Page 148 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

• L2: 150 ~H

• DI, 02: lN914
• D3, D4, D5: 1N270 or lN34
• QI: 2N5089
• Vel: two-gang variable capacitor 140 pF and 60 pF

Figure 10-2 presents the schematic for the one-transistor radio. As shown in the
schematic, the RF signal from the tuned circuit consisting of VC1 RF and Ll is
connected to the base of Q1. The collector of Ql is connected to a secondary
winding of the oscillator coil Tl that feeds energy back to its primary winding to
provide a reliable oscillation signal from 1 MHz to 2 MHz at the emitter of Q1. At

the low side of the primary winding of T1, two diodes are wired back to back to
provide amplitude limiting of the oscillator signal because the operating collector
current of Ql is biased to a much higher than "normal" direct-current (DC) current.
With a local oscillator signal at the emitter of Ql and an RF signal at the base of
Q1, a multiplying effect of the two signals occurs to provide an IF signal at 455 kHz
at the collector of Q1. The first IF transformer T2 provides extraction of the IF
signa:1 but still contains some of the local oscillator's signal. For further attenuation

of the local oscillator signal while passing the IF signal, a series resonant circuit
formed by L2 and CS is connected to a second IF transformer T3 that passes
signals around 455 kHz and provides further attenuation of the local oscillator
signal, whose frequencies range from 1 MHz to 2 MHz. The signal from the IF
transformer T3 provides further attenuation of the local oscillator signal and is

connected to D3 for envelope detection of the 455-kHz amplitude-modulated (AM)
signal. The detected signal now is an audio signal that is connected to the primary
winding of audio transformer T4, whose secondary winding is connected in series
with the secondary winding of L1.