Best not to attempt fixing your power supply by yourself unless you absolutely know what your are doing. Need to make sure caps are fully discharged and of course unit is unplugged before work. might be easier/quicker to replace the powersupply, especially if you have a spare ATX powersupply around the house. You can get ATX adapter.
http://amigakit.leamancomputing.com/catalog/product_info.php?products_id=998For the reason why the fuse blows, could be powersupply or what ever is connected to it. (motherboard, etc). Might be a good idea to check with the motherboard schematic and go through the affected area with a test meter to test for continuity.
http://www.amigawiki.de/doku.php?id=en:service:schematicsInspect for obvious shorts. i.e. - metal case, or rogue screw touching somewhere it shouldnt on motherboard etc..
