The main performance of HX 108-2 seven-transistor radio is the frequency range: 525 ~1605 kHz; Output power: 100mW (maximum); Speaker: φ57mm, 8 Ω; Power supply: 3V (No.2 battery of No.5 battery); Volume: 122×66×26. The electrical schematic diagram is shown in Figure 2. 1. As can be seen from the figure, the whole machine contains seven triodes, so it is called a 7-tube radio. The triode V 1 is a frequency converter, V2 and V3 are intermediate amplifier tubes, V4 is a detector tube, V5 is a low-frequency preamplifier tube, and V6 and V7 are low-frequency power amplifier tubes. Select the required radio signal from the antenna loop and couple it to the base of the inverter tube V 1 through the transformer Tr 1 (or B 1). At the same time, the feedback local oscillator of common-base-emitter transformer, which consists of frequency converter V 1, oscillating coil Tr2, double coaxial variable capacitor C 1B and other components, has its local oscillator signal injected into the emitter of frequency converter V 1 through capacitor C3. The radio signal and the local oscillator signal are mixed in the frequency converter tube V 1. After mixing, the collector current of V 1 transistor will contain a series of combined frequency components, including the difference frequency (465KHZ) component between the local oscillator signal and the radio signal. The required intermediate frequency (465KHZ) component will be selected by the medium period Tr3 (including the resonant capacitor) and coupled to the base of the intermediate amplifier tube V2. In the figure, the resistor R3 is used to further improve the anti-interference performance, and the diode VD3 is used to limit the amplitude of the intermediate frequency signal after mixing (that is, the secondary AGC). The amplifier is a two-stage small signal resonant amplifier composed of V2, V3 and other components. The intermediate frequency signal obtained after mixing is amplified by two intermediate amplifiers and sent to the next detector. The detector is a large signal envelope detector composed of triode V4 (equivalent to diode) and other components. The detector restores the amplified intermediate frequency AM signal to the required audio signal, and sends it to the subsequent low frequency amplifier for amplification through the coupling capacitor C 10. In the detection process, not only the required audio signal is generated, but also the DC component reflecting the intensity of the input signal is generated. This component is taken out from both ends of C7, one of the detection capacitors, and filtered by a low-pass filter composed of R8 and C4.

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As AGC voltage (-UAGC), it is applied to the base of intermediate amplifier V2 to realize reverse AGC. That is, when the input signal is enhanced, the AGC voltage decreases, the base bias voltage of the intermediate amplifier tube V2 decreases, the working current IE decreases, and the gain of the intermediate amplifier decreases accordingly, so that the output level of the detector can be maintained in a certain range. The low amplifier consists of a preamplifier and a low frequency power amplifier. The transformer coupled preamplifier composed of V5 amplifies the audio signal output by the detector and sends it to the power amplifier for power amplification through the input transformer Tr6. The power amplifier consists of V6, V7 and other components. They form a transformer coupled analog-to-digital power amplifier. After the power of the audio signal is fully amplified, it is coupled by the output transformer Tr7 to drive the speaker to sound. Among them, R 1 1 and VD4 are used to provide appropriate bias voltages for power amplifier tubes V6 and V7 to eliminate cross distortion. This machine is powered by 3V DC. In order to improve the output power of the power amplifier, the 3V DC voltage is decoupled and filtered by the filter capacitor C 15, and then directly supplied to the low-frequency power amplifier. The previous circuits are all powered by simple voltage-stabilizing circuits composed of R 12, VD 1, VD2 (the stable voltage is about 1.4V). The purpose is to improve the stability of static working points of circuits at all levels.