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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Improvements to the VHF FM receiver MAMVO. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Miniature portable radio receivers of the MAMVO type and similar are designed to receive VHF FM radio stations on head phones (headphones). They are small, affordable and therefore quite widespread among radio listeners. They have a fairly simple design, since they are assembled on one SC1088 chip or similar, and one or two transistors. But due to the simplicity of circuit solutions, these receivers have certain disadvantages that are easy to eliminate, in addition, depending on the operating conditions, these receivers can be modified accordingly. Increasing economy One of the serious shortcomings of the radio receiver "MAMBO" and others like it is a significant current consumption (40 ... 60 mA), which does not depend on the volume level, i.e. remains practically constant. This is due to the fact that the UZCH is made according to a simple scheme (Fig. 1), and therefore the current it consumes depends on the resistance of the load connected to the telephone jack. Typically, such a load is stereo headphones (with a resistance of about 32 ohms each), connected in parallel, which have a total resistance of about 16 ohms. This is the reason for the indicated value of the consumed current. When the headphones are turned off, the current consumption is only about 6 mA.
If the receiver has a margin for volume control, then a load with high resistance can be connected to it. This will reduce the volume and maximum output power, but the current consumption will decrease. This can be done by turning on the headphones not in parallel, but in series. Due to this, the total UZCH load resistance will increase to 64 ohms. For the author, this made it possible to reduce the current consumption from 50 to 22 mA. The service life of the batteries also increases proportionally (more than twice!) It will be possible to implement such a refinement if a simple cable adapter is made, the diagram of which is shown in Fig. 2. It includes an XP1 plug that connects to the receiver and an XS1 socket that plugs in headphones. No refinement of the headphones themselves is required.
It will not be superfluous to also include a tuning resistor in series with the headphones (Fig. 3). At the same time, the volume decreases even more, but the current consumption also decreases. With an acceptable volume, the current consumption with the help of this device was reduced to 16 mA. In the device according to Fig. 3, you can use both parallel (as shown in the figure) and series connection of two headphones (as in figure 2).
By experimenting, you should choose the best option. It is useful to shunt resistor R1 with a 470 uF capacitor (plus to the left output of the resistor according to the circuit), the volume will increase. Such adapters can be used depending on the situation, choosing what is more important, volume or low power consumption. Work in stationary conditions The output power of the UHF receiver is quite enough to sound a small room. This allows you to use the receiver in stationary conditions, for example, in the country. But if you connect a loudspeaker with a conventional dynamic head with a low resistance (4-8 Ohms) to its output, this will lead to an increase in the current consumption up to 80 ... 90 mA, and the ultrasonic frequency converter will work with noticeable distortion. Therefore, dynamic heads with a resistance of at least 16 ohms can be connected directly to the receiver. To connect heads of lower resistance, a matching transformer should be used. If there is no mains supply or no mains power supply at the location where the receiver is installed, it will be important to reduce the current consumption at the same time. In such a situation, it is recommended to use the wiring diagram shown in Fig. 4. Resistor R1 made it possible to reduce the current consumed by the receiver from 50 to 15 mA, but the volume, as noted above, will also decrease.
This device uses an output transformer from transistor radios ("Selga", "Alpinist", etc.), a tuning resistor of the SPZ-19 or SP-3 type, a capacitor - K50 or a similar imported one. Dynamic heads can use any of the available power of 0,5-3 watts. If there is a choice, it is recommended to choose the best head according to the criterion of maximum volume. If the task of reducing energy consumption is not worth it, then the elements R1 and C1 can be excluded. The receiver (RPU) uses a telephone cord as an antenna (see Fig. 1), and in our case, a conductor connecting it to a loudspeaker. Therefore, to improve the quality of reception, the length of this conductor should be chosen as a multiple of a quarter of the wavelength of the received radio station, for the range of 88 ... 108 MHz it is about 750 mm. A counterbalance conductor of the same length, connected to one of the contacts (preferably "+") of the battery compartment, will also help improve reception quality. Both wires should be oriented in opposite directions from the receiver (Fig. 5). Since in most cases broadcasting VHF FM radio stations operate with vertical polarization of the emitted radio waves, it is better to place the wires vertically.
Changeover to other bands The radio receiver can be reconfigured to receive signals from FM radio stations operating in other frequency bands, for example, CBS (27 MHz) or amateur 144 ... 146 MHz. To tune to the 144 MHz range, it is necessary to halve the number of turns of the inductors of the input L1 and heterodyne L2 circuits (the marking of the elements is printed on the board). However, since the sensitivity of the receiver is low, it will be possible to receive only powerful radio stations, and even then with low volume, due to the fact that the frequency deviation of the emitted signals from amateur radio stations is several times less than that of broadcasters. To tune the receiver to the frequencies of the CB band (27 MHz), you can replace the existing ones or install additional capacitors in parallel with the coils L1 and L2 (Fig. 6). In this case, small-sized capacitors for surface mounting K10-17v or similar imported ones can be used.
Please note that the receiver is designed to operate in automatic scanning mode. Therefore, tuning to the frequency of radio stations in the amateur bands will be difficult due to the fact that the receiver will be tuned during the transmission pause. The way out of this situation may be the introduction of manual settings, similar to how it is described in the article by V. Kvashenko "Manual tuning in the scanning receiver"in "Radio", 2003, No. 5, p. 20. When repeating this refinement (see the figure in the mentioned article), it is necessary to include a constant resistor with a resistance of 3 ... 51 kOhm in the wire going to the offset of the variable resistor R100 in order to avoid a short circuit of the power source in the lower (according to the circuit) position of the engine and accidental pressing to the SA1 button. Author: I. Nechaev, Kursk
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