VHF prefix to the DV-SV receiver. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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VHF prefix to the DV-SV receiver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Radio receivers, having only the ranges of long and medium waves, in connection with the development of ultra-shortwave broadcasting, seemed to be out of work. Even if you are a radio amateur with little experience, then you have a chance to give your receiver a second life - enter the VHF band into it.

A significant number of household transistor radios manufactured 10 ... 15 years ago or more have the LW and MW ranges, therefore, due to the lack of the VHF FM band, they are now obsolete. They are rarely used, although they are quite efficient and could still serve. Using the K174XA34 chip, you can assemble an attachment to a similar VHF receiver and place it directly in its case.

The most effective in this regard can be the refinement of receivers with larger dimensions, for example, "Alpinist", "Giala" (all modifications), etc. Their dimensions allow you to easily place additional parts, as well as install a whip telescopic antenna with a length of 0,6, 1...XNUMX m.;

It is necessary to embed the set-top box in such a way that it is possible to make maximum use of the controls of the radio receiver (knob and tuning scale, volume control and power switch). The attachment diagram is shown in Fig.1. The basis of the set-top box, as already noted, is a single-chip VHF FM receiver on the K174XA34 (DA1) chip. To increase the sensitivity of the receiver, UHF is used on the transistor VT1. Reception is carried out on a whip telescopic antenna WA1. To suppress signals with frequencies below 60 MHz, a high-frequency filter - C1L1C2 is used at the UHF input.

VHF prefix to DV-SV receiver

For tuning, a variable capacitor of the main receiver is used. In the proposed version, an extended VHF band is installed, which includes both VHF bands, as well as providing the reception of sound accompaniment of some television programs (second - fifth channels).

To connect the set-top box, the main receiver undergoes the following modifications. The wire from the range switch to the variable capacitor is cut near the latter, and switch SA1.2 is included in this gap. The RF-IF path is disconnected from the power bus and connected to the SA1.1 switch in accordance with Fig. 1. The output of the AF set-top box is connected to the volume control of the receiver. They finalize the case for installing the set-top box (the window for the SA1 switch), and also, if desired and possible, install a whip telescopic antenna.

In the position of the switch SA1 "DV/SV" the radio receiver operates in the normal mode, the set-top box is de-energized. In the "VHF" position, the HF-IF path of the receiver is de-energized, and the set-top box is tuned in frequency by a variable capacitor of the receiver. Naturally, the "frequency reading" is on the scale of the receiver.

The prefix should be placed in the immediate vicinity of the variable capacitor - as close as possible to the section that will rebuild the prefix. This is necessary in order not to disturb the configuration of the receiver itself. That is why the design of the set-top box depends on which receiver you will install it in. The design option given below is intended for installation in radio receivers of the Alpinist-405, Alpinist-407 and similar series.

All parts of the attachment, except for the antenna, are placed on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in Fig. 2, and the placement of elements on it is shown in Fig. 3.

VHF prefix to DV-SV receiver

The second side of the board is left metallized and is connected along the edge with a common wire in several places (shown in Fig. 3 by curved arrows). The option of placing the set-top box in the receiver is shown in fig. 4. Corner 1 made of an insulating material, for example, organic glass, about 1 mm thick, is glued or mechanically attached to the attachment board, on the side free from parts. Using this corner, the board is glued directly to the variable capacitor 2 of the receiver, so that the switch slider SA1 through the window 3 in the back cover 4 of the receiver is accessible from behind.

VHF prefix to DV-SV receiver

After that, the attachment is connected in accordance with the scheme. It should be noted here that the body of the variable capacitor in this receiver is connected not to the common wire (minus of the power supply), but to the plus of the power supply of the RF-IF path, therefore the common wire of the set-top box should not have galvanic contact with the body of the variable capacitor. All connections to the common wire and the variable capacitor should be made with conductors of the minimum length. In addition, it is necessary to install a ceramic capacitor with a capacity of several thousand picofarads between the body of the variable capacitor and the common wire of the set-top box (the leads are of minimum length).

The following parts can be used in the attachment: transistor - KT368A, KT368B, KT399A; resistors - MLT, P1-4, S2-33; capacitor C12 - K50, K53; the rest - KM, KLS, K10. The switch will fit any small-sized one with two positions and two directions. The coils are wound with PEV-2 0,4 wire and contain: L1 - 8,5 turns on a 5,5 mm mandrel, and L2 - 5,5 turns on a 3,5 mm mandrel.

With serviceable parts and proper installation, setting up the set-top box comes down to setting the RF amplifier mode for direct current and setting the boundaries of the restructuring carried out by selecting capacitors C15, C16 and coil L2. This can be done both with the help of a generator and when receiving radio stations in the VHF bands. First, with a minimum capacitance (with some margin) of the variable capacitance capacitor of the receiver, by selecting the number of turns of the L2 coil or by shifting and expanding its turns, they achieve tuning to the highest frequency radio station in the VHF band or to a frequency of 110..112 MHz signal generator. Then, in the position of the maximum (with some margin) capacitance of the variable capacitor, by selecting C16, they achieve tuning to the lowest frequency radio station in the VHF range, or to a frequency of 66 MHz. It is advisable to repeat this operation several times. The more accurately it is made, the smoother the tuning will be and you will be able to receive all stations. After adjusting the turns of the L2 coil, it is desirable to fill it with paraffin, this is necessary in order to eliminate the so-called "microphone effect" that occurs during mechanical action on this coil.

As an antenna, you can use a piece of wire 0,5 ... 1 m long, using the antenna socket already in the receiver to connect it. But a whip telescopic antenna is preferable, since, depending on the signal level, its length can be changed. In urban conditions, with a high signal level, in most cases it is not necessary to put it forward. Outside the city, away from the transmitters, you have to use the entire length of the antenna.

Author: I. Nechaev, Kursk

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