Replacing the chip in the tape recorder. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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In many models of household cassette tape recorders of the 2nd and 3rd complexity groups, produced until the end of the 80s, the K237GS1 hybrid microcircuit was used. It contains in its composition a voltage regulator and elements for constructing an erasure and bias current generator (GTS). Often, the output of the stabilizer is not connected to the GTS circuits, but provides voltage to a pre-amplifier made on another microcircuit of the same series.
As shown by the long-term operation of the tape recorders of these groups, the K237GS1 chip often fails. Malfunctions happen in each of its constituent parts. If the stabilizer fails, the tape recorder loses the ability to play back phonograms, and if the transistors used for the GTS break down, it will erase the old recording and make new recordings. Today, finding such a microcircuit, if it is necessary to repair the device, has become a problem, since at one time, when developing new products, it was supplanted by the K157XP2 microcircuit.
At the same time, it is possible to restore the operation of tape recorders in which the K237GS1 chip is used. To do this, it is recommended to use discrete elements with minimal modification of the printed circuit board.
Figure 1 shows the equivalent circuit of a microcircuit using the example of a cassette tape recorder of the second complexity group "Spring-202" - the elements and circuits of the tape recorder are marked in black (in accordance with the factory diagram of this product), red - newly introduced elements.
To replace part of the A1.2 chip used in the GTS, you will need two powerful high-frequency transistors (they are designated VT1 and VT2 in the diagram). It is desirable to use transistors with the same electrical parameters. The required offset is set by resistors R1 and R2 to replace the voltage regulator of the A1.2 microcircuit, a DA1 microcircuit stabilizer of the KR142EN5A type is taken. In those models of tape recorders where the voltage of the stabilizer should be 5,4 ... 6 V, it is advisable to use KR142EN5B. As a rule, pin 10 of the K237GS1 microcircuit is connected through a capacitor (C59) with a capacity of about 100 microfarads to a common power rail. When installing a microcircuit stabilizer, the terminals of this capacitor must be closed with a jumper.
Figure 2 shows a fragment of the printed circuit board of the tape recorder after installing the newly introduced elements (not to scale). The whole refinement consists in removing the K237GS1 chip and drilling one hole for the emitter terminal of the VT1 transistor. The DA1 microcircuit stabilizer should be installed on contact pads 9, 10 and 11. Since the GTS unit is located in close proximity to the tape drive mechanism, when installing the KR142EN5A chip, it must be placed horizontally to the board, but in such a way as to exclude possible touching of the elements of the tape drive mechanism on the chip body.
Transistors VT1 and VT2 should also be placed horizontally. To do this, it may be necessary to extend the terminals of the bases of the transistors with small pieces of wire. Resistors R1 and R2 are installed vertically between the printed tracks for chip pins 12 and 13; 14 and 13.
After such refinement for more than four years, the tape recorder worked without failures.
Author: I. Tsaplin, Krasnodar
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