Re: No power
First thing to look at when the main fuse blows instantly (ok second after disconnecting the outputs, but you've already done that!), is the crowbar device across the 24VAC input.
This is a small device that usually looks similar to a diode and will be located near the fuseholder. It's purpose is to go short circuit if the machine is plugged directly into a 100V outlet and blow the fuse instantly, hopefully protecting the rest of the electronics from any overvoltage damage. It's not resettable and if it has tripped will need to be removed or replaced.
Next check is the bridge rectifier diodes, if any of them go short circuit, the fuse will blow instantly on power up, and they can easily be checked using the diode test on a multimeter.
If these checks fail, then usually the best bet is to shotgun all the semiconductors on the board.
