Range 88...108 MHz in old receivers. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Range 88...108 MHz in old receivers. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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When Japanese radio receivers appeared in the USSR with a VHF band of 88 ... 108 MHz (VHF2). which was not broadcasting at that time, radio amateurs began to adapt them to receive radio stations in the range of 64 ... 73 (VHF1). For this, converters of varying complexity were proposed [1], as well as various ways of changing the parameters of the circuits of these receivers.

At present, the situation has changed. In some regions, broadcasting in the VHF1 band has been discontinued, but many stations have appeared on VHF2.

Transistor receivers for receiving stations in the VHF2 band can be rebuilt by changing the number of turns of the loop coils in the VHF unit. The resonant frequency of an oscillatory circuit is inversely proportional to the square root of the coil's inductance. At the same time, the inductance is proportional to the square of the number of turns. Therefore, the frequency of the circuit depends linearly on the number of turns. If the average frequency of the VHF2 range is taken equal to 98 MHz, and VHF1 - 70 MHz, then the conversion factor for the input coils and URF coils will be KP1=98/70=1,4. and for local oscillator coils - Kp2 \u98d (10.7 + 70) / (10,7 + 1,35) \uXNUMXd XNUMX.

Since the range of received frequencies needs to be increased, the number of turns of the coils decreases. This circumstance makes it possible to simplify the restructuring and not solder the coils from the board. The coil leads are simply cut off so that the ends are left, to which it will be possible to solder the leads of the shortened windings. After unwinding the turns, the coil end may be in a different place, and in order to solder it to the remaining end, you will have to shorten or lengthen the turn, which will not correspond to the calculated value. In this case, when setting up a converted receiver, it may be necessary to replace the ferrite cores in the coils with brass ones or vice versa, as well as the selection of capacitors connected in parallel with the coils. If the coil is with a tap, then the windings are shortened on both sides in proportion to the number of turns before the tap.

Retro lovers continue to use tube receivers. In most of these receivers, tuning to the VHF station is carried out by variometers, the windings of which are fused into polystyrene frames. There are designs where the turns are laid in grooves, and the input coils are printed. It is impossible to rebuild such a block without serious changes. To receive stations in the VHF2 range on these receivers, you can connect a converter to them that converts VHF2 frequencies to VHF1.

The proposed converter consists of a local oscillator on a field-effect transistor VT2 according to the inductive three-point circuit and a mixer on a transistor VT1. The converted signals are taken from the VT1 collector and fed to the VHF receiver unit. The antenna is a flexible wire 0,75 m long or half of the dipole antenna built into the receiver. The converter is powered by the filament circuit of the lamps through a rectifier on the diode VD1. To reduce the "creeping" of interference from the VHF1 range, the converter is located near the VHF antenna jacks, and its common bus contacts directly with the receiver chassis. The output of the converter is connected with a short wire to one of the antenna sockets, and the second socket is connected to the common wire of the converter. If these measures are not enough, then the converter will have to be shielded.

Range 88 ... 108 MHz in old receivers

The desired section of the VHF2 range is set using the L2 coil core. The L1-C2-C3 circuit is set to the middle of the range. To adjust L1, it is convenient to use a test stick, which is made of a 3 mm diameter vinyl chloride tube. A ferrite core HH100 with a diameter of 2,8 mm is inserted into it on one side. and on the other - a piece of aluminum or copper wire. If the volume increases with the introduction of ferrite into the L1 coil, then it is necessary to compress the turns. With increasing volume from the metal, the coil is stretched. If the receiver has an optical tuning indicator, a voltmeter (at the limit of 300 V) can be connected to its anode and, according to the maximum readings, a more accurate setting can be made. If VHF1 stations also work at the reception point, then the converter can be supplemented with a switch that will turn off the power and switch the antenna from the converter input to its output.

It should be noted that the converter converts a part of the VHF2 range equal to the receiver's bandwidth (73-64=9 MHz). In the converter, the local oscillator frequency is selected below the frequency of the received signal, which simplifies setup and reduces installation requirements. On the other hand, the second harmonic of the local oscillator can get into VHF1 and suppress the signal of the desired station. By a slight change in the local oscillator frequency, the interference can be shifted to a free area. In this case, the harmonic and received stations on the scale of the receiver are shifted in opposite directions. The converter can also be connected to a transistor receiver without altering the latter.

Details. Coil L1 is wound on a mandrel with a diameter of 5 mm and contains 5 turns of PEL wire 00,68 mm. Coil length -10 mm. Coil L2 is wound turn to turn on a 05 mm frame, contains 2,5 + 7,25 turns of PEL 00,43 mm. Core - carbonyl, M4. Transistor VT2 can be taken with any letter. Its channel resistance is at least 200 ohms. With less resistance, the voltage drop across the resistor R5 will increase, and the local oscillator may not be excited.

Literature

I. Alexandrov. VHF converter. - Radio, 1992, No. 8, S. 44.

Author: P. Sevastyanov, Tashkent, Uzbekistan.

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