Repair of imported handset. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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In recent years, many cheap foreign-made handsets have appeared in our country. They cause a lot of trouble to their owners, as they often fail. The main reason for this is the difference in the parameters of domestic and foreign exchanges. The repair of one of the options for such tubes is described in the proposed article.
I want to share my experience of repairing a handset assembled on a KS5853 chip. Its scheme is shown in the figure. Lever switch SA2 shown in on-hook position.
Only the call worked in the device, when the handset was picked up, the line was not seized, i.e., the voltage at the emitter of the transistor VT4 was equal to the voltage in the telephone line. To analyze the cause of the failure, the device had to be disconnected from the telephone line and connected to a +15 V DC voltage source. The positive output was connected to point A, and the negative output was connected to the common wire. Measurement of the operating modes of all semiconductor devices revealed that the transistor VT2 is closed, since the potential of its base (point B) was almost zero. There is a malfunction of the conversational node, assembled on transistors VT2, VT3, VT4. This was the result of the failure of part of the DD1 chip that controls the conversational node.
I soldered the DD1 chip with a needle from a medical syringe and, bending the 16th leg, soldered it back to its original place. After that, it turned out that all the functions of the device were restored, except for dialing, since the dialing signals came from the 16th output of the microcircuit to the VT2 base.
Having disconnected the handset from the telephone line and connected it to the power source, as indicated above, using an oscilloscope, it was possible to establish that noise with an amplitude of 16 mV was observed at the 1th pin of DD2. When pressing any number on the keypad, it turned out that there was a useful dialing signal on this pin. On the oscilloscope screen, this looks like an alternation of noise and a useful signal in the form of a low logic level. After the phone was finalized (newly introduced elements are shown in the diagram with thick lines), dialing was restored. It turned out that you can do without replacing the microcircuit with KR1008VZH1.
Resistor R14 must be selected so that the transistor VT5 is in saturation mode when there is no dialing signal and in cutoff mode when this signal is present at pin 16 of DD1. Resistor R16 and zener diodes VD8, VD9 are necessary to protect against increased ringing voltage in case the handset is picked up during its operation. The VD7 zener diode serves to protect the microcircuit, and the VD10 diode ensures the reliability of opening the VT5 transistor during the absence of a useful signal.
Refinement of the handset was carried out by hanging installation. By the way, the HL1 LED, signaling the pickup and occupation of the telephone line, also turned out to be faulty. It had to be replaced with the AL307BM LED without any alteration.
Transistors VT5, VT6 can be replaced by KT503E. Diode VD10 - any silicon.
Author: A.Soldatenko, Maikop
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