I know this is old, and I'm not responding to the direct question, but I just got the tx 390 and had the same issue as Mike did. There's a kind of gravity switch that's a safety switch to keep the unit off when it's folded up, if that switch doesn't turn all the way it won't turn on. IDK if the previous owner had a problem with it and replaced the screw, but the one it pivots on is a tapered black wood/plastic screw like the others that hold the plastic panels in place, not a bolt or something else more logical o.0...
Brian
July 2011