A simple device for checking phones. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple device for checking phones. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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The proposed device is simple and cheap. Nevertheless, it can be used to check and repair telephones, using an oscilloscope in difficult cases.

The diagram of the device is shown in the figure. It consists of the following nodes: call node, dial control node, power node. The call node consists of capacitor C3, resistor R5, button SB1 (non-latching) and normally closed relay contacts K1.1. The dialing control unit includes relay K1, zener diode VD3, diode VD2, LEDs HL1, HL2, resistors R1-R4, transistor VT1. The HL2 LED controls the opening of the line when dialing, and HL1 - the circuit. The power node contains a network transformer T1, a diode bridge VD1 and a capacitor C1.

A simple device for checking phones

When the telephone is on-hook (hereinafter TA), its DC resistance is high, relay K1 does not work, LED HL2 is off, and HL1 is on. When the SB1 button is closed, the alternating voltage from the winding III is supplied to the TA. The caller of a healthy SLT should sound the call. Capacitor C3 is needed to limit the ringing current, and resistor R5 is needed to discharge this capacitor.

The dialing control node operates as follows. When the TA tube is raised, its resistance sharply decreases, the K1 relay is activated, the HL2 LED turns on, the normally closed contacts of the K1.1 relay open and the ringing signal to the TA will be prohibited. The HL1 LED will continue to shine, since the voltage at the output of the device will be set within 8 ... 15 V, and it is enough to open the transistor VT1. A hum with a frequency of 100 Hz will be heard in the telephone capsule of the SLT, which makes it possible to dispense with the use of an additional audio frequency generator to test the telephone capsule of the handset of the SLT.

When dialing the number, the HL1 and HL2 LEDs should blink. During pulse dialing, the TA periodically closes and opens the subscriber telephone line. When opened, HL2 goes out, when closed, HL1 goes out. Resistor R3 is chosen so that the HL1 LED goes out when the output terminals are closed and lights up in all other cases.

To test the microphone, you must connect an oscilloscope with a closed input to the output terminals. Saying various words into the TA tube, check the microphone with an oscilloscope. However, practice shows that such a malfunction of electronic TA is rare.

Any low-power mains transformer with winding voltage II 25...30 V at a current of 50...100 mA and winding voltage III 50...70 V at a current of 50...100 mA is suitable for the device. Thus, the voltage at the output of the device with the telephone set on-hook (or its absence) should be about 40 V.

Capacitor C1 - any oxide. It is undesirable to use a larger capacitor than indicated in the diagram, as this will reduce the ripples, and they are necessary to test the telephone capsule. Capacitors C2 and C3 are non-polar, for example, MBM. Relay K1 - RES22 (passport 131) with a winding resistance of 750 Ohm or another with an operating voltage of 22 ... 30 V and a response current of 20 ... 35 mA. Diode bridge VD1 - any for a voltage of at least 50 V and a current of 100 mA. Zener diode VD3 can be replaced with KS147A. Diode VD2 must withstand a reverse voltage of 100 V and a forward current of at least 50 mA. Transistor KT503D can be replaced by KT503E.

When setting up the device, it is necessary to select the resistor R3 so that the HL1 LED reliably goes out when the output terminals are closed and glows when two silicon diodes connected in series (anodes to VD2) are connected to them.

Author: K. Movsum-Zade, Tyumen

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