On an SPF the main fuse is in the drop down electical tray and may look like an old wire, its 80 amps if I remember correctly. Depending on your serial number the main fuse is just 2 studs glued to the electrical tray with a metal stip between them which is the fuse. The glue on these can come lose and the stud can contact frame grounded parts ;-(
more info:
Main Fuse in Early Mark III
Good luck!