VHF receiver from ready-made blocks. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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VHF receiver from ready-made blocks. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Using functional units from obsolete radios and televisions, it is possible to assemble a fairly good VHF receiver at minimal cost and time. How to do this, and this article tells.

To manufacture a structure with a reception range at frequencies of 65 ... 74 MHz (monophonic version), you can use an audio unit from almost any lamp or lamp-semiconductor black and white or color TV and a VHF unit from any tube receiver, radio or radio (hereinafter according to the text just a tube receiver) of old models.

The VHF block of tube receivers is a functionally and structurally complete assembly, enclosed in a metal case, inside which are mounted input circuits with tuning elements, a high-frequency amplifier, a local oscillator and a frequency converter.

After converting the radio frequency signal received by the VHF unit, a signal with an intermediate frequency of 6,5 MHz was received at its output in older models. With the same frequency, the sound accompaniment units of all TVs worked (and still work), which in their composition have all the necessary components for amplifying the radio signal, decoding it and amplifying the selected sound accompaniment signal. This circumstance made it possible to combine blocks of various radio-electronic devices in relatively simple ways to create a VHF radio receiver.

Connecting a VHF unit (of any type) from a radio receiver and an audio board from a TV (also of any type) is the same and consists in supplying them with a 6,3 V incandescent alternating voltage and the corresponding anode constant +150 and +250 V, as well as connecting the antenna to input of the VHF unit, a loudspeaker with a matching transformer to the output of the sound card and interconnection with a 75 Ohm coaxial cable.

A schematic diagram of a possible receiver option is shown in the figure. As nodes, a high-frequency unit of the VHF-IP-2 type (unit A), which had the most widespread use, and a sound board (unit B) from a ZULPT-50-III TV ("Record-V312", "Spring-308") . The ordinal indexing of the elements on the diagram below is indicated in accordance with their factory diagrams for each specific block. Additionally, several radio elements are used to build a power node and connect a loudspeaker, the indexing of these elements is separate.

(click to enlarge)

A variable resistor (R3) with a resistance of 47 kOhm (type B regulation characteristic) from the specified TV was used as a volume control. It is possible to replace it with another variable resistor with a resistance of up to 150 kOhm. Resistor R4 PEV-7,5 is also available on the TV; in the absence of it, it can be replaced with three MLT-2 12 kOhm resistors connected in parallel.

Resistor R1 - PEV-7,5 type or three MLT-2 resistors of 8,2 kOhm each, connected in parallel.

Capacitor C1 is paper, for example, BMT-2 0,047 μF (400V), when installed, it is soldered directly on the contacts of the VHF unit. The capacitances of the oxide (electrolytic) filter capacitors (C2-C4) are not critical and can be changed both up and down in depending on what is available.

As a power transformer (T1), you can use a transformer from any tube receiver. The output transformer (T2) from almost any TV (in the specifications of television radio elements they were referred to as TVZ with the corresponding ordinal number) or from any other equipment, for example, a tube receiver or a tape recorder.

If the components used are serviceable and the installation is correct, then the adjustment of the assembled receiver can be minimal. It comes down to setting the required supply voltages depending on the actual consumption currents of the blocks used. After turning on the receiver, the voltage of +250 V is set by selecting the resistor R2, +150 V - R1 and +20 V (power supply of the transistor part of the audio board) - resistor R4.

The design of the receiver can be arbitrary. It all depends on the dimensions of the elements used (mainly boards, transformers and oxide capacitors) and the taste of the radio amateur. It is only necessary to take into account the following: the sound block board must be placed with the lamps up so that they do not heat up the rest of the installation elements, and the power and output transformers should be spaced as far as possible and positioned so that their magnetic cores are mutually perpendicular (with network interference will be minimal).

Following these recommendations, you can quickly assemble and adjust the electronics of the receiver, with good results. With some complication of the design and, accordingly, the cost of time and money, it is possible to perform the stereo version in the same way. You only need two audio boards and an additionally manufactured stereo decoder.

Author: S. Savinov, Novosibirsk

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