Soldered in an expansion pak only to have an N64 that normally powered on fail. Spent an hour yesterday desoldering, cleaning it all up, and tracing the grounds so i'd have a better idea of where the fault could be. re-soldered last night, failed to boot again. Followed my guide to figure out which should be grounded and which weren't, found the fault (an extremely thin bit of solder shorted with a nearby contact, nearly invisible to the naked eye), corrected it, and BOOM! - Works! so, to everyone else out there having problems along these lines, here's to you.