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.
The transistor can be the BD135 and this circuit, found in a 1978 publication, can be powered by...
This interesting circuit measures the distortion of the sinusoidal voltage of the power grid. We...
This sophisticated circuit is from a 1975 documentation. The components are from the time and the...