In sci.electronics.repair Arfa Daily wrote:
Has anyone seen any good write-ups on, or figured out for themselves, just
how the self-oscillating dirty - i.e. smoothing cap-less - switchers that
you find in use as lamp ballasts (or 'electronic transformers' that they
seem to be sometimes known as) work ?

I recently acquired a fairly sophisticated disco lighting fixture that has
such a supply to run the 24v 150 watt lamp. It seems to have two main
switching devices, which I suspect are FETs or IGBTs, but can't tell for
sure so far, as the numbers have been ground off them. On the mains side,

That's pretty high tech for a product from the 1970s to use a switching
power supply.