First of all, a word of warning. As you probably already know, opening and working inside a compact Macintosh computer can be very dangerous because of the high voltages involved. Charges can remain in various sections and components (CRT and capacitors) even after the computer has been switched off and disconnected from the mains. All work must be carried out by someone with the necessary expertise.
The broken fuse indicates a short-circuit condition. There could be a number of reasons for this. An inspection might reveal a discoloured or cracked diode, SCR, transistor or electrolytic capacitor. Damaged circuit board traces could possibly be found in the problematic area.