canon old supertelephotos (1 Viewer)

Question to those who have experienced with the first non is versions of Canon 300/2.8 or 500/4.5. Can a Canon EF 300mm 2.8 L USM/Canon EF 500mm 4.5L USM be used in manual focus if the AF is broken? Thanks!

I haven't owned those lenses personally but I used to own the, similar, vintage EF 400 F2.8. They have a sort of "Fly by Wire" (for want of a better description) focus system which I thinks FAR better than the modern lenses if you need to focus manually.
Unfortunately, as I understand it, if AF dies then MF does as well also parts are few and far between. I wouldn't necessarily let this put you off these excellent lenses - just bear in mind that theyprobably will be unrepairable (though they may work for many years) when you consider what one would be worth to you.

Thank you very much! How is the manual focus with fly by wire system and is this electronical manual focus?

The "OLD" manual focus is electrical and does not function without a camera acting as the power supply. In use it is simply superb and far in advance of anything we have today. The action is extremely smooth with no change in friction as you rotate the focus ring and also offers 3 different ratios for quick, medium or very fine focusing.
I don't know why Canon no longer use this system - perhaps there are technical reacons?