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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING VHF receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception
If you still have a subscriber loudspeaker, then using its case with a dynamic head, you will manufacture the simple network VHF radio receiver offered here at no special cost. This does not require scarce or expensive parts. As you know, one of the most complex components of a VHF receiver is the radio reception path. But with the advent of specialized microcircuits, for example. TDA7021 (domestic analogue of K174XA34) and K174XA42. making such a knot is not so difficult. The circuit diagram of the receiver is shown in fig. 1. It consists of a radio frequency amplifier based on a VT1 transistor. an FM signal processing unit on a DA1 chip, an AF power amplifier on VT5-VT11 transistors and a power supply.
Radio station signal received by antenna WA1 through antenna socket XW1. capacitor C1 and coils L1 and L2, enters the base of the transistor VT1. In this case, the input oscillatory circuit L1C3 is tuned to the middle of the VHF-2 range to provide a slight rise in the frequency response in the high-frequency region of the operating range. This is necessary to compensate for the decrease in sensitivity with an increase in the frequency of the applied microcircuit. The radio signal amplified by the VT1 transistor is emitted on the L3 coil and, through the capacitor C8, is fed to the input of the DA1 microcircuit. The inclusion of the microcircuit is standard, it was cited in the Radio magazine. 1995. No. 10. p. 62; No. 11, p. 45. The tuning indicator is made on transistors VT3, VT4 and LED HL4. The signal is supplied to it from pin 9 of the DA1 chip. The sensitivity threshold of the indicator is set by a tuning resistor R4. When finely tuned to the station, the HL4 LED lights up. Tuning at the radio station is carried out by changing the resonant frequency of the oscillatory circuits of the local oscillator of the DA1 microcircuit. The VHF-2 range circuit includes an L5 coil with VD3 varicaps. VD4, VHF-1 range - L4 coil. varicap VD2 and capacitor C22. The coils are switched by switch SA1. Section SA1.2 through resistor R11, simultaneously with switching ranges, lights up one of the LEDs HL2 or HL3. signaling the inclusion of one or another VHF range. The tuning element in the receiver is a variable resistor R6. included in the voltage regulator circuit on the LED HL1. From the engine of this resistor, the tuning voltage through the resistor R9 is supplied to the varicaps VD2 - VD4. changing their capacity. The power supply of the URF and the DA1 microcircuit is carried out through a voltage regulator on the VT2 transistor and the VD5 zener diode. The HL1 LED also serves as an indicator of the inclusion of the receiver in the network. The audio frequency signal (3H) from pin 14 of the QA1 microcircuit, through the capacitor C23 and the tuning resistor R10, is fed to the buffer amplifier on the VT5 transistor. It increases the amplitude of the 3H signal to the level necessary for the operation of the audio frequency power amplifier (UMZCH). UMZCH is made according to a push-pull circuit with a direct connection between the cascades on transistors VT6 - VT11. At its input there is a volume control - a variable resistor R18. The C33R26R27 feedback circuit is used to correct the frequency response of the amplifier, providing a more pleasant sound. The amplifier is loaded on the dynamic head BA1 through the capacitor C35. The power supply of the receiver is made on a step-down transformer T1 with a rectifier on a VD9 diode bridge. The output voltage is stabilized by a device based on a VT12 transistor and a VD8 zener diode. Instead of the TDA7021 chip. as already noted, in the receiver you can use its domestic counterpart - K174XA34. KT1A, KT3127 can work as a transistor VT363 in the URF. KT337. KT3123 with any letter indexes. The remaining transistors VT3-VT6. VT8 - any of the KTZ102, KT315 series; VT7 - VT9 - KT361, KT3107. Transistor VT2 - any of the KT502 series. KT815; VTIO - KT814. KT816; VT11. VT12-KT815. KT817 with any letter indexes. In place of the diodes VD1. VD7 can be installed KD522B, KD521B, and VD6 - D2B, D9B. HL1-HL4 LEDs will fit any of the AL307 series or imported ones with the appropriate recommended parameters. Varicaps VD2 - VD4 - KB 109. KB 132 with any letter indices. Zener diodes VD5 - KS162A. KS168A, KS 147A, VD8 - D814D. KS512A. KS213J. Capacitors in the VHF unit, it is desirable to use miniature ceramic KD-1, KD-2, KM-4. K10-23; tuning C1. C3 - KPK-MN. It is possible to use other types, for example. CT4-21. CT4-25. CT4-23. but you will need to change the PCB pattern. Variable resistor R18 - SP-1. SPZ-30 with functional characteristic B; R6 - with linear movement of the SPZ-23a slider, 86 mm long and with a resistance of 10 - 100 kOhm (functional characteristic A). All tuned resistors are SPZ-38. SPZ-27. Dynamic head BA1 - any power up to 3 watts. For example. 1GDSH-9. 1GDSH-11. 2GDSH-7. ZGDSh-22 and others. Coil L1 has 3,5 turns of 0,3 mm PEV wire. L2 - 2,5 turns of wire PEV-0.1 mm. the winding is ordinary, and both are located on the same frame made of polystyrene with a diameter of 5 mm. Coil L3 has 20 turns of 0,5 mm PEV wire. wound coil to coil on a mandrel with a diameter of 2 mm. The local oscillator coils are frameless, L4 has 7 turns of 0,8 mm PEV wire. its inner diameter is 6 mm. and the L5 coil - 5 turns of PEV wire 0.5 mm. inner diameter 4 mm. As a range switch SA1, a sliding switch from an imported radio tape recorder is used, but any suitable one can be used, for example P2K. The power transformer T1 is used from an imported network adapter, but is applicable to any power of 5 - 10 W with a voltage on the secondary winding of 12 ... 15 V. Most parts of the receiver are mounted on three printed circuit boards: the RF part - fig. 2, UMZCH - fig. 3 and power supply - fig. 4.
These boards are installed in a case, which, as already mentioned, can be used as a subscriber loudspeaker case, but you can also make a home-made one, as shown in the photo in the splash screen and in fig. 5.
An antenna socket and a block for installing a fuse are placed on the side walls of the case. The rest of the controls and settings are on the front panel. Moving the engine of the resistor R6 is made from the tuning knob through the vernier device. All connections between the individual boards are made with a flexible mounting wire. The VHF board is connected to the antenna socket with a piece of coaxial cable. If the receiver is made as a wall-mounted receiver, then on the rear wall of the housing (removable) it is necessary to provide holes for mounting on the wall. Setting up a receiver is easy. First, a trimming resistor R19 sets half the supply voltage on the collectors of transistors VT10, VT11, by selecting resistor R16 - also half the supply voltage on the collector of transistor VT5, trimming resistor R2 - the voltage drop across resistor R3. equal to 0,5 V. Then, setting the engines of the tuning resistors R4 and RW to the middle position, they lay the boundaries of the received ranges by stretching or compressing the turns of the heterodyne coils L4 and 15. You may have to reduce or add one turn each. Having tuned in to the radio station, set the resistor R18 to the upper position according to the diagram and the trimming resistor R10 sets the maximum sound volume at which distortion is not yet noticeable. After that, the tuning resistor R4 achieves a clear ignition of the HL4 LED when finely tuned to the station and its extinction - when detuned. The final adjustment procedure - by rotating the rotors of the tuning capacitors C1 and C3, they achieve maximum receiver sensitivity when receiving weak radio signals. Author: I.Potachin, Fokino, Bryansk region
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In the relatively recent past, a wired broadcasting network with subscriber loudspeakers was widely developed in our country. Today, radio listeners prefer VHF FM radios, especially since the number of radio stations transmitting in the VHF-1 (65,8 ... 74 MHz) and VHF-2 (88 ... 108 MHz) bands is increasing every year. The author of the published article talks about how outdated wired devices can be turned into VHF receivers.
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