bibilit,

thanks! I finally managed to open it. It seems like the front frame may be a little damaged and the back is not fitting anymore in the correct way, as I noticed from the beginning that, when the Mac is closed, the bottom right corner of the machine is a little "open", in a non-symmetrical way if you look at the bottom left corner.

Anyway, since I managed to open it, I decided to put on a little experiment and try swapping the logic board with another Macintosh Plus ED (european version) that I know working for sure.

The results: the ED with the swapped logic board booted up just fine!

So I think we can definitely rule out the logic board, there must be something wrong with the analog board.

I tried to have a look at the top of the board, being careful not to touch the CRT to prevent accidental damage to the tube (and avoid getting a potential shock, though the computer has been unplugged for over 10 days now). From this quick look the capacitors seems to be still in nice shape and they don't seem bulged; there's something that resembles white glue around some of them, but it doesn't seem like any of them is leaking (though a better examination is necessary).

When I've got enough time I'll try to take the protective cover off the analog board and reflow the power connector that goes to the logic board and see if it gets better.

Thanks for the help guys!