on.
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Using a Patch
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For efficient use, a patch should have a telephone connected
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in parallel with it. This enables the operator to dial, answer,
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and monitor calls to and from the patch, as well as use the
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handset for joining in conversations or giving IDs.
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One useful modification to the control telephone is adding a
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mute switch to the handset transmitter. This allows monitoring
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calls without letting room noise intrude on the line. It's also
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a good modification for high noise environments, where ambient
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noise enters through the handset transmitter and is heard in the
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receiver, masking the incoming call. Muting the transmitter
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makes calls surprisingly easy to hear. The mute switch can be a
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momentary switch used as a "Push-To-Talk" (PTT) or a Single Pole
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Single Throw (SPST) mounted on the body of the phone for long-
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term monitoring. The switch should be wired as Normally Closed,
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so that the transmitter element is muted by shorting across it
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(see fig.4). This makes the mute "clickless." If the monitor
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phone uses an electret or dynamic transmitter it should still be
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wired as shown in fig.4.
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Transmit and receive levels on the phone line are a source
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of confusion that even telephone companies and regulatory
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agencies tend to be vague about. The levels, which can be
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measured in various ways, vary. But all phone companies and
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regulatory agencies aim for the same goals; enough level for
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intelligibility, but not enough to cause crosstalk. The most
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trouble-free way to set the outgoing level on the patch is to
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adjust the feed onto the phone line until it sounds slightly
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louder than the voice from the distant party on the phone line.
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If the level out from the patch is not high enough, the distant
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party will ask for repeats and tend to speak louder to compensate
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for a "bad line." In this case, adjust the level to the patch
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until the other party lowers his or her voice. The best way to
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get a feel for the level needed is to practice monitoring on the
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handset by feeding a broadcast station down the phone line to
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another Amateur who can give meaningful signal reports. It's
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difficult to send too much level down the phone while monitoring
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because the signal would simply be too loud to listen to
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comfortably. The major problem is sending too little signal down
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the line.
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Coupling the phone line into the radio transmitter is not
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much more difficult than adjusting a microphone to work with a
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radio transmitter. Depending on the setup, the RF output
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indication on a wattmeter, the ALC on the transmitter or even
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listening to the transmitted signal on a monitor receiver will
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help in adjusting the audio into the radio transmitter. Phone
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lines can be noisy, and running too much level into the
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transmitter and relying on the ALC to set the modulation can
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cause a fair amount of white noise to be transmitted. Watching
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the RF output while there are no voice or control signals on the
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line will help in adjusting for this. VOX operation can
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alleviate the problem of noise being transmitted during speech
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pauses.
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A hybrid patch used for VOX operation needs to be adjusted
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carefully for good performance. If it has a null adjustment,
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this should be set before adjusting the VOX controls. Using a
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separate receiver/transmitter setup is the easiest to adjust the
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patch. The phone line should be attached to a silent
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termination: the easiest way to do this is to dial part of a
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number; another way to do it is call a cooperative friend. Tune
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the shack receiver to a "talk" broadcast station or use the BFO
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as a heterodyne. With the transmitter keyed into a dummy load,
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set the null adjustment potentiometer R11 (R12 for European
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phones) for a minimum RF output on the transmitter. Using a
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transceiver, place an oscilloscope or audio voltmeter across the
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microphone input terminals and, while receiving a signal, adjust
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for the lowest voltage. For proper operation, it's important
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that the phone be connected to the patch during these adjustments
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since the hybrid relies on all inputs and outputs being
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terminated.
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Reference
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1. Julian Macassey, N6ARE, "Understanding Telephones," ham radio,
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September 1985, page 38
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Bibliography
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Rogers, Tom, You and Your Telephone, Howard W. Sams & Co., Inc.,
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Indianapolis, Indiana 46206. ISBN No. 0-672-21744-9.
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Bell System Technical Reference 48005; Telephones, January, 1980.
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British Standard Specification for General Requirements for
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Apparatus for Connection to the British Telecommunications Public
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Switched Telephone Network. BS 6305.
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Certification Standard for Voice-Type Terminal Equipment and
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Connectors, No.CS-01 and No.CS-03, Department of Communications,
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Government of Canada.
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FCC Rules and Regulations: Part 68 - connection of Terminal
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Equipment to the Telephone Network, United States Government
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Printing Office, 1982.
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End of Text
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----------------------------
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Fig 1. Simple Phone Patch
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Tip \ C5
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O----.\---o---------o----||----------O
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. | |
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. | /
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S1. | R6\ Shielded
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. | / Wire
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Ring \ | | C5 To Transmitter
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O---- \--------o----o----||---o------O
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| | -----
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| | ---
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| | -
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| | C5
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| |---------||---o------O
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| \ Shileded
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| R7/ Wire
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| \ To Receiver
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| C5 |
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---------------||---o------O
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-----
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---
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-
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-------------------------------
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Fig 2. Basic Phone Patch
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Tip \ R1A
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O---o.\o--/\/\/-----o----- -------o----o----O
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. | | | | | To Tx
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. | | | | |
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. | )||( | |
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. --- )||( C1--- -----O
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S1. MOV ^ ^ T1)||( --- To Rx
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. --- )||( |
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. | )||( | Shielded
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. | | | | Cable
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Ring\ R1B | | | | Common
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O---o \o--/\/\/-----o----- -------o----o----O
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-----
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---
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-
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------------------
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Fig. 3 Improved Phone Patch
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----
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Tip \ | | R1A
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o----o.\o o---/\/\/--o-------||(-----------------o
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|. | | |||(
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|. | | |||( 8 Ohms To RX
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|. | | |||( Shielded cable
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--|. | | |||(------------o----o
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| . | | T2 )|| |
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| |-----| MOV --- )||(----o----o |
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| |. ^ ^ )||( | | |
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| |. S2 Hksw --- )||(600 | C1 \ | R10
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-------- | 600 )||( --- /<------o
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|. | | Ohms |||( --- \ | To TX
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|. | | |||(Ohms| / | Shielded
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Ring|\ | R1B | |||( | | | Cable
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o----o \o o--/\/\/---o------- (----o----o--o----o
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| | |
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---- -----
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---
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-
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NOTE: S2 Hook Switch is also a polarity reversal switch.
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-------------------------
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Fig 4. Typical U.S. Network (425B). Note: Circled letters are
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marked on Network Interconnection block terminals. Component
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values may vary slightly between manufacturers.
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|-------------------|
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..|...................|
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. | .|
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Sidetone balancing. | C3 .|
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impedance & loop . | | | .|
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compensation. >>> . o----| |-------o .|
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. | | | | .|
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. | | .|
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. | |<| VR2 | .|
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. o----| |-------o---.|
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. | |>| |.|
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. | |.|
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. | R4 |.|
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. o---\/\/\/-----| |.|
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..|..............|..|.|
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| . (GN)| | |
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(R) -----)||(------|-------o-----|
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TA1 1)||(5 TC | | | | |
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Loop )||( | | | | |
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TIP \ Compensation 2)||(6 | | | --- |
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o-----o.\----------o---------)||(------o | | ^ ^ RX O
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. | (RR) . || | | | --- |
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. | || | | | |VR60 |
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. \ 180 || C2 --- | | | |
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. / Ohms || --- | |--o-----o
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. (F) C4 \ || | | | |
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S1 . o--||---| || | | | |
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HKSW . --- . || . | | o |
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. ^ ^ -----)||(------o--- \ TX O
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. VR1 --- | 3)||(7 S3| |
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. | |TA2 )||( TB | |
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RING \. | | 4)||(8 R3 | |
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o----o \-----------o---------)||(---o----/\/\/-o------
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(L2) | (C) | (B)
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^ | |
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Hookswitch ------------
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-------------------------
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Fig. 5. Typical European Network
|
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A \
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o--o.\---------o----o----o-------|
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. | | | |
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. | C4 | | |
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. | --- \ |
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. | --- / R5 |
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. | | \ |
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. | | | |
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. | -----o----)|||
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. | )|||
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S1 . | )||o------o-----
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HKSW . | 200 )||| VR | |
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. TX O Ohms)||| 60 | |
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. | )||( ----- |
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. | )||( ^ ^ O RX
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. ---------------|||( ----- |
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. 50 )||(60 | |
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. Ohms)||(Ohms | |
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B \. )||(------o-----
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o--o\-------------------------)||
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----------------------------
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|
Fig. 6. Deluxe Phone Patch
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|-------------------|
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| |
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| |
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| C3 |
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| | | |
|
|
o----| |-------o |
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| | | | |
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| | |
|
|
| |<| VR2 | |
|
|
o----| |-------o--- |
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| |>| | |
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| | |
|
|
| R4 | |
|
|
o---\/\/\/-----| | |
|
|
| ^ or R11| | |
|
|
| |-------| | |
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|
| . (GN)| | |
|
|
(R) -----)||(------|-------------
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|
TA1 1)||(5 TC | | | |
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|
)||( | | | |
|
|
TIP \ R1A 2)||(6 | | | R12/ To TX
|
|
o-----o.\--/\/\/---o---------)||(------o | | \<---------
|
|
. | (RR) . || | | | /
|
|
. | || | | | |
|
|
. \ R2 || C2 --- | |--o----|---|------
|
|
. / || --- | | | ---
|
|
. \ || | | | R12\ -
|
|
S1 . | || | | | /<---------
|
|
HKSW . --- . || . | | --- \ To RX
|
|
. ^ ^ -----)||(------o--- --- |
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|
. VR1 --- | 3)||(7 C1 | |
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. | |TA2 )||( TB | |
|
|
RING \. R1B | | 4)||(8 R3 | |
|
|
o----o \---/\/\----o---------)||(---o----/\/\/-o------
|
|
(L2) | (C) | (B)
|
|
| |
|
|
------------
|
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|
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|
|
Note: T1 600 Ohm 1:1 Transformer would be between R1 and the
|
|
line.
|
|
|
|
|
|
-------------------------
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|
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Parts List
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|
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|
|
Item Description
|
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|
|
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
|
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|
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|
|
END
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--
|
|
Julian Macassey, julian@bongo.info.com N6ARE@K6VE.#SOCAL.CA.USA.NA
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742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495
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