Years ago I had a Canon 300D which died, suddenly and completely after plugging in the CF memory card. The same card then went on to kill another camera before being binned. So the first message is: don’t give the card a second chance, they’re much cheaper than the camera.
The problem turned out to be a blown fuse on the power board immediately underneath the power/mode switch. I replaced it successfully and the camera went on for ages…for longer than the web page actually, which I subsequently lost in a server crash. I was going to rewrite it, but then I found this excellent guide. There’s a spare fuse pad next to the one which blows. He just bridged it which of course leaves the board unprotected: I soldered on a couple of wires and lead them out forward to where there there was some space in the casing with a 250mA in line fuse on the end (available from RS).
One more thing: the large (black) capacitor on the left hand side stores charge for the flash and has a high voltage on it, which is still there when the battery is removed from the camera. Quite rightly, he warns you not to touch the pins. I discharged it, as soon as I could reach the pins by attaching insulated leads to a 1kR resistor and then holding the ends across the capacitor terminals for about a minute.
I’ve never done the shutter pin on a 300D but I’ve used 5Ds a lot for weddings (so I know their little foibles) and I’ve rebuilt old Hasselblads, Olympus and Nikon 35mm cameras. If you’ve got a camera problem, feel free to email me (I hate to see them broken).