Six-tube superheterodyne transistor radio principle circuit household appliance circuit

Shown is the principle circuit of a six-tube superheterodyne transistor radio . The circuit principle is analyzed as follows.
    (1) The high-frequency AM wave signal emitted by the broadcast station of the antenna tuning input loop is selected by the magnetic antenna L1, c], and C tuning loop, and then
sent to the base of the frequency conversion stage VT1 through S and K coupling.
    (Z) Mixing and oscillation circuit of this stage The circuit composed of an oscillation loop and a mixing stage is also called a frequency conversion stage. The transistor VT1 serves as both an oscillation loop and
a mixer. The oscillation loop consists of VT1, L3, G, and C2. , Chu, L4 composed. It can generate a constant amplitude oscillation signal 465kHz higher than the input signal frequency
. C2 is the base bypass capacitor. G is the coupling capacitor. The local oscillation signal is coupled to the emitter of VT1 through the middle tap and G, forming
a positive feedback. After being amplified by VT1, it is output from the collector micro to the oscillation resonant circuit, thereby generating a local oscillation signal. The radio signal and the local oscillator
signal are mixed in VT1. Due to the nonlinear effect of the transistor, signals of multiple frequencies will be generated, one of which is
an intermediate frequency signal with a difference of 465kHz between the local oscillation frequency and the radio frequency. Since the resonant frequency of the intermediate frequency transformer T3 is 465kHz, only the 465kHz intermediate frequency signal
can produce a voltage drop in this parallel resonant circuit, while other frequency signals are almost short-circuited. During tuning (tuning), Ch and Cb use
coaxial double-connected variable capacitors so that the local oscillation frequency and the resonant frequency of the input loop change at the same time, and are always
465kHz higher than the resonant frequency of the input loop. This requires careful coordination (or tracking).
    (3) Intermediate frequency amplification stage The intermediate frequency amplification stage generally consists of two elements. VT2. VT3 is used for intermediate frequency amplification, and the intermediate frequency transformers T4 and T5 of the intermediate loop circuit
resonate at 465kHz. Since there are two stages of mid-amplification, it has better sensitivity and selectivity. In the figure, G and CL{ are neutralizing capacitors. Use it to
eliminate the parasitic oscillation of the intermediate frequency amplifier circuit. Some radio circuits do not include a neutralizing capacitor. When parasitic oscillation occurs, a 1-3pF capacitor can be
connected to the base of the intermediate amplifier tube and the lower end of the intermediate primary winding to eliminate the parasitic oscillation. If you do not have a 1 to 3pF capacitor on hand, you can use two sections of thin insulated wires to
replace the capacitor.
    (4) The detection stage is composed of diodes VD, Cl6 and potentiometer RP. Its function is to detect audio signals from AM waves.
    (5) The low-frequency preamplifier stage is composed of VT4, which functions as voltage amplification and provides an
audio In order to obtain a larger power gain, its output adopts transformer coupling. At the same time, in order to adapt to the push To meet the needs of the power stage,
the secondary side of transformer T6 has a center tap. The output signal of this stage is divided by the center tap into two signals of equal size and opposite phase, which drive the push-pull
tubes VT5 and VT6 to work respectively.
    (6) The low-frequency power amplifier stages T6, T7, VT5, and VT6 form a transformer-coupled push-pull power amplifier circuit. VT5 and VT6 respectively
amplify half the cycle of the audio signal, that is, one tube is on and the other tube is off and works alternately. The output transformer T7 is coupled to the speaker through the primary and secondary sides
to obtain a complete audio signal.
    In order to avoid differences in receiving strong and weak signals, as shown in Figure 1-1z. The green harvester circuit shown uses an automatic gain control circuit (AGC circuit
). It is a DC negative feedback circuit composed of G and R9. A part of the audio signal after detection is sent back to the base of VT2 through the wind. Since G
is equivalent to a short circuit to the AC signal (audio signal), its DC component is sent to the base of VT2. When a strong station is received, the audio signal output by the detector increases, causing the base potential of V rz to increase, the collector voltage to decrease, and the gain of VT2 to decrease (the polarity of this control voltage is opposite to the original bias voltage
of the VT2 substrate)
), thereby keeping the detection output signal size basically unchanged, thus achieving the purpose of automatic gain control.