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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING VHF radio receiver in the Selga-405 case. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception In the "Selga-405" case, the author assembled a mains-powered VHF radio using some parts from the original device. In the last century, the domestic industry produced a large number of battery-powered portable radio receivers with LW, MW, and less often HF bands. Today, these ranges are practically empty - some of the radio stations have migrated from the air to the Internet, some have closed. For this reason, the surviving copies of such radios have become useless. At the same time, VHF broadcasting bands are still "alive", so such radios can be converted to work on VHF. One of the possible methods is shown on the example of the domestic radio receiver "Selga-405" of 1984 release. Of the several available circuit solutions, the choice fell on the option with the domestic KR174XA34 microcircuit. This microcircuit is a low-IF FM path for receiving and processing signals with frequency modulation in the VHF band [1, 2]. In the 1990s, the author assembled about 1066 radio receivers on this microcircuit, as well as other similar ones with a low IF KS1XA142, K42XA20. These microcircuits are distinguished by high reliability - not a single failure, high sensitivity, simplicity of the switching circuit, and the negative effect described in [3] was not noticed in all manufactured receivers. The scheme of the VHF radio receiver is shown in fig. 1. From the original device, there was a case, a variable capacitor with a vernier device, a variable volume control resistor, a circuit board and a dynamic head. The VHF radio signal from the telescopic antenna WA1 through the coupling capacitors C13, C15 is fed to the UHF, assembled on a high-frequency low-noise transistor VT2 according to a common emitter circuit. Diode VD6 protects this transistor from damage. The bias voltage to the base VT2 comes from the output of the collector of this transistor through resistors R6, R7. Capacitor C14 prevents the formation of RF feedback by increasing the gain of this stage at high frequencies. The resistor R8 serves as a transistor load. The UHF cascade is fed with a voltage of about 4,1 V through the L3C16 filter.
The amplified RF signal through the coupling capacitor C18 is fed to the input of the DA2 microcircuit (pin 12). At the radio station, the receiver is tuned using a variable capacitor C33, which adjusts the operating frequency of the oscillatory circuit. Capacitor C31 - stretching. The parameters of the oscillatory circuit are chosen so as to cover the frequency range of 63...110 MHz. A slightly extended range of received frequencies was chosen so that when the elements age, strong changes in the ambient temperature, the radio stations located at the edges of the range do not end up DA1 APL1117-ADJ overboard. The low-frequency audio signal from the DA2 output (pin 14) through the filters R11C35, Z3, the separating capacitor C37 and the closed contacts of the switch SB1.2 goes to the volume control - a variable resistor R14. Voltage is removed from pin 9 of DA2 to control the LED indicator of the tuning signal level to the radio station. This voltage is fed through the filter C29Z2 to the emitter follower VT1. When the radio station is fine-tuned and the signal level is high, the LED goes out. The A1 VHF module is assembled on a separate printed circuit board made of fiberglass foiled on both sides with dimensions of 65x28 mm and a thickness of 2 mm. The module is shielded with tin plate, UHF on the VT1 transistor is shielded from the DA2 chip. The variable capacitor is also shielded. The bottom layer of foil is also used as a screen. The lower foil layer is connected to the common wire of the upper layer by 15 jumpers evenly distributed over the board. On the upper layer of the foil, the conductors are cut out with a hand cutter to fit the existing parts (Fig. 2).
The VHF module receives power at about 4,1 V through the Z1 filter from the DA1 voltage regulator and remains operational when the supply voltage drops to 1,9 V. Since the radio station is tuned by a variable capacitor, it does not go astray due to a change in the supply voltage. A similar module was used in the conversion of radio receivers "Selga-404", "Yunost KP-101", "Signal RP-204" and for the modernization of the radio receiver "Russia RP-303", as well as, with some changes, in other designs. The audio frequency amplifier is assembled on an integrated circuit DA3 (LM386N-1). Resistor R15 eliminates zero volume operation, reducing the chance that the radio will operate when it is not needed. The load of the amplifier DA3 is the dynamic head BA1, connected to the output of the microcircuit through a decoupling capacitor C44. The damping circuit R16C42 prevents self-excitation of the DA3 chip at ultrasonic frequencies. Since many mobile digital devices have poor sound quality, it is advisable to connect them to an external UHF. To this end, the new radio is equipped with an XS1 socket. Switch SB1 select the mode of operation "Radio" / "Amplifier". Resistors R12, R13 summarize the stereo signal into monophonic, capacitor C36 prevents the input of UZCH radio frequencies. The sensitivity of the UZCH is enough to work with any digital multimedia device. Today it is not customary to "walk" with such radios, so the device does not provide autonomous power supply. But if necessary, you can connect an external source of autonomous power supply with a voltage of 3,3 ... 12 V, for example [4-6]. Schottky diode VD5 protects the device from power supply voltage reversal. Instead of a battery of galvanic cells or batteries, a mains power supply is built into the receiver housing. The mains voltage of 230 V is supplied to the primary winding of the step-down transformer T1 through the closed contacts of the switch SA1, the safety resistor R1 and the thermistor RK1 with a positive temperature coefficient of resistance, which operates as a high-voltage resettable fuse. In the case of an increase in current through the primary winding of the transformer, for example, at an abnormally high mains voltage, the thermistor heats up, its resistance increases from 20 ... 30 ohms to tens or even hundreds of kilo-ohms, which prevents damage to the transformer. From the secondary winding T1, an alternating voltage of about 8,5 V is supplied to a bridge diode rectifier assembled on Schottky diodes VD1-VD4. Capacitor C6 smooths out the ripple of the rectified voltage. A voltage regulator of +1 V is assembled on the DA1117 chip (APL4,1-ADJ). The output voltage is set by selecting the resistor R4 - the lower its resistance, the lower the output voltage. LEDs HL1, HL2 shine when there is a supply voltage above 5 V, they are designed to illuminate the tuning scale. A view of the layout of nodes in the housing is shown in fig. 3. Key power switch KCD-2011 (SA1) is located on the rear wall of the radio receiver next to the mains transformer, it can be replaced, for example, with MRC-101-6A, KCD1-101. Switch SB1 - RS10. Resistor R1, capacitor C1 and thermistor RK1 are placed on a separate 35x20 mm board. Diodes VD1-VD4, capacitors C2-C6 are mounted on a board with dimensions of 35x24 mm. Resistors R12, R13, capacitor C36 and socket XS1 are mounted on a 33x18 mm board. The VHF module is glued to the main board of the device in such a way that the wire from the capacitor C31 to C33 is as short as possible. UZCH and voltage stabilizer nodes are located on the main board. Installation - hinged. Do not neglect the correct wiring of the common wire for power, signal and high-frequency circuits.
Instead of the KR174XA34 chip, you can use K174XA34, KR174XA34A. A copper heat sink with a cooling surface area of at least 386 cm is glued to the LM1N-3 chip.2. Instead of an ultrasonic frequency converter, this microcircuit can be used to assemble another amplifier that operates at a supply voltage of 3 ... ADJ. Any similar stabilizer with a low voltage drop in the appropriate switching circuit is also suitable. Instead of a high-frequency transistor SS12, any of the 1117SC1117, 1117SC1117, KT9018, KT2, 1730T2, 1395T368, KT399T2, KT399, KT2T372, KT372,2, 382T382 series is suitable. We can replace the KT325,2B transistor with any of the KT325, KT355, KT2, SS355, PN3102, BC312, BC315 series. The transistors mentioned in the replacement options have differences in the pinout. Instead of Schottky diodes EC31QS04, you can install SB140, SB150, SB160, 1 N5819, MBRS140T3. Diode 1 N4148 can be replaced with PMLL4148, PMLL4446, PMLL4448, KD503A. LEDs HL1, HL2 - super-bright, for surface mounting, yellow glow (from the backlight of the car radio buttons). LED RL32-SR114S - red glow, can be replaced by any continuous glow without built-in resistors, preferably with the lowest possible operating voltage. Coil L2 - frameless, contains 19 turns of winding wire with a diameter of 0,39 mm, wound on a mandrel with a diameter of 3 mm. Coil L4 - frameless, contains six turns of winding wire with a diameter of 0,39 mm, wound on a mandrel with a diameter of 3 mm. A piece of foam rubber is inserted inside this coil, which is then impregnated with paraffin. The capacitor C31 is also filled with paraffin. Choke L1 - ready, industrial production, wound on an H-shaped ferrite magnetic circuit, winding resistance - no more than 1 ohm, inductance - the more the better. Inductor L3 is similar, with an inductance of 100 ... 1000 μH and a winding resistance of 3 ... 15 Ohms. The power supply unit uses a transformer TS6-2. The secondary winding is rewound, it contains 115 turns of winding wire with a diameter of 0,33 mm. Winding - turn to turn, not a single turn should be overlapped, otherwise the winding will not fit in the window. Instead of such a transformer, for example, the unified TP-112-1 is suitable. WA1 - rotary telescopic antenna 56 cm long. Domestic dynamic head 0,5GD-37 differs from similar ones of the same size in good sound quality and high sensitivity. Can be replaced by a similar 1GDSH-6. The housing of the dynamic head is connected to a common wire. As mentioned above, a standard variable resistor is used, the power switch in it is not used. This switch cannot be used to switch the mains voltage of 230 V. Any variable resistor with a resistance of 4,7-22 kOhm will do. The thermistor ZPB53BL200C (RK1) is used from the funai TV kinescope degaussing unit. Can be replaced by ZPB53BL300C or other 20...30 ohm resistance at room temperature, or SF250-080 polymer resettable fuse. Resistor R1 - imported non-flammable or discontinuous. The remaining resistors are of any type for general use, it is desirable to use surface-mounted resistors in the VHF module. Capacitor C1 - ceramic for a rated voltage of at least 1000 V DC or 250 V AC. Capacitor C38 is a small film capacitor. Oxide capacitors - imported analogues K50-68, K53-19. The remaining non-polar permanent capacitors are ceramic, for a rated voltage of at least 25 V. Capacitor C31 should be with the lowest possible TKE. Filters Z1-Z3 - DST9NC52A222Q55B or DST9HB32E222Q55B - capacitors with a capacity of 2200 pF, on the terminals of which ferrite tubes are put on. They can be replaced with 2200pF ceramic capacitors. Adjustment basically comes down to setting the boundaries of the range. By selecting the capacitance of the capacitor C31, the lower limit of the received VHF band is set. Stretching and compressing the turns of the coil L4, set the upper limit of the range. By connecting the receiver to an external VHF antenna and tuning it to a local radio station, the resistor R10 is selected, focusing on its greater resistance so that the HL3 LED does not light up. This completes the device setup. The radio receiver, made according to the scheme of Fig. 1, receives all local radio stations on a piece of wire 10 cm long connected as an antenna, reception is carried out from a distance of about 30 km from the transmitting antenna. At maximum volume, the device consumes a current of 230 mA from the 16 V network. When the device is powered from an external 6 V DC source, the current consumption is about 80 mA at maximum or 20 mA at minimum volume in the "Radio" mode or 6 mA in the "Amplifier" mode in the absence of a signal. Literature
Author: A. Butov
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