A circuit for controlling the temperature of a heating element was found in a Mullard transistor manual from 1960. Transistors are types from the time.
This crystal oscillator generates a 20 MHz signal. Equivalent transistors such as BF494 and 2N2222...
The interesting circuit shown in the figure, can have its gain digitally controlled and present two...
We found this circuit in a 1970’s Philips documentation. The circuit uses components from that time....