In use telephone indicator (CB8657E)

Found in a 90s Popular Electronics, this circuit keeps an LED on when a phone is in use. Can be used with extensions.

Relative impedance meter (CB7487E)
Found in a 1960 Sylvania manual, this circuit can be implemented with a modern equivalent...
Logarithmic Converter (CB19486E)
This configuration, already presented in other points of this section and website, is of a...
2 Meter Crystal Oscillator (CB4051E)
We found this precise crystal oscillator for the 144 MHz band, depending on the crystal used. The...