Purpose of the work

The purpose of the work is to deepen knowledge about the design and principle of operation of the TA-68 telephone set, a theoretical study of dialing signals and information acoustic signals.

Theoretical part

1. Generalized block diagram and operating principle of a telephone set

To the generalized block diagram TA systems of the Central Bank ATS (Fig. 1) include: speaking devices (RGP), call receiver (CR), lever switch (RP) and dialer (DN). RGPs consist of acoustoelectric and electroacoustic transducers. Microphones and electromagnetic telephones are used as converters, which for convenience are combined into one unit - a microtelephone handset.

Figure 1 – Generalized block diagram of the telephone apparatus of the Central Bank ATS system

2. Rotary dialer device

The disk dialer is a spring mechanism that creates direct current pulses when dialing a number to control devices on the telephone exchange. The dialer device is shown schematically in Fig. 2. Winding disc 19 with emphasis 20 has holes under which the numbers 1, 2, ..., 9, 0 are written. On the axis 8 reinforced end of the return coil spring 9, segment 10 for switching contact springs 1…5 and big gear 11. On the second axis 12 placed small gear 13 with a dog 14 and ratchet 15 , worm wheel 16 , impulse washer 17 , which serves to open the contact springs 6  7. Regulator 18 axis-related 12 worm gear, maintains the required speed of the disk when it returns to its original position.

Figure 2 – Rotary dialer device

3. Carbon microphone

Figure 3 – Carbon microphone: A- principle of the device and circuit diagram; b- sketch of the MK-16 microphone; V- symbol;

1 - fixed electrode; 2 - coal powder; 3 - movable electrode; 4- membrane; 5 - frame; 6 - backfill limiter; 7 - insulating sleeve.

4. Electromagnetic telephone

An electromagnetic telephone with a simple magnetic system (Fig. 5) consists of a permanent magnet 1 , pole extensions 2 , windings 3 , membranes 4 , anchors 5 and rod 6 .

Figure 4 – Electromagnetic telephone with a simple magnetic system

5. Anti-local telephone circuits

There are two classes of counter-local schemes: pavements And compensatory. The principle of constructing a bridge circuit is shown in Fig. 5 A. It is formed by: three-winding transformer Tr, microphone R m, phone Z t, line input impedance Z k and resistance Z b. Such a circuit can be represented in the form of a canonical AC bridge, in one diagonal of which a microphone is included as an alternating current generator, and in the other - a telephone.

Figure 5 – Anti-local pavement ( A) and compensation ( b) telephone diagrams

5. Telephone set TA-68

Let's consider the operation of the TA-68 apparatus, the schematic diagram of which is shown in Fig. 6. The device operates according to a counter-compensation type scheme. The circuit includes: lever switch RP, bell Zv, capacitor WITH 1, NN dialer, telephone T, microphone M, transformer Tr, balanced circuit (capacitors WITH 1 and WITH 2 and resistors R 1 and R2). The transformer has three windings: linear I, balanced II and telephone III.

Figure 6 – Schematic diagram of the TA-68 telephone set

Conclusion

In this laboratory work We studied the circuit and operating principle of a telephone set. We looked in detail at how a rotary dialer and a carbon microphone work, and also studied the construction of two classes of anti-local circuits: bridge and compensation circuits of telephone sets.

For the telephone to operate, two conditions must be met: to provide power to the conversation circuits with a constant voltage of 1.5 - 9 volts (depending on the type of device) and to provide power to the call circuits with an alternating voltage of 40 - 60 volts, 25 - 50 Hz. Based on the power supply principle, telephone sets are divided into two groups. The first group includes local battery (MB) devices, in which all power sources are located inside: a galvanic battery for powering conversation circuits and a manual AC inductor for sending a call to the subscriber. Such telephones include field military devices TAI-43 and TA-57. The second group includes central battery (CB) devices, the circuits of which are powered from a central station or automatic telephone exchange; these devices do not have their own power sources. These phones include all devices with dialers and some others. public use type: TA-68, TAN-70, VEF TA-12, Aster, etc. When the devices of the first group are connected to each other by a two-wire line, they immediately begin to work without any problems, since they are MB devices with a local battery. In order to make two connected devices of the central bank of the second group work, I assembled a special device. There are quite a few descriptions of such devices, but all of these schemes, as written earlier, have a significant drawback - a three-wire line is required to connect the devices. The device I assembled provides operation over a two-wire line.

The power supply itself is located on the side of one of the subscribers and consists of a step-down network transformer Tr1. The secondary winding of the transformer provides two voltages: 40 and 15 volts. An alternating voltage of 40 volts provides the calling circuits. The second voltage is rectified by the CC bridge and stabilized by a stabilizer on the ROLL - it is used to power the conversation circuits. The stabilizer and capacitor C1 are needed to reduce the background alternating voltage during a conversation. The stabilizer can be neglected if the background is not large. KN buttons are used without fixation and are mounted in telephone bodies. The TA2 device is connected to the TA1 device and the telephone device using a two-wire TRP 1 x 2 wire. The lower contacts of the switches KN1 and KN2 according to the diagram are grounded. Grounding can be a water supply pipe, heating pipe, or a metal pin driven into the ground. I used the ground contact of a Euro socket.

Operation of the circuit. When you press the KN1 button on the TA1 device, an alternating voltage of 40 V from the transformer winding through the closed contacts of the EH1 button is supplied through the line, the normally closed contacts of KN2 to the ringing device of the TA2 device. (when the handset is on the device, a ringing device is connected to the line). From the device through the line, capacitor C1, to the second horse winding 40 V. The phone TA2 rings. When the telephone handsets are lifted in both devices and the buttons KH1 and KH2 are pressed, the intercom circuits of the devices are connected to the line. In this case, the 12 volt DC power supply is connected in series with the telephone sets. By circuit: Capacitor C1 plus power supply, connecting line, spoken circuit of the TA2 device, closed contacts of the KN2 button, line, closed contacts of KN1, spoken circuit of the TA1 device, minus the power supply. The scheme works similarly when sending a call from the TA2 telephone set. When the KN2 button is pressed, the ringing alternating voltage of 40 V from the transformer winding through the grounding and closed calling contacts of KN2 enters the line and through the contacts of KN1 to the telephone ringer TA1 and the second end of the 40 V winding Tr1. The conversation of subscribers takes place according to the circuit described above. In my use case of this device at the installation point of the TA2 telephone there were no lines at all except grounding and TV cable cable television going to TV. Laying a new line through the building was far away and expensive, and the television cable ran not far from the installation of the TA1 telephone. As a result, I was able to connect the TA1 and TA2 telephone sets using the already installed RK75 television cable without disturbing the operation of the TV. For these purposes, I installed special isolation filters on the cable.

Chokes Dr1 and Dr2 serve to suppress high-frequency television signals from penetrating phones and at the same time maintain the physical circuit between the devices. Wound on MLT 100 resistances with PEL 0.2 wire until filled. The screen braid of the RK75 cable is used as the second wire of the line. Capacitors C1 and C2 prevent voltage from penetrating elements of television equipment, but in turn they transmit radio frequencies well television signal. Everything works stably.

At telephone conversation a person speaking in front of a microphone hears his speech in the telephone of his own device. This is explained by the fact that the current from the microphone of the speaking subscriber enters the telephone of this device, and not only into the line and the device of the interlocutor. For the same reason, the subscriber hears surrounding noise through his phone. When a subscriber listens to local noise and his own speech during a conversation through his phone, it is called local effect. TA schemes in which measures are taken to reduce the influence of local effects are called anti-local.

There are two classes of counter-local schemes: pavements And compensatory. The principle of constructing a bridge circuit is shown in Fig. 5 A. It is formed by: three-winding transformer Tr, microphone R m, phone Z t, line input impedance Z k and resistance Z b. Such a circuit can be represented in the form of a canonical AC bridge, in one diagonal of which a microphone is included as an alternating current generator, and in the other - a telephone.

Rice. 5. Anti-local pavement ( A) and compensation ( b) telephone diagrams

When the bridge is in equilibrium, the current in its diagonals is zero and, therefore, your own voice will not be heard in the phone. The equilibrium of the bridge is achieved under the condition Z 6 Z 1= Z l Z 2 where Z l - line input resistance, Z 1 and Z 2 - winding resistance I And II transformer, Zb is the resistance of the balanced circuit. However, complete suppression of the local effect cannot be practically achieved, since the balanced circuit, which usually contains a small number of elements with lumped parameters (from one to five resistors and capacitors), cannot reproduce the full frequency dependence of the line input impedance Z L. It should also be taken into account that the length and type of lines under operating conditions vary. Therefore, when developing TA regimens, they do not strive to completely suppress the local effect, but only to significantly weaken it.

The principle of constructing the TA compensation scheme is shown in Fig. 5 b. Let's explain its work. Let the instantaneous current values i = i 1+i 1 created by the microphone R m, have the directions indicated by the arrows. Flowing through the windings of an autotransformer I And II currents i 1 And i 1 induce him into III EMF winding e1 And e2. The circuit parameters are selected so that the absolute value of the EMF e1 there were more e 2, then in the winding III induced by EMF e = e 1-e 2. At the same time, part of the conversational current i K creates a voltage drop across the compensation resistance U K = i K Z K . For the condition of complete counter-terrain it is necessary that the induced EMF e and voltage drop U K were equal in magnitude and opposite in phase. In this case, the phone Z T will be connected between points with the same potential and, therefore, there will be no current in the phone.

Telephone set TA-68

Let's consider the operation of the TA-68 apparatus, the schematic diagram of which is shown in Fig. 6. The device operates according to a counter-compensation type scheme. The circuit includes: lever switch RP, bell Zv, capacitor WITH 1, NN dialer, telephone T, microphone M, transformer Tr, balanced circuit (capacitors WITH 1 and WITH 2 and resistors R 1 and R2). The transformer has three windings: linear I, balanced II and telephone III.

Rice. 6. Schematic diagram of the TA-68 telephone set

To protect the subscriber's ear from acoustic shocks and reduce the force of the click when winding and returning the dialer, a varistor consisting of two diodes is installed D 1 and D 2 connected in parallel to the phone. The lever switch of the device has contacts 1-2-3 RP and 4-5-6 RP. When the handset is on the cradle, to the line terminals 1 And 2 the call receiver is connected via the circuit: terminal 2 , Sv , contact 3-2 RP , WITH 1, terminal 1 . Capacitor WITH 1 blocks the path of direct current from the central bank of the automatic telephone exchange. The call signal from the telephone exchange is supplied by alternating current with a frequency of 25 Hz. In this case, the capacitor WITH 1 ensures the passage of alternating current and the polarized call VO notifies the subscriber of the arrival of a call.

When the handset is removed, the contact closes 5-6 RP, contact 1-2 the calling circuit is opened. Contact 5-6 RP closes the DC circuit: terminal 2, contact 5-6 RP, winding I Tr, microphone M, LV contacts, terminal 1 . The closure of the DC circuit is perceived at the station as a call signal.

The diagrams below may be useful when restoring any SLTs with a rotary dialer.

The simplest telephone apparatus consists of a handset with a microphone and telephone capsules, a dialer, one lever switch contact, a bell and a capacitor (Fig. 55). If we exclude the bell and capacitor from this set, and attach the dialer to the handset, then we will get an indispensable tool for a telephone lineman.

The diagrams of household appliances produced by industry differ from the diagram in Fig. 55 by the presence of a differential transformer and an RC circuit designed to eliminate the local effect (“self-listening”) and to match the line.

From the late 60s to the early 80s, the Riga VEF plant produced the most popular model of household telephone in the USSR - TA-68. The device has relatively good characteristics, and its circuit diagram (Fig. 56, Fig. 57) actually became the basis for subsequent, more modern, devices of the company. The TA-72M telephone set (Fig. 60, Fig. 61) has only a modified body shape; the handset, bell and other components are the same as those of the TA-68M.

For clarity, all diagrams show a typical connection of a two-wire socket cord. At the same time, connection diagrams for different dialers are shown.

The most vulnerable point of the TA-68 and TA-68M devices is the top cover of the case. As a rule, when struck by a fall, the mounting bushings break off, as well as the clamps of the lever switch pressure plates. PS, dichloroethane or epoxy glue is suitable for gluing the body.

Only elastic adhesives such as “Moment” or “Phoenix” cannot be used. It should also be noted that if the case is dirty, it cannot be cleaned with acetone or other solvents, but only with warm soapy water or diluted shampoo.

If, after connecting the telephone set to the PBX network, a strong rustling and crackling noise is heard in the handset, try pressing the twisted microtelephone cord to the handset, making a small loop approximately the same way pop singers do. Then carefully feel every centimeter from the body of the device to the tube. Usually damage occurs either at the tube itself or directly at the body. Of course, it is best to replace the twisted cord with a new one, but not everyone has one. In this case, you need to cut off the damaged piece of the cord, carefully strip the wires and install them on them first.

terminals removed from the cut cord. Since the microtelephone cord consists of tinsel wires, which are made by spirally winding a narrow and very thin copper tape onto a silk or nylon thread, they cannot be soldered to the terminals. The terminals bend over, trapping the insulation. This will make your microtelephone cord a little shorter, but will still work without replacement.

In a similar way, you can restore the functionality of the outlet cord. If it was not possible to detect a malfunction of the microtelephone cord, or after replacing it with a new one, the same symptoms are observed when shaking the handset, it is necessary to replace the microphone capsule (MC). Sometimes, to restore the functionality of the microphone, it is enough to dry it on a radiator for 24 hours.

Pay attention to the markings on the capsule body. Currently, carbon microphones of the MK-16 type are produced with a resistance of 20...40 Ohms for short MB lines and a resistance of 180±80 Ohms for all other devices. Microphones MK-16-U are resistant to climatic conditions, designed for temperatures from -50° to +50°. MK-16-N is manufactured for normal climatic conditions (-10° to +45°).

It is necessary to pay attention to the correct connection of the dialer, socket and microtelephone cords. The diagrams specifically indicate the colors of the cord cores to make it easier to find errors. If everything is connected correctly, then when “blowing through” the microphone you should not hear much noise or your voice in the handset. Otherwise, you need to check the connection of the colored wires of the microtelephone cord under the microphone (MK) in the handset. The green wire in all telephones made in the USSR must be connected only to the MK contact. The white wire connects to both capsules.

The bell and telephone capsule rarely fail completely, so to check them, it is enough to measure the winding resistance with a tester. The bell should have 2400 Ohms, and the TC winding should have 60...70 Ohms. The telephone capsule can be of the TA-4 or TK-67-NT type. For the bell, it is necessary to check the stroke of the striker and, if necessary, adjust it by turning the eccentrically fixed bell cups in one direction or another, so that in the position for maximum volume the striker almost touches the cups (the gap should be from 0.1 to 0.2 mm). The firing pin is anchored, stroke

which is selected within 0.4±0.1 mm. As the armature stroke decreases, the sensitivity of the bell increases.

If a working bell does not work, you need to check whether a jumper is installed between contacts K2 and K5 (Fig. 56). Next you should check the contacts of the lever switch. In all diagrams they are shown in a state where the tube lies on the levers of the device. The distance between open contacts must be at least 0.4 mm. By removing the clear dust cover on the lever switch, you can bend them if necessary. Clean contacts with alcohol.

The most complex device in a telephone is the rotary dialer. It is impossible to properly adjust it at home, so it is better to immediately replace the dialer with a new one. To get acquainted with this important unit, let’s briefly look at its main characteristics. Pulses are sent to the automatic telephone exchange during the reverse (free) movement of the disk. The duration of the opening-closing cycle of pulse contacts (IR) of the dialer is 90...110 ms (or 10±1 pulse/s). The ratio of the opening duration to the closing duration of the IR lies in the range of 1.4...1.7 and is called the impulse coefficient. The distance between open contacts must be at least 0.3 mm. To avoid listening to clicks in the telephone while dialing, old dialers had an additional group of contacts S2-3 (see Fig. 56), which bypassed the telephone capsule while dialing. If you want to install a dialer with a five-wire cord instead of a dialer with a three-wire cord, then the green and black wires must be isolated and not connected anywhere.

The above diagrams are very useful in practice when repairing telephone sets.

Telephone sets intended for operation in telephone networks include the following mandatory elements: a microphone and a telephone combined into a handset, a ringing device, a transformer, an isolation capacitor, a dialer, and a lever switch. On fundamental electrical diagrams The telephone set is designated by the letter E.

Let's briefly look at the purpose of the main elements of a telephone.

The microphone is used to convert the sound vibrations of speech and the electrical signal of sound frequency. Microphones can be carbon, condenser, electrodynamic, electromagnetic, piezoelectric. They can be classified into active and passive. Active microphones directly convert sound energy into electrical energy. In passive microphones, sound energy is converted into a change in some parameter (most often capacitance and resistance). To operate such a microphone, an auxiliary power source is required.

In mass-produced telephone sets, as a rule, carbon microphones are used, in which the electrical resistance of the carbon powder located under the membrane changes under the influence of sound waves. The most widely used microphone capsules are types MK-10, MK-16, which have a fairly high sensitivity (the described devices mainly use carbon microphones). On circuit diagrams the microphone is designated by the Latin letters VM.

It should be noted that recently a number of telephone sets are also equipped with condenser microphones of the MKE-3, KM-4, KM-7 types.

A telephone is a device designed to convert electrical signals into sound and designed to operate under conditions of stress on the human ear. Depending on their design features, telephones are divided into electromagnetic, electrodynamic, with a differential magnetic system and piezoelectric. In telephone sets, electromagnetic type telephones are most widespread. In such phones, the coils are fixed. Under the influence of the current flowing in the coils, an alternating magnetic field arises, driving a movable membrane, which emits sound vibrations. In modern telephone sets they are used in

mainly telephone capsules of the TK-67 type, and in devices of outdated designs - also TK-47 and TA-4.

The operating frequency band for microphones and telephones used in telephone sets is approximately 300...3500 Hz. On circuit diagrams, the telephone is designated by the Latin letters BF.

For ease of use, the microphone and telephone are combined in a handset.

The ringing device is used to convert the AC ringing signal into an audio signal. Electromagnetic or electronic ringing devices are used. The first of these is a single or double coil bell. Beep is formed as a result of the striker hitting the bell cups. The current flowing in the coils with a frequency of 16...50 Hz will create an alternating magnetic field, which sets the armature with the striker in motion. As a rule, in phone calls They use permanent magnets that create a certain polarity of the magnetic circuit, which is why such calls are called polarized. The resistance of the bell windings to direct current is 1.5...3 kOhm, the operating voltage is 30...50 V. On the circuit diagrams, the bell is designated by the Latin letters HA.

An electronic ringing device converts the ringing signal into an audio tone that can imitate, for example, a bird singing. A telephone or a piezoelectric ringing device VP-1 is used as an acoustic emitter. Such ringing devices are used, for example, in modern telephone sets TA-1131 "Lana", TA-1165 "Stella", etc. Electronic ringing devices are made using transistors.

The telephone set transformer is designed to connect individual elements of the speaking part and to match their resistances with the input resistance of the subscriber line. In addition, it allows you to eliminate the so-called local effect, which will be discussed below. Transformers are made with separate windings or in the form of autotransformers.

The separating capacitor serves as an element for connecting the calling device to the subscriber line in standby and call receiving mode. This ensures an almost infinitely high resistance of the telephone to direct current and low resistance to alternating current. In telephone sets, isolation capacitors of the MBM and K73-P types are used with a capacity of 0.25...1 µF and a rated voltage of 160...250 V.

The dialer supplies dialing pulses to the subscriber line in order to establish the required connection. The pulses are used to periodically close and open the line. Modern telephones use mechanical and electronic dialers. A rotary mechanical dialer has a disk with ten holes. When the dial rotates clockwise, the spring of the dialer mechanism is wound up. After releasing the disk, it rotates in the opposite direction under the action of a spring, and the contacts that switch the subscriber line periodically open. The required speed and uniformity of rotation of the disk is achieved by the presence of a centrifugal regulator or friction mechanism. The formation of pulses with free movement of the disk ensures their stable frequency and the required interval between pulse parcels corresponding to two adjacent digits of the dialed number. The required interval is ensured due to the fact that the number of openings of the pulse contacts is always selected one or two more than the number of pulses required to be supplied to the line. This ensures a guaranteed pause between bursts of pulses (0.2...0.8 s). In this case, these extra pulses do not enter the line, since at this time the pulse contacts are shunted by one of the groups of dialer contacts. There are also contacts that close the phone when dialing a number to eliminate unpleasant clicks. The frequency of pulses generated by the dialer should be (10±1) pulses/s. The number of wires connecting the dialer to other elements of the telephone can be 3 - 5.

Electronic dialers, which are equipped with many modern telephone sets (for example, TA-5, TA-7, TA-101), are made on integrated circuits and transistors. The number is dialed by pressing the keyboard buttons - the so-called keypad. Since the speed of pressing buttons can be as high as desired, on average 0.5 seconds are saved on dialing one digit of a number. In addition, keypad dialers provide users with various time-saving conveniences:

remembering the last dialed number, the ability to remember several dozen numbers, etc. Electronic dialers are powered both from the subscriber line and from a 220 V network through a power supply.

The lever switch provides connection to the subscriber line of a telephone ringing device in an inoperative state (the handset is on) and conversation circuits or a dialer in a working state (the handset is off-hook). A lever switch is a group of several switching contacts that are activated when the telephone is picked up.

In addition to the listed elements, the telephone set also includes resistors, capacitors, diodes, and transistors that form the speaking circuit of the device.

Let's consider the device of the telephone set (TA) as a whole.

When the telephone is operating in conversational mode, a local effect occurs, i.e. listening to your own speech on your phone. The local effect is explained by the fact that the current flowing through the microphone flows not only into the subscriber line, but also into your own telephone. To eliminate this undesirable phenomenon, anti-local devices are used in modern telephone sets.

There are various types similar devices. Let's consider one of them - a bridge-type anti-local device (Fig. 1).

Microphone VM1, telephone BF1, balanced circuit Zb and line Zl are interconnected by the windings of transformer T1: linear I, balanced II and telephone III. During a conversation, when the resistance of the microphone changes, conversational audio frequency currents flow through two circuits: linear and balanced. It is clear from the diagram that the currents flowing through windings I and II are summed with opposite signs, so there will be no current in winding 111 if the currents in the linear and balanced windings are equal in magnitude. This is achieved by appropriate selection of elements of the balance circuit Zb, the parameters of which depend on the parameters of the line Zl. The line resistance contains active and capacitive components, so the balanced circuit is made of resistors and capacitors.

Complete elimination of the local effect is achieved only at one specific frequency and certain line parameters, which is impossible in real conditions, since the speech signal contains a wide range of frequencies, and the line parameters vary widely (depending on the distance of the subscriber from the telephone exchange, transition resistances and capacitances in the cables etc.), therefore in practice the local effect is not completely destroyed, but only weakened.

Let's consider the diagram of the TA-72M-5 telephone set (Fig. 2), designed for operation in urban networks. Its switching and calling part consists of a lever switch SA1, a bell HA1, a separating capacitor C1 and a dialer SA2. The speaking part of the telephone consists of telephone BF1, microphone VM 1, transformer T 1, balanced circuit (capacitors C1 and C2, resistors R1-R3) and limiting diodes VD1, VD2. The speaking part is made according to a counter-bridge type scheme.

In the initial state of the contacts of the lever switch SA1 and the dialer SA2, shown in the diagram, the bell HA1 and capacitor C1 connected in series are connected to the line, and the speaking part is turned off. When ringing voltage appears at terminals 1 and 4 of the telephone set, current flows through the circuit: terminal 1 - jumper - terminal 3 - bell winding - normally closed contacts SA1.2 of the lever switch - capacitor C1 - terminal 4. (The direction of the current is chosen conditionally - with this it could also be considered flowing from terminal 4 to terminal 1.) Having heard the call, the subscriber picks up the phone. In this case, contacts SA1.1 and SA1.2 switch to another position, turning off the calling circuit and connecting the speaking circuit to the line. The DC resistance between terminals 1 and 4 varies from very high (hundreds of kilo-ohms - mega-ohms) to relatively small (hundreds of ohms), this is recorded by the telephone exchange devices, and they switch to conversational mode.

When dialing a number, the contacts SA2.1 of the dialer are in a closed state during forward and reverse rotation of the disk, which provides bypassing of the conversational circuit and eliminates the possibility of listening to clicks on the phone. When the dialer rotates back, the SA2.2 contacts break linear circuit, and station devices record the number of the called subscriber based on the number of such openings.

Diodes VD1 and VD2 limit voltage surges on the phone windings and eliminate sharp sounds, unpleasant to the ear.

To operate in manual telephone exchange networks, telephone sets without a dialer are used. The diagram of one of these devices (type TA-68CB-2) is shown in Fig. 3. Its main difference from the previous device is the absence of dialer contacts and one group of lever switch contacts, therefore the bell and capacitor C1 remain connected to the line in conversation mode. However, they have virtually no effect on the operation of the telephone in this mode.

In devices telephone communication which are described in this book, you can use industrially produced telephone sets both with a dialer (TA-68, TA-72M-5, TA-1146, etc.) and without it (TA-68CB-2 and others similar). But telephone sets without a dialer are only suitable for telephone switches with manual control. If a radio amateur has a telephone set at his disposal, in which only the handset and bell are working, it can also be used. In this case, the elements are connected in accordance with the diagram shown in Fig. 4. Capacitor C1 - type K73-17, MBM, MBGO. It should be noted that in such a telephone set the local effect will be fully manifested, but for the sake of simplicity, you can sacrifice some convenience.

Let's take a brief look at how telephone lines are switched in city PBXs. Since 1876, when the Scotsman A.G. Bell invented the world's first two-wire telephone, the principle of telephone communication has not undergone significant changes.

The diagram for organizing telephone communication between two subscribers is shown in Fig. 5. Power supply current for telephone sets El, E2 pro-

goes through chokes L1 and L2. Chokes are necessary to prevent the conversational (alternating) current from shorting through the DC power source Upit, the internal resistance of which is very small and amounts to fractions of an ohm. The direct current source is usually called the central battery (CB). Chokes L1 and L2 have a relatively low DC resistance (usually no more than 1 kOhm). The inductance of the chokes is quite large and in the frequency range of conversational currents (300...3500 Hz) it will create such significant resistance to the conversational (alternating) current that it practically does not branch into the central bank and flows in the circuit between devices E1 and E2. On automatic telephone exchanges, the windings of two-winding relays are usually used as chokes, and these relays simultaneously serve to receive a signal about a call to the station by the subscriber and a signal to end the call (hang up).

The inductor generates an alternating ringing voltage with a frequency of 16...50 Hz, which activates the ringing device of the desired telephone set.

Subscriber switching was initially performed manually on the PBX, then step finders began to be used, and currently switching is carried out quasi-electronically or electronically. PBX switching devices controlled by impulse

direct current signals, which are created by the telephone dialer when the subscriber dials the digits of the called subscriber’s number.

Figure 6 illustrates the simplest principle of establishing a connection on a PBX. The telephone set of the first subscriber E1 is connected to the central bank (Upit) through the windings of the two-winding relay K1. When the first subscriber picks up the handset of the E1 device, relay K1 is activated and contacts K 1.2 supply power to the winding of relay K2. This relay is designed in such a way that the armature is not released immediately after the voltage is removed from its winding, but with some delay (in this case, this delay is about 0.1 s). Relay contacts K2.2 prepare the power circuit for the stepper short circuit detector. When the E1 subscriber dials the number of the called subscriber, the power circuit of the K1 relay windings will be interrupted by the contacts of the dialer of the E1 telephone set (this occurs when the dialer dial moves back). Contacts K1.1 supply power pulses to the winding of the step-by-step fault detector according to the number of the called subscriber. After the E1 telephone dialer has finished rotating, the step finder contacts will connect the caller's line to the callee's line, after which the subscribers will be able to carry on a conversation.

When, at the end of the conversation, the subscriber puts the handset on the E1 device, relay K1 will release, its contacts K 1.2 will open the power circuit of relay K2, which will also release after 0.1 s. In this case, through contacts K2.1, KZ.4 and KZ.3, power will be supplied to the winding of the stepper short circuit detector. Contact KZ.4 slides along the solid lamella of the step finder and opens only when the step finder returns to its initial state. Contact KZ.3 is a self-interrupting contact of the stepper finder, which interrupts the power supply circuit of the winding of the stepper finder when the armature is attracted to the core.

nick. Thanks to this contact, a series of pulses are formed on the short-circuit winding, which sequentially set the short-circuit contacts.1 and short-circuit.2 to their original position.

The accuracy of the operation of the subscriber relays and the step finder depends on the opening time of the dialer contacts, which should not exceed 0.1 s. Otherwise, when contacts K 1.2 open, relay K2 will not be able to hold the armature, and the connection will not occur. Therefore, the parameters of telephone dialers must meet the following requirements:

1) dialer pulse frequency 10±1 pulse/s;

2) pulse repetition period 0.95...0.105 s;

3) a pause between series of pulses of at least 0.64 s;

4) the ratio of the opening time to the closing time of the dialer pulse contact, called the pulse coefficient, depending on the type of telephone exchange 1.3...1.9.

The central battery of the telephone exchange supplies the subscriber lines with a constant voltage Upit = 60 V. When the handset of the telephone set is removed, the telephone exchange line becomes loaded with the internal resistance of the telephone set, as a result the voltage at the line terminals drops to 10...20 V (depending on the distance of the subscriber depending on the telephone exchange and the type of device used). The internal resistance of a telephone set when the handset is off-hook can be 200...800 Ohms, and the operating (conversational) current through the device can be 20...40 mA. The resistance of the telephone exchange brought to the subscriber sockets, which includes the resistance of the line, relay windings K1 (see Fig. 5) and the internal resistance of the central battery, can range from 600 Ohms to 2 kOhms.

For a telephone with a rotary dialer, dialing a subscriber's number is carried out as follows: by rotating

dial clockwise to the finger stop, the contacts of the dialer close the line, and during reverse rotation the line opens the number of times that corresponds to the dialed digit. In Fig. Figure 7 shows a time diagram of the operation of the telephone.

The PBX uses an alternating voltage of 80...120 V with a frequency of 16...30 Hz as a ringing signal.

In the telephone communication devices described in the book, two methods are used to connect telephone lines: parallel and serial (Fig. 8).

The circuit with parallel connection of telephone sets was discussed above (Fig. 5). The difference between the diagram shown in Fig. 8a, is that instead of two inductors, a current stabilizer CT is switched on, i.e. a two-terminal network, the current through which remains unchanged when the parameters of the external circuit change within certain limits.

In any case, the relation L1 + L2 = L= const is valid. therefore, a change in current in the circuit of the first subscriber causes exactly the same change in current in the circuit of the second subscriber, but with the opposite sign. This ensures the highest possible conversation volume. In practice, in intercoms, instead of a current stabilizer, you can use a resistor with a resistance of 1...5 kOhm, however, it should be taken into account that the volume of the conversation will decrease somewhat.

In Fig. 8.6 shows a diagram of the serial connection of telephone sets. With this connection, the conversational current of one device flows completely through the second device, which ensures the maximum possible conversation volume (under given conditions).

It should be noted that in city PBXs the serial method of connecting telephone lines is not used due to the complexity of switching the devices. (In the book this method used in intercoms and manual switchboards.)

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