Found this page on my dead PSU https://pa0fri.home.xs4all.nl/Diversen/EP925/ep925eng.htm which makes it sound like one of these beefy 2N3055 transistors has probably died.
No sign of scorch marks etc though! 
I still think dumping the full 24 volts across the terminals is a rubbish failure mode.
pa0fri.home.xs4all.nlEP-925 modifications
Dead transistor now identified!
Attached is how my cheapo component tester said it behaves now. Basically a short from collector to emitter, and that’s how I found it (meter on continuity mode, testing across disconnected transistors).
Tempted to replace all of them as they’re not too expensive.
