Repair of imported telephone sets. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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In recent years, a large number of simple foreign-made telephone sets have appeared on store shelves, for which there is no technical documentation. The author offers diagrams and recommendations for the repair of two models of telephones - VT60M and VT960R.
In the process of repairing each telephone set (TA), one has to study its principle of operation. Consider first the BT60M model. The scheme of this phone is shown in fig. 1.
(click to enlarge)
Lever switch SF1 is shown in the diagram in the on-hook position. The TA is assembled on two printed circuit boards - the main one (Fig. 2) and the keyboard board, which are interconnected by a ribbon cable.
The following defects were found in this device: when dialing a number, impulses were heard, but dialing into the line was not processed; during a conversation, the interlocutor on the other end of the wire complained about the low level of the audio signal.
During the repair process, it turned out that the VT3 transistor, which acts as a pulse key, was faulty, and a collector-emitter leak of the VT5 transistor was also detected. After replacing these transistors, the phone began to work normally.
The UM91610A chip can be replaced with the VT91611.
On fig. 3 shows a schematic of the BT960P telephone. Lever switch SF1 shown in on-hook position. Instead of the IS2 chip (UM9151-3), the KR1008VZH17 or FT9151-3 with an appropriate selection of master oscillator circuits is suitable. On the IS1 chip (KR1436AP1 or FT2410 are also suitable), a ringing signal generator is assembled.
(click to enlarge)
During the repair of the device, the following malfunctions were discovered: the ringing device did not work, during a conversation there was poor audibility at the other end of the telephone line. Transistor Q2 has been broken. On the main board (Fig. 4) there was no resistor R3, indicated by a dashed line, although there is a seat for it. When repairing the ringing device, it turned out that the resistor R3 is installed when the KA2411 chip is used.
There are two groups of chips for calling devices. In the first group, the RBC(R3) resistor connected between the BC input and the common wire makes it possible to forcibly turn off generation at a microcircuit supply voltage equal to the operating voltage (25,1 ... 28,9 V). This group includes the following microcircuits: KR1436AP1, FT2410, KA2410, ML8204, DBL5001, etc. The type of zener diode used depends on the operating voltage of the microcircuit [1].
In the second group, the resistor RBC(R3) connected between the input BC and the case allows you to change the input resistance of the microcircuit [2]. This group includes microcircuits KR1436AP2, FT2410, KA2410, ML8205, DBL5002.
Since the chip installed in this phone belongs to the first group, the resistor R3 must have a resistance of 220 kOhm (at an operating voltage of 27 V) and it should be connected between the first (U) and second (BC) pins of the IS1 chip.
Chips of calling devices of the first and second groups are identical in terms of technical parameters, but the control input (BC) for each group performs its functions. Details about the operation of these microcircuits can be found in [2].
Literature
- Integrated circuits. Microcircuits for telephony. Issue. 1. - M.: DODEKA, 1994, p. 256.
- Kizlyuk A. Device and repair of telephones of foreign and domestic production. - M.: Biblion, 1997
Author: S. Derevyanko, Cherkessk
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