The schematic and circuit description were provided by Ronald Rissel:
It has been interesting
analyzing these radios. You have to give the Japanese credit for
even making a radio that can drive a speaker with only 2 transistors!
Basically, the first transistor (Q1) performs double-duty. It first
acts as an RF amplifier, with some of the signal being fed back to the
antenna coil to provide some regeneration for better selectivity and sensitivity.
(This also results in a non-linear amplification of the signal which results
in noticeable distortion.) The RF (radio frequency) signal is then
rectified ("detected") by the diode, and then the resultant audio signal
is fed back to the base of the first transistor where it acts as the first
AF (audio frequency) amplifier. (This is called a "reflex" circuit.)
The audio signal is then fed through an interstage transformer to Q2 (the
second transistor). The transformer provides impedance matching.
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