Using the 723 integrated circuit, this circuit was found in an article on power supplies in the August 1971 Radio Electronics magazine. The transistor may be the BD136 on a heatsink.
Figure bellow shows another circuit that allows adjustments in the duty cycle of the output...
This circuit is from an American publication of 1978. It can be assembled with modern transistors, since...
Found in a documentation for radio amateurs from the 90s, this circuit is suitable for RF...