We found the circuit of this Japanese transistorized commercial receiver in a documentation from the 1960s. It is a radio for the medium wave range with 6 transistors and 9 V power.
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....