This circuit is from a 1967 documentation. It makes use of varicap and diode tunnel in addition to currently uncommon FET. It serves as a reference.
This circuit generates sinusoidal signals between 300 and 3000 Hz. We find a 303 Circuits from the 90's....
This amplifier for audio transmission lines was found in a 1961 American publication. The critical...
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...