This card-type radio is fashionable for the first time, and its biggest disadvantage is that the adjustable power is easy to wear, and there is obvious noise after less than a month and it is easy to run. The radio circuit part adopts a miniature radio integrated circuit 7642, similar to which there are several models such as 85414, IN414, MK484 and NT484. These types of integrated circuits can generally be used universally. The main parameters are shown in the attached table.
Due to the low operating voltage, high frequency bandwidth and high gain of this circuit, it was decided to transform it into a superheterodyne radio.
The modified circuit is shown in the figure. BG1 is a frequency conversion tube, which plays the role of local oscillation and frequency mixing. In order to work properly at low voltages, a bias circuit composed of R1 and D is used. Even if the voltage drops to 1V, it can still work normally. 7642 and peripheral components form a 465kHz intermediate amplifier and detection circuit, and its gain can be as high as 73dBo. In order to improve the selectivity of the whole machine, the common method of motorcycle is used, that is, two 3L465 are used in series. BG2 and BG3 form a simple low-amplifier circuit that can directly drive an 8Ω load.
The double variable capacitors and oscillating coils used in the circuit, and the intermediate frequency transformer are all supporting components removed from an old superheterodyne radio. The antenna part uses a 55mm medium wave flat magnetic holder, L1 uses φ0.07x3 to wrap around 110 turns, and L2 uses a towel 0.07 & TImes; 7 to wrap around 11 turns. But with different doubles, the number of turns is different.
The debugging of this machine is very simple. First adjust R2 to make the working current of BG1 0.5mA. Then turn Cl to find a radio signal, adjust the T2 core to make the loudest sound, then turn C1 to find the low-end radio signal, if you can't tune, you can turn the T1 core to make it appear, then turn C1 to find the high-end radio, fine-tune C3, make the sound clearest. Move L1 and L2 to make the low-end radio the loudest, and then adjust C2 to make the high-end radio the loudest. After repeated several times, it can be put into use.
Because the gain of 7642 is high, the sensitivity of this machine is very high.
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