As far as I'm aware the PS120M has the same/similar circuitry as the PS120Mii version, details of which can be found here :-
(Helpfully there's a schematic too.)
I'm not sure about the 2N3055 output transistors but these may have been used in this earlier model.
(Unless retro-fitted by a previous owner?)
I'm not keen on these type of linear power supplies as a quick look at the circuitry shows that if the output transistors blow short circuit then the whole unregulated voltage appears at the outputs!!!
I think that's pretty much what happened from your description.
For me I'm a fan of switch mode power supplies, (owning a Sharman SM-30), as the design of such items means that if anything goes wrong you usually get no voltage at the outputs at all.
Earlier linear power supplies sometimes included clever circuitry to prevent such things like over-voltage 'crowbar' protection but often this would not be fast enough to protect a radio or whatever connected to it before the fuse blew. None too good.
It might be worth an attempt at a repair or modification Paul but nowadays switch mode power supplies are so cheap and plentiful that it might cost you more cash in the long run.
Up to you mate.
All the best,