Repair of LPM car radio SONY XR-2750. Scheme, description

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Repair of LPM car radio SONY XR-2750. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The Radio magazine has already published articles on repairing the cassette loading mechanism and refilling the tape in VCRs, as well as on replacing the microcircuit that controls the LPM engine. A similar problem arose during the repair of the Sony XR 2750 car radio, manufactured in 1998. In it, loading the cassette and turning on the rewind, playback and tape straightening modes (by briefly turning on the rewind in both directions alternately) are carried out by a motor controlled by a microcircuit 718.1322. At the moment when it is turned off, there is no supply voltage on the microcircuit, since the D1802 transistor, controlled by the processor, is closed. When the engine is turned on, the collector of this transistor always has the voltage of the on-board network (12 ... 13 V), but when the transistor opens, there will be only about 9 V at its emitter, since the voltage at the base is limited by the zener diode.

When looking for a defect, it turned out that not only the 718.1322 chip, but also the D1802 transistor. and a zener diode, the type of which remained unknown, are faulty. Simultaneous failure of the transistor and the microcircuit is possible during prolonged engine overload or overheating (the radio tape recorder is located in the car near the heating system). It should be noted that if there is no signal from the LPM limit switches for some time, the cassette loading is blocked by locking the transistor. For this reason, it was not possible to "deceive" the processor, simulating the operation of the engine by manually rotating the pulley, in order to set the sequence of signals at the inputs of the microcircuit and the signals received by the processor from the LPM programmer.

The faulty zener diode and transistor were replaced by domestic KS191A and KT805AM. The transistor had to be fixed with a screw in an additional drilled hole in the board.

To replace the microcircuit, I developed its analogue (see diagram in the figure), which is easy to manufacture and reliable in operation. It can be made in small dimensions using elements intended for surface mounting. The analogue is a phase inverter in the bridge version, in which the control voltage can be applied simultaneously to two inputs. Due to the absence of resistors between the bases of the transistors of the final stage, a short circuit and failure of the transistors are excluded. The voltage on the transistors of the final stage is quenched by resistors R3, R4, R8, R9. Resistors R2, R5, R7, R10 are used to balance the cascade and, if necessary, an accurate zero setting can be achieved by selecting one of them.

Repair LPM car radio SONY XR-2750

The analogue of the microcircuit is mounted on a printed circuit board. The external pins are numbered according to the chip pins. The board is installed between the housing wall and the LPM control motor and fixed with MZ screws on the side wall through 4 mm thick dielectric gaskets. Inputs 2 and 4 are connected by a conductor to the terminals of resistors with a nominal value of 1 kΩ, from which the signal was sent to the same-name terminals of the microcircuit. Power pins 3, 7 are soldered to the emitter of the previously installed KT805AM transistor. Conclusion 1 is connected to the body in any convenient place. Conclusions 6 and 8, bypassing the connector, are connected to the engine.

The output transistors of the analogue of the microcircuit, as well as the installed KT805AM, do not need a heat sink. The smoothness of the loading mechanism depends on the speed of rotation of the motor shaft, which is changed by selecting a zener diode for a voltage in the range of 7 ...

Author: A.Kuzmin, Lipetsk

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