The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
This circuit with 5 times gain was found in European documentation from the 1980s. The input and...
We find this simple tone modulated signal generator in a 1987 publication. It uses common...
This circuit is from an ancient American documentation. The coil is frequency dependent and the...