Sony Xperia LT30at TL No Power. Here's a fix for the no power no charge Xperia won't turn on issue guys. This won't solve the triangle firmware error (you need to have a firmware restore performed by Sony or read forums and download PC Companion to repair it).
Hopefully this helps anyone that needs it. I operate a cell phone and computer repair shop, so I have a few years ripping apart phones. This is no easy feat, so put on your Superman cape before attempting this repair. First off, my Sony Xperia TL LT30at died one night. It would not charge, blinked a few times every time I pressed the power button, and had a variety of responses every time I tried hard resetting with the up volume key and power button. I was frustrated, and about ready to throw my new brick of a phone through my window into the neighbor's pool.
You have to dismantle the Xperia. Using a safe pry tool (plastic prying tools so you don't destroy the phone) Remove the top and bottom housing pieces. They are a pain to pop off, pry from the edges and slowly lift and slide out away from the phone. I thought the top one was going to break for sure but then it released. Once those are off remove the series of screws at top and bottom. Then remove the main housing assembly. The battery is your target. You will see a flex cable connected to the main logic board. Remove it. Then plug in your charger. If you have a phone that didn't go swimming already, you should see the screen display "SONY," then blink off. Then back with a "SONY." Hold down the power button push pad (taking care not to damage the flex cable you are pressing on) plug the battery back in. If you have crossed your toes and dotted your eyes correctly your Xperia should lose the "SONY" logo and turn black, but the red charging light will start back up at the top of the screen and the battery will charge again.
Ten minutes later press the power button and voila, you should feel the vibration and things are back up and running.
Hope it helps!