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.
July 2017 - Mouser Electronics announced the availability of Littelfuse's GEN2 series Schottky...
This configuration is suggested by Mullard (Philips) in an old manual of 555 integrated circuits....
We found this circuit in a Popular Electronics of the 90s. Its purpose is to detect when it locks...