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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING TECSUN receiver upgrade. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Chinese-made miniature radios are popular because of their low cost, pleasant appearance and a large number of operating ranges. The author of the published article managed to improve the quality of their work and change the operating frequency ranges to those that are more consistent with Russian conditions. Most receivers made in China are assembled on common Korean and Japanese-made microcircuits, which allow you to get good parameters. I would like to highlight the receivers with the trade name "Tecsun" and, in particular, the model "R1012". His radio frequency channel. made on the SHA1191M chip from SONY, allows you to receive radio stations in 12 bands: three VHF - TV1, FM and TV2 (hereinafter, their name is given as indicated on the scale and in the accompanying documentation - ed.), respectively, with frequencies of 48. ..84; 84...108 and 170...230 MHz. one medium wave and eight short wave. Switching coils of oscillatory circuits of the ranges is electronic, using diodes. During the operation of the receiver, it was noticed that in the FM band (corresponds to the domestic VHF-2 - ed.), the reception frequency changes spontaneously, and it is quite difficult to tune in to the stations precisely - the setting is very sharp and the slightest rotation of the control knob leads to loss of the radio station signal. When analyzing this defect, it turned out that the oscillatory circuits of the FM range are affected by the capacitance of the switching diodes of the TV2 range. Given that the operation of the receiver in the TV2 range was practically not used, it was decided to abandon it. The following radio elements were removed from the main board of the receiver (the designations are given on the printed circuit board of the receiver itself, since there is no electrical circuit diagram): diodes D3, D5, D6. capacitors C8. C11, C12. Coils L2 and L5 are replaced by jumpers. After that, the stability of the FM band tuning became much better. Tuning stability was further improved by introducing an extended AFC capture bandwidth. The fact is that the current AFC system is ineffective, especially when working in the FM band, where the tuning on the station, as already noted, is very sharp. It was possible to achieve an expansion of the AFC capture bandwidth in a simple way. On the printed circuit board of the receiver, from the side of the conductors, an AFC circuit was assembled. shown in fig. 1.
The upper output of the capacitor C1 according to the diagram is connected to the section of the block of capacitors of variable capacitance (KPI) of the receiver, operating in the local oscillator circuit of the VHF bands. Of the four KPI conclusions, this is the lower right, when viewed from the side of the printed conductors. The free terminal of the resistor R1 should be connected to terminal 21 of the CXA1191M chip (it is also located on the side of the printed conductors). A changing constant voltage is formed at this pin of the microcircuit - with an untuned receiver, its value is about 0.9 V, with a slight detuning from the carrier frequency of the radio station upwards, it drops to 0.4 V, and with a detuning towards decreasing frequency, it increases to 1,8 V. Change The voltage at this pin, together with the varicap VD1 and the capacitor C1, allows, within certain limits, to maintain the tuning frequency of the local oscillator circuit unchanged. All these measures ensured reliable and stable reception of radio stations in the VHF bands, tuning turned out to be much easier than before the rework. It is desirable to use miniature parts for all parts, VD1 varicap - any of the KB 109 group, although you can try to use other types. None of the Tecsun series receivers we sell, including the Tecsun R1012, has a long-wave range, since it is practically not used in Asian countries. Therefore, instead of the excluded TV2 range, it is advisable to introduce a long-wave band, which is still used in some regions of Russia for broadcasting. Moreover, the length of the ferrite antenna (about 100 mm) allows you to do this. With such a refinement, no additional switches will have to be installed - the existing ones will come in handy. The scheme of changes and additions in the "R1012" model is shown in fig. 2. On it, a dash-dotted line separates the radio elements located in the radio receiver itself. It was decided to abandon the coupling coil on the magnetic antenna, so the L1 coil consists of the WA1 magnetic antenna on the ferrite rod. series-connected loop coil and coupling coil. An additional coil L2 of the DV range is wound on the free end of a ferrite rod and consists of 350 turns of PEV wire with a diameter of 0.1 ... 0.2 mm. wound in seven sections of 50 turns each. Pre-ferrite antenna must be wrapped with a layer of paper.
The connection of the loop coils L1 (SV) and L2 (DV) to the input of the receiver is direct, through a source follower on a field-effect transistor VT1. Moreover, in the CB range, both coils are connected in parallel, and in the DV range, only L2 works. The connection of the L1 loop coil to the power plus is made by the SA1 VHF band switch (conditional designation) installed in the receiver, in the AM station reception mode. The heterodyne circuit on the main board is used on both bands. Later, simultaneously with the connection of the L1 coil, additional capacitors C3 and C4 are electronically connected or disconnected to the input and heterodyne circuits, respectively (KPE sections C5.1 and C5.2). The connection is made using diodes VD2 - VD5 and switch SA1. A few words about the points of connection of additional radio elements in the receiver, since the reference designations of the KPI and the SA1 switch on the receiver boards were not found. The VHF band switch is installed on the control and indication board (it is mounted separately under the front panel of the receiver), the pin numbering is counted from the control buttons located nearby. Conditional point 1 is pin 3 (middle) of switch SA1; point 2 - output 4 of the switch (it was not used in the original version, therefore it was shortened): point 3 - output 1 of the switch (it was used before the alteration to turn on the TV2 range). To determine where the KPI is located (symbol C5) and where its conclusions are located, in my opinion, will not be difficult for radio amateurs. Point 4 is one of the KPI pins (upper right, when viewed from the side of the printed conductors); to this point, before the alteration, one of the terminals of the coil of the input circuit of the CB range was connected. Point 5 - KPI conclusion, located to the left of the above. Conditional point 6 was also used in the original version, one of the outputs of the AM range switch was connected to it (when viewed from the printed conductors) - to output 10. Counting from the left. Parts used for recommended modifications, it is desirable to use miniature: resistors - 0,125 W or less; diodes VD2-VD5 - any of the groups KD521, KD522; transistor VT1 - any of the KP303 group. The installation of additionally used elements is mounted on the side of the printed conductors, it is better to install the VT1 transistor from the end of the WA1 magnetic antenna on the left, near the terminal for connecting the telescopic antenna. After the rework, the functions of the VHF band switch have changed. In the mode of receiving ultra-shortwave radio stations in the right position according to the scheme, the receiver operates in the FM band, in the middle and left - work in TV1. In AM mode, with the AM band switch in the MW position, the right position is not used, in the middle - work in the MW band, in the left - in the LW band. When working in any of the eight KB ranges, the SA1 switch must be in the middle position. Establishing the receiver is reduced to the selection of the capacitance of the capacitor C4 to set the limits of reception in the LW range. Given that the newly introduced range is slightly narrower than the standard one - 160 ... 250 kHz, by selecting the capacitance of the capacitor C3, maximum sensitivity is achieved over the entire LW range. Instead, it is convenient to temporarily turn on a tuning capacitor with a capacity of 5 ... 25 pF. The conjugation of the input and heterodyne circuits turned out to be acceptable over the entire range of DV. In the case of self-excitation of the receiver at the low-frequency edge of the range, it is necessary to select a resistor R2 with a lower resistance or replace R3 with a tuning one and reduce the transfer coefficient of the source follower. In the MW range, the sensitivity increased slightly due to the parallel connection of coils L1 and 12. In this mode, you may have to slightly adjust the loop coil of the local oscillator (it is marked "AM" on the board and has a red trimmer). These alterations did not affect the operation of the remaining ranges. It seems that in receivers of other models it is also possible to enter the LW range. using existing ones or by installing an additional switch. Author: I.Potachin, Fokino, Bryansk region
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Comments on the article: Andrei I read the commentary and remembered how we at the Litiy plant (Bryansk) improved the bands and rewound the coils so that the VEF receivers caught the 19, 16 and 13 m band, but that was a long time ago, and the desire to have a modern multi-wave receiver remained. I do not know what year the note is, but it would be interesting to chat with the "neighbor". if he is interested in receivers. I'm looking at Tecsun 660 and 3000. If possible somehow contact here or ace 326775724.
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