I am trying to understand this power supply from an old piece of electronics, specifically the step-up based around the transformer. It's a derivation of a flyback converter but I'm having trouble understanding how it works, how it starts up, how the switching is regulated, feedback, etc..

 

It's a non-isolated boost, with a feedback winding. Q43 would turn on due to output voltage reaching limit - otherwise W44 is allowed to turn on, but feedback winding would turn it off (v=L di/dt) as inductor saturation of main winding occurs (I reaches limit).

Ciao, Tim

 

very nice, thank you very much; was always looking at it as an isolated boost so would get confused with what's going on with the second winding.