Quoted from frunch:

If I'm not mistaken, the small 100uf/25v (on same circuit as fuse F1) electrolytic capacitor sitting near the big heat sink is often found to be leaking. I believe it's capacitor C2 on the schematic. If it's original, I'd replace it regardless of whether it fixes the problem. Good luck!
(Edit: The other electrolytic caps may also be due for replacement... At least those on the 12v and 5v supplies)