This circuit identifies the polarity of DC signals and if the signal is AC. The circuit is from 1969 US EDN documentation. Operational may be equivalent.
This interesting adjustable power supply uses the transistors themselves as controllers and...
This alarm from an old 1978 publication uses PNP transistors and driver and output transformers that are...
This is a typical source of the time (1989) using the well-known regulator integrated circuit 723....