This simple audio and RF signal tracker was found in a Japanese publication in 1967. The transistors are made of germanium and the headphone is made of crystal.
This oscillator from a 1970 documentation uses an operational amplifier that is no longer common on...
The integrated circuit used in this 90's circuit is not common today, but the configuration can be...
Found in a 70s RCA manual, this circuit uses a transistor of the time for an UHF L band...