Many people, especially inexpensive, telephones foreign production designed for the line voltage is 48 V. And although the sellers of such equipment claim that it is adapted to the domestic telephone network, failure in the work very much. Most often fail transistors of the so-called "current key" and the dialer chip.
Most of the vehicles collected under the scheme, a fragment of which is shown in the figure. Typically, the output NSI chip dialer DD1 protect the Zener diode VD1 (indicated on the diagram by the dashed line) with voltage stabilization 13...15 V. However, if fails the transistor VT1, for example, breaks his collector-junction diode, Zener diode does not save. When you try dialing all the current the line passes through the chip, and it also fails.
This can be avoided by including the resistor 4,3...10 kOhm and power 0,125 W, as shown in the diagram by a dotted line. In an emergency he will limit the current of the chip, and in the normal - almost no affect. On boards of some telephones even have holes for the resistor, but they soldered the jumper, which of course must be deleted first.
Operating experience several improved apparatus for proven reliability.
Author: V. Chudnov, Ramenskoye, Moscow region.