Rebuilding VHF units to FM. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Rebuilding VHF units to FM. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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About ten ... twelve years ago, amateur radio magazines often published articles on restructuring imported receivers with the FM band (88 ... 108 MHz) to the VHF-1 range (65,8 ... 75,0 MHz). At that time, broadcasting was carried out exclusively in the VHF-1 band.

Now the situation has changed dramatically. The air in the range of 100 ... 108 MHz is filled almost everywhere. On sale there are many imported and domestic radio receivers with a VHF-2 range or with common ones (VHF-1 and VHF-2).

Since the VHF-1 range was actually "orphaned", a giant fleet of old radios and radio tape recorders remained "out of work". You can give them a second life by relatively simple modification of the VHF units of these receivers. In doing so, the following points should be noted. Modification of inexpensive portable receivers ("VEF", "Sport", "Sokol", "Ocean", etc.) should be minimal and ensure the reception of 3 ... 7 VHF-2 broadcasting stations in the region. For stationary devices of a higher class with an external VHF antenna, it is desirable to keep all its technical parameters (sensitivity, local oscillator stability, wide scale, etc.).

Usually, the VHF radio receiver unit contains an input circuit, 1-2 UHF cascades, a local oscillator, a mixer, and IF cascades. As a rule, these are 4 (less common 5) LC circuits. Having a basic (even better, mounting) diagram of a radio receiver, it is easy to determine all the necessary nodes (inductors, capacitances, etc.). The first circuit of the IF and all subsequent cascades do not need to be altered.

It is clear that for the range of 100 ... 108 MHz, the capacitances and inductances of all LC circuits of the VHF-1 unit must be reduced. Theory and practice state that the capacitance of the circuit changes in proportion to the wavelength, and the number of turns of the inductor - the square root of this value.

When moving from the VHF-1 range to the VHF-2 range and with constant inductances (the number of turns of the inductors does not change) - this is an option for portable receivers for medium frequency ranges (69,0 MHz and 104,0 MHz) - we obtain the following relationship for containers:

С_UKB-2 \u0,44d XNUMX * C_VHF-1.

where C_VHF-1 - the total total capacitance of the VHF-1 range circuit; FROM_VHF-2 - the same capacity of the VHF-2 range. In a real VHF block circuit, these capacitances include capacitors soldered into the circuit, parasitic mounting capacitances, the interturn capacitance of the inductor, and the input capacitance of transistors.

With this in mind, in practice the following ratio of capacities is more suitable:

С_UKB-2 = (0,3...0,35)*С_VHF-1.

In addition, in VHF units, it is possible to change the inductance of the loop coils within certain limits by rotating the tuning cores. Usually, the local oscillator of the VHF-2 block for the range of 100 ... 108 MHz should be tuned within 110 ... 119 MHz (with a margin) at IF = 10,7 MHz, and within 106 ... 115 MHz at IF = 6,5, 1 MHz, i.e. above the signal frequency. On the circuit diagram of the VHF-XNUMX block, we mark those capacities that will be completely soldered from the circuit, as well as those capacities that will be replaced by others with a lower rating. Usually these are miniature disk ceramic capacitors.

Capacitors must be selected in advance, cleaned and tinned, shortening them to a minimum. If there is no device for accurately measuring capacitance, the table below will partially help to solve the problem, where the size and color of the capacitor will suggest the limits of the nominal capacitance.
Table 1
TKE group, body color Limits of nominal capacitances (in pF) with case diameter Marking dot color
4mm 5mm 6mm
P120, blue 1,0 ... 2,2 2,7 3,9 ... 4,7 ... 7,5 -
PZZ, gray 1,0 3,9 .. 4,7 7,5 ... 8,2 10 ... -
M47, blue 1,0 4,7 .. 5,1 10 ... 11 15 ... -
M75, blue 1,0 11 .. 12 24 ... 27 39 ... Red
H700, red 10 18 ... 20 33 ... 36 56 ... -
H1300, green 18 47 ... 51 82 ... 91 130 ... -
H70, orange 680, 1000 1500 2200 -

For clarity, you can compare the capacitance ratings in the "VEF-221" and "VEF-222" radios, which are built according to the same circuits with the same inductors ("VEF-221" has a range of 87,5 ... 108 MHz, " VEF-222" - 65,8...74,0 MHz). These data are taken from the factory operating manual (Table 2). Capacitance ratings are given in it in picofarads.
Table 2
Receiver type Capacitive input circuit divider UHF loop series capacitance Parallel capacitance of the local oscillator circuit Series capacitance of the local oscillator circuit Capacitance in AFC circuit Parallel UHF loop capacitance
С3 С4 С6 С13 С14 С15 С19
VEF-221 8,2 33 33 2/10 62 5,1 -
VEF-222 33 82 47 22 75 12 15

Similar schemes of VHF units are used by the VEF-215 radio receiver and the VEF RMD-287S radio receiver, so the data in Table 2 are also suitable for reworking the VHF units of these devices.

Another example is a removable auto-receiver of the Ural-auto-2 type (input circuit, two UHF stages on GT322A transistors, a local oscillator on a microcircuit of the 224th series with the ZHA1 or XA1 index). In the input circuit in the capacitive divider C1-C2, we change C1 \u22d 5,1 pF by 6,8 ... 2 pF, C33 \u12d 5 pF - by Yu ... 7pF. Capacitors C14, C33 and C1 of 2 pF each (series capacitances with KPI of the 12st, 13nd stages of UHF and local oscillator) are changed to 0 ... 2,88 pF. In the local oscillator circuit, the tuning core made of ferrite (3 101 mm) is changed to brass with a thread (diameter 368 mm). Another example is the tuner "Radiotechnika T-339-stereo" (VHF unit on transistors KT111A and KT3A, restructuring - varicaps KVS15A). Parallel capacitances C14 = 15 pF (input circuit), C18 = 9,1 pF (UHF), C4 = 130 pF (local oscillator) are dismantled. Serial capacitances C13 = 130 pF, C43 = 47 pF (input circuit and UHF) are changed to 15 ... 82 pF, and C27 = 33 pF (local oscillator) - to 1,5 ... 1 pF. To stretch the scale, carefully unsolder the loop coil of the local oscillator and unwind 0,9 turns from the top of the coil, 1,2 turn from the bottom (the tap from XNUMX ... XNUMX turns as it was). Then carefully solder the coil into place.

It is convenient to divide the process of reworking blocks of VHF receivers into several stages

We provide access to the VHF unit both from the side of the parts and from the side of the printed conductors by removing the covers of the receiver and the VHF unit.
We determine the LC circuits of the input circuit, UHF, local oscillator, mixer, and the first circuit of the IF (the last alteration does not apply).
Carefully unsolder the containers to be replaced and dismantled.
We solder new containers prepared in advance (with cut and tinned leads) for each individual circuit of the VHF unit.
After making sure that there are no errors, and the circuit is not broken (there are no bad solderings, short circuits of printed tracks, etc.), we turn on the power of the receiver and try to hear at least one powerful (in this place) VHF station. At the same time, we rotate the receiver tuning knob and the local oscillator core. It is very useful to have an industrial receiver with a VHF-2 range nearby. This will help to immediately identify the desired station in the tuned receiver. Having heard at least barely the station, the tuning cores of the coils and the tuning capacitors of the input circuit, the UHF and the mixer, we achieve a loud reception of this station. At this stage, you can determine whether you need to change the cores from ferrite to brass and vice versa.
By rotating the core of the local oscillator coil, we set the required place for this station on the receiver scale (focusing on an industrial receiver with a VHF-2 range). Usually, the section of the scale of the tuned receiver, where the stations of the range 100 ... 108 MHz are located, occupies a very small part of the constructive scale of the receiver (about one third).
We carry out conjugation of the circuits of the input circuit, UHF and the local oscillator of the tuned VHF unit. In the area near 100 MHz, we achieve the highest volume of the stations by rotating the tuning cores of the input circuit, UHF and mixer, and in the area near 108 MHz - by rotating the rotors of the tuning capacitors of the same cascades (in this case, you need to monitor the position of the receiver tuning knobs - the maximum capacitance of KPI or varicaps at the beginning of the range and their minimum capacitance at the end). We repeat this operation 2-3 times. In conclusion, it is necessary to reduce the capacitance in the AFC circuit by 2 ... 2,2 times (if its value exceeds 5 ... 6 pF). The last stage must be carried out in the assembled VHF unit through the holes in the covers to adjust the capacitances and inductances with a dielectric screwdriver.

These general rules for reworking VHF units should be followed for various schemes and designs of units. Briefly about receiving antennas. Obviously, directional antennas provide excellent reception quality, but they need to be rotated. The author uses a single square for the rebuilt tuner "T-101-stereo" (in parallel, two copper wires with a diameter of 1,8 mm with a distance between them = 15 mm and with a perimeter of slightly less than 3 m). The wave impedance of the square is about 110 ohms, so it is powered by a PRPPM cable - 2 x 1,2 (wave impedance is about 135 ohms). The height of the mast on the five-story building is approximately 9 m. The plane of the square is perpendicular to the line Chisinau - Bendery - Tiraspol - Odessa. As a result, more than 10 stations from Chisinau and 3-4 powerful stations from Odessa are heard.

Literature

A brief guide to the REA designer (edited by R.G. Varlamov). -M.: Sov. Radio, 1972, pp. 275,286.
V.T. Polyakov "Direct Conversion Transceivers". - M.: 1984, p.99.
PM Tereshchuk and others. Handbook of a radio amateur, part 1. Kyiv: Technique, 1971, S.Z0.
"VEF-221", "VEF-222". Manual.
Radiotechnika (T-101-stereo tuner). Manual.
A.N. Maltese, A.G. Podolsky. Broadcast reception in a car.- M.: Radio and communication, 1982, p.72.
V. Kolesnikov "Antenna for FM reception". - Radiomir, 2001, N11, p.9.

Author: A. Perutsiy, Bendery, Moldova; Publication: radioradar.net

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