I'm posting at the same time as PB, with a similar suggestion:
Measure the DC resistance between the two non-ground prongs of the power cord. It should be infinite with the power switch off, and about 13 ohms with the switch on. If you get less than that, then there's a short in the power cord or IEC wiring. You can pull the cord from the IEC connector and repeat the test to check the cord itself. What's left is very simple, the IEC socket, power switch, and the power transformer input terminals 1 and 2 - knowing where to look is more than half the battle!
Check the wiring of transformer terminals 6-7-9-10 as shown on page 26. If one of the pairs is reversed, the fuse will still blow immediately even after you fix the "power switch has no effect" problem.
