About paired telephone sets. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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About paired telephone sets. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Immediately make a reservation that you should not confuse the paired inclusion of TA with parallel.

It is known that on telephone lines for paired switching on of TAs, diode circuit separation attachments are used, which are installed near the subscribers themselves (usually in shields on the landing). Their schematic diagrams are shown in the figures below. The principle of operation of the diode separation attachments is based on blocking the power supply circuit of the current TA that is not working at the moment and at the same time providing power to the working TA. This is achieved by including blocking diodes in the linear circuits of telephone sets in polarity opposite for one and for the other SLT.

In addition, to ensure the normal operation of polarized telephone calls, the set-top boxes are equipped with a discharge circuit for generating a second half-wave of current through the bell during those half-periods of the PBX ringing signal when the blocking diodes are closed, i.e., the capacitor cannot be discharged through the PBX due to the diodes on .

Ris.1.Scheme of the prefix PDT-1
Ris.2.Scheme of the prefix PDT-2

Such telephone lines have many disadvantages, and one of them is a reduced ringing level. Because of this, the ringing devices of some SLTs (especially foreign ones), in which the circuit is designed for the rated voltage of the ringing signal, may not work when the SLTs are paired. The circuits for discharging a capacitor in the bell circuit are also not without drawbacks. Which, in my opinion, is the main reason for the inoperability of most electronic ringers of telephone sets, because they are quite high-resistance compared to electro-mechanical ones.

On the pages of amateur radio literature, including on the Internet, This problem has come up repeatedly. Everywhere it was proposed to install additional devices that not only complicate the connection of TAs to them, their placement, but also lead to a weakening of the signal, and sometimes they are completely inoperable in real conditions. And the prices for some of them reach 10 dollars!! This is - sorry, madhouse!

There are several different PBXs in our city, and when faced with this problem, it turned out that the same set-top boxes behave differently on different PBXs. Therefore, the prefix should be configured specifically for your PBX. It is on this occasion that I would like to share my thoughts. And give some helpful tips.

At first

If you have a PDT-1 prefix (fig.1.), then it should be replaced, because its setting is too time-consuming (selection of a transistor). In this case, I recommend removing all parts (except diodes) from the removed set-top box and soldering them using any of the typical schemes shown on pic.2 or pic.3. (I recommend Fig.3.)

Secondly

I would like to make one comment right away. The value of the resistor R3 according to the circuit in Fig. 3 should be increased to 24 kOhm, as well as according to the circuit in Fig. 2. You should not reduce the resistance of these resistors below 18 kOhm, as this may lead to failures on some exchanges. Although according to the typical scheme, its nominal value is 11 kOhm.

I made a device containing two identical prefixes on a common printed circuit board according to the PDT-3 scheme. The printed circuit board drawing is shown on pic.4, and mounting - on pic.5. To connect wires to the device, metal contact brackets with a bolt in the middle from the old SLTs are installed on the board. The assembled board is placed in a plastic case of suitable dimensions. The device used KT940A transistors, MLT-0,5 resistors, and KD105G diodes, for which the printed circuit board was designed. The new device was installed instead of two set-top boxes PDT-1. In the PDT-2, PDT-3 circuits, KT940A transistors were tested. In the PDT-2 attachment, KT605BM works better. Recommended resistors MLT-0,5; 0,25.

The setup of all consoles is identical and is as follows. Measure the voltage of your telephone line. Make measurements with a pointer voltmeter - a digital one is not suitable here due to its large inertia. Then, temporarily, instead of resistor R2, connect a variable with a resistance of 20 ... 30 kOhm, setting its slider to the position of minimum resistance. Now apply to the input of the set-top box a constant voltage greater by 3 ... 5V than previously measured on the telephone line.

Connect a voltmeter to the output of the set-top box and slowly increase the resistance of the variable resistor, follow the readings of the voltmeter. It is necessary to find such a position of the variable resistor engine at which the voltmeter readings will begin to decrease. Fix this position of the variable resistor, disconnect it and measure the resulting resistance. And in its place, solder a constant resistor of the same rating. The device is still functioning normally! (fax, modem, radiotelephone.)

Literature

"Telephone sets and payphones". I.M. Gubrenko, E.V. Kuchumov. Radio and Communication 1989.

Author: Yu. Zeshkov, Pervouralsk, Sverdlovsk region; Publication: N. Bolshakov, rf.atnn.ru

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