This rectangular signal generator was found in an Electronics World of 1966. The transistors are from that time and modern equivalents can be tested.
In the figure we have the way to implement a 5 to 45 V zener diode with 50 mA of maximum current...
We found this circuit in a Radio Shack documentation from 1977. The circuit serves for a link of...
This circuit was found in the 1977 National Semiconductor FET Databook. The FET can be the BF245...