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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Modernization of radio receivers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Domestic "pocket" radios, which are on the shelves of many radio amateurs, produced in the 70-80s, can provide better reception of broadcast radio stations than their modern imported counterparts. Simple improvements, which are described in this article, will give them a "second life". Comparison of modern imported radios (mostly Chinese-Hong Kong) with domestic past production years leads to interesting results. In the ranges of SV, DV and KB, the quality indicators of old domestic receivers are much better. Thus, the dual-band "QUARTZ-302", manufactured in the late 80s, had a real sensitivity of 0,4 mV / m, which is unattainable for imported analogues, excluding, of course, expensive digital and professional models. The parameters of the receivers of those years were subject to the domestic GOST 5651-82, which strictly normalized the sensitivity, selectivity and other characteristics depending on the complexity group (class). Without going into a detailed analysis of the electrical path, we only note that modern small-sized radio receivers are produced mainly in a vertical design, in which the small horizontal size of the radio receiver does not allow placing a magnetic antenna (MA) of sufficient length. With an MA length of only a few centimeters, the signal level at the input of the first stage is low, and the signal-to-noise ratio is poor. As a result, outwardly attractive and seemingly comfortable "Tecsan", "Manbo", etc. in the medium wave range are very "noisy" and do not provide an acceptable reception quality. In the VHF band, the performance is somewhat better, but even here only local reception is possible with good quality. Due to the characteristics of the propagation of radio waves in this range and the low efficiency of the whip antenna, the VHF band (on the receiver it is designated as FM) is often useless at a considerable distance from the transmitting centers. Under these conditions, it is much more expedient to have an old MW-LW-HF receiver, having upgraded it according to the method proposed below. A favorable feature of modern radios is the power supply from two finger-type batteries with a total voltage of 3 V. Domestic models worked mainly from the Krona nine-volt battery. The advantages of a three-volt power supply are obvious: the capacity of AA type galvanic cells (domestic version - size 316) is several times higher, and the cost of even two pieces is lower than one Krona battery and its analogues. The service life of the latter at an average sound volume does not exceed 20 ... 30 hours. Due to the owner's understandable reluctance to frequently change an expensive battery, quite serviceable domestic radios lie idle. Alternative power options also have disadvantages: batteries are expensive and require periodic charging, and mains power negates portability, a major advantage of pocket radios. The way out is to transfer the receiver to a three-volt battery supply. One way to do this is proposed in [1]. It consists in using the conversion of the voltage of the AA elements to the receiver supply voltage of 9 V. However, this does not completely eliminate interference. The best and, perhaps, the easiest way is to make changes to the circuit of the radio itself in such a way as to ensure the normal operation of all stages at a supply voltage of 3 V. This is quite possible, and with a competent approach, the receiver parameters (except output power) practically do not deteriorate. Let's consider modernization on the example of the "QUARTZ-302" receiver. Its circuit is typical for the receivers of this group and is shown in Fig. 1 (it does not show the elements of MA, input circuits and local oscillator circuits, which are not touched at all during refinement). In later models of this and other radio receivers, instead of FSS on inductors, a piezo filter was used, which, however, does not affect the further refinement technology, as well as other minor differences in the circuits of transistor receivers.
The first stage on the transistor VT1 is a mixer with a combined local oscillator. The mode of the transistor VT1 is set by a base bias through the resistor R2 and is stabilized by power supply from the parametric stabilizer VD1, R11, C22. The stabilization voltage is 1,44 V, in connection with which it is possible to maintain it when the total supply voltage is reduced to 2 ... 3 V. To do this, it is enough to reduce the resistance of the ballast resistor R11 to 1 kOhm. It is important to note that the first stage largely determines the operation of the receiver as a whole. Transistor VT1 type KT315 is not optimal here: it has a high noise level, significant transition capacitance and low gain. Much better results are obtained with microwave transistors of the KT368, KT399A types. Although their parameters are normalized at higher frequencies, the noise minimum area extends "down", up to a frequency of 0,5 MHz (KT399A) - 0,1 MHz (KT368), i.e., it also captures the MW range. The gain of these transistors is less dependent on the supply voltage, which is also important in this case. The author used the KT399A transistor, while the noise level turned out to be so low that in the absence of tuning to the station it is even difficult to determine whether the receiver is on or off. Thus, replacing the transistor VT1 guarantees an increase in sensitivity, limited by noise. To ensure the normal operation of the local oscillator (at an emitter current of about 1 mA), the resistances of the resistors R3 and R5 should be reduced to 620 Ohm and 1,5 kOhm, respectively. In the original circuit, the RF-IF path and the first UZCH stage are fed through the R10C13 decoupling filter. A voltage drop of about 10 V is formed across the resistor R1, which is undesirable. To avoid voltage losses, the R10 resistor should be replaced with a small-sized DPM-3 choke from unified TV blocks of the 3rd and 4th generations or, in extreme cases, just a wire jumper. True, in the latter case, the absence of self-excitation when the batteries are discharged is not guaranteed. In the IF path, it is highly desirable to replace the VT3 transistor of the KT315B type with KT3102E, KT3102D or KT342B, KT342V with a gain of 400 ... 500. This is necessary in order to increase the IF gain and thus maintain the gain limited sensitivity, as well as to ensure the effective operation of the AGC. The signal of the latter through the filter R13C23 is fed to the base of the transistor VT3, and therefore it is important to correctly set its operating point by reducing the resistance of the resistor R12 to 30 kOhm. In UMZCH, it is also necessary to reduce the resistance of the resistor R8 to 39 kOhm, and bring the total resistance of two resistors R21, R23 connected in parallel to 1 ... 1,5 Ohm. Why replace the resistors R21, R23 with one wire resistor of the specified resistance. This UMZCH provides for adjusting the quiescent current with a tuning resistor R16. To avoid distortion and obtain acceptable efficiency, the quiescent current should be within 5 ... 7 mA. For the battery, a shell with spring contacts is made, into which two AA elements should fit tightly. The design of the shell can be any, in the author's version it is made of double-sided foil fiberglass and tin, the parts are connected by soldering. The dimensions of the shell allow it to be placed in the Krona battery compartment. The receiver is tuned with a fresh battery, the voltage under load of which is at least 3 V. First, you should check the operating modes of all stages: for transistors VT1-VT3, voltage measurements are taken at their collectors, for transistors VT4-VT7 - at emitters (see table) .
In practice, it may be necessary to adjust the mode of the transistor VT3, the voltage on the collector of which in the absence of a signal should be 1,4 ... 1,6 V and be regulated by the selection of resistor R12. The remaining modes are usually set automatically if the above operations are observed. Further, if possible, a signal from the 2H generator is fed to the input of the UMZCH (VT3) and, observing the output signal on the oscilloscope, by selecting the resistor R8, the symmetry of the half-waves of the sinusoid is achieved, and by the resistor R16 - the absence of "step" type distortion. Then measure the total current consumption in silent mode, which should be 10 mA, and, if necessary, adjust it with a tuning resistor R16. As can be seen, the proposed modernization is simple and does not require large expenditures of time and money. The achieved result is impressive - the sensitivity of the receiver does not decrease (and even increases slightly), the selectivity remains the same, the maximum current consumption at signal peaks does not exceed 20 mA, the performance is maintained when the supply voltage is reduced to 1,8 V, the life of the radio receiver from one set of elements AA - at least 80 hours, and with good quality of the latter - more than 100 hours. The only parameter that deteriorates during rework is the output sound power, which drops to 20 ... 30 mW. As a rule, this is quite enough, since the characteristic sensitivity of the BA1 head is very high. The majority of imported receivers have the same output power, but subjectively the sound quality of the converted one is better due to the better acoustic properties of the case. If desired, modernization can be continued by assembling a more powerful UMZCH bridge. In this case, one should not "reinvent the wheel" and manufacture it on discrete elements, although such schemes have been published. There is a large range of specialized microcircuits - ready-made high-quality amplifiers with low-voltage power supply. Figure 2 shows a diagram of one of them - UMZCH on the TRA301 chip. Here are some of its characteristics: output power at a supply voltage of 3,3 V, KHi = 0,5%, F = 1 kHz, RH = 8 Ohm - 250 mW; quiescent current - less than 1,5 mA; the width of the reproducible frequency band at maximum output power is 10 kHz. Monoamplifiers based on TRA311, TRA701, TRA711 microcircuits have similar parameters and switching circuits [2]. All microcircuits are equipped with protection against thermal and electrical overloads. A typical scheme for their inclusion with the necessary additional surface-mounted elements makes it possible to manufacture a new amplifier in the form of a miniature block. The old UMZCH is dismantled, leaving only the preamplification stage on the VT2 transistor, and the new one is assembled by surface (or any) mounting on a separate board according to the diagram in Fig. 2 from [2].
The board is mounted on brackets to the main board in the place where the previous UMZCH was dismantled. The input signal is supplied from the collector of the transistor VT2 (see Fig. 1), plus the power supply is from the battery, the capacitance of the capacitor C31 is increased to 220 microfarads. The integrated UMZCH does not require settings. It may only be necessary to adjust the pre-amplification stage on the transistor VT2 according to the collector voltage indicated in the table by selecting the resistor R8. Literature
Author: A.Pakhomov, Zernograd, Rostov region
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