Page 147 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

FIGURE 10-1 Block diagram of a one-transistor reflex superheterodyne

radio.
The radio station's RF signal is received by antenna coil Ll, which couples the RF
signal via a secondary winding of Ll to converter circuit Q1. The output of Ql feeds
an IF transformer and an audio transformer. The output of the first IF T2
transformer then is coupled with a second IF transformer T3, which provides

sufficient rejection of the oscillator's signal for envelope detection of the 455-kHz IF
signal. Detector D3 then provides an audio signal that is fed back to the converter
oscillator transistor's input for audio amplification. Audio transformer T5 is
connected to the output of the converter oscillator transistor Ql for extraction of
the ampl:ified audio signal. This amplified audio signal then has sufficient current to

drive a low-impedance earphone.
Parts List
• Cl: 100 IJF, 16 volts

• C2, C3, C4, C9: 0.01 I-IF
0
• cs: 820 pF, 1 /0 silver mica
• C6: 0.0039 IJF

• C7, C8: 1 IJF, 35 volts
• RI: 180

• R2: 390 k

• R3: 47 k

• R4: 20 k

• VR1: 50 k

• TI oscillator coil: 421Fll0
• T2, T3 IF transformer: 42IF103
• T4 audio-drive transformer: 10 k

primary, 10 k

secondary
• TS audio output transformer:: 1 k

primary, 8

secondary

• LI antenna coil: 600 IJH to 680 IJH primary, 10 turns for secondary winding