This Japanese commercial radio with 5 transistors is from the 60s and is powered by 6 V. It is a conventional superheterodyne configuration with good performance.
This circuit for frequencies from 1Hz to 100 kHz was found in the June 972 Radio Electronics...
This circuit generates short pulses. The transistor can be a 2N2222 or BC548 and the frequency is given by...
We found this filter in a Popular Electronics from 1993. The operational amplifier can be the 741....