Encyclopedia of radio electronics and electrical engineering / radio reception

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Among the whole variety of pocket receivers on sale, there are stereo options, designed to work only with stereo headphones, and combined ones, to work with phones and built-in dynamic heads. But the first of them usually have smaller dimensions and weight. Such devices include, for example, the Chinese stereo radio receiver "MANBO" model S-202. It has an operating range of 88 ... 108 MHz, the radio path and the stereo decoder are assembled on the SHA1538M chip (SONY), and the stereo ultrasonic path is on the TDA7050 chip (PHILIPS).

The receiver comes with higher quality stereo headphones than many other receivers. All this provides the device with excellent stereo sound quality. But as is often the case, this receiver is not without flaws. How to eliminate them, or at least minimize them, is described below.

Since in this receiver, as in similar ones, the antenna is a wire going to the head phones, the reception quality is sometimes not satisfactory - the RF amplifier included in the CXA1538M chip does not always cope with the reception conditions for such an antenna.

You can improve the sensitivity of the receiver by introducing an additional simple radio frequency amplifier (URCH) into it. Its scheme is shown in Fig. 1 (newly introduced elements are marked with an apostrophe). The amplifier is assembled on a single VTT transistor. All parts are installed in a hinged way on the receiver's printed circuit board, above the installation site of the СХА1538М chip (it is located on the side of the printed conductors). The output of the capacitor C10 installed in the receiver (with a capacity of 33 pF), going to the choke coil L5, is soldered. The URC is included in the gap: the capacitor C1 'to the inductor L5, and the collector of the transistor VT1' to the capacitor C10.

After installing such a URF, the sensitivity of the receiver increased noticeably. Of course, this amplifier is very simple, it does not have a single oscillatory circuit and filters at the input. Therefore, if during its connection interference from nearby powerful KB stations is observed, then an additional high-frequency filter with a cutoff frequency of 55 ... 60 MHz should be installed at the input.

The next disadvantage of this receiver is that the volume control is used no more than one third of a turn. With a further increase in volume, the audio frequency amplifier (UCH) on the TDA7050 chip is overloaded and strong distortions appear. This is due to the lack of signal level matching between the stereo decoder outputs and the UZCH inputs.

To eliminate this drawback, it is enough to break the printed conductors going to the R9 volume control from the outputs of the right and left channels of the CXA1538M chip (pins 5 and 6), and solder additional resistors into the gap, the refinement circuit is shown in Fig. 2. In the above diagram, this is the resistor R1'. For example, only one channel is shown, in the other - the changes are the same.

Now the volume control has become much more convenient to use, it is used more fully. At maximum volume, there is no distortion, since the signal level is now limited by the resulting divider R1 R9.

Even in this receiver, you can enter a loudness circuit when the volume is reduced. On fig. 2 is the capacitor C1' (HF boost) and the elements R2', C2', R3' (LF boost). Moreover, the rise of LF and HF components is carried out not only at low and medium levels, but also at the maximum signal level, since the loudness circuits are connected directly to the outputs of the stereo decoder and the mid-frequency components of the signal pass through the input circuits more attenuated than the LF and HF components.

All additional radio elements are mounted on the receiver board from the side of the printed conductors, so they must be selected from among the small ones. After carrying out the alterations described above, it became much more convenient to use the receiver - the sensitivity increased, the sound quality became more pleasant due to the rise in low and high frequencies.

Author: I.Potachin, Fokino, Bryansk region

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