This is the power supply circuit for the amplifier in the previous circuit. The source is adjustable and uses power transistors of the time. The AC transistor must be mounted on a heat sink.
Found in a Hands On Electronics of the 80s, this circuit makes use of a gas sensor of the time....
This control for a heating element was found in an old documentation. The current in the element...
This variable gain circuit for analog video signals was found in a 90s Linear Technology...