Hi, I know this post is from a very long time ago but I have just fixed a Sony ps-lx310bt with no power and managed to find a fault that may be common to these players.
Upon opening the bottom of the player I started looking for voltage going through the main board, which wasn't present anywhere. After working around the board I traced a short circuit to a Zener Diode on the board labelled ZD001. After identifying the component and sourcing a replacement, resoldering the new diode on to the board fixed the power issue and the player booted back to life!
Hope this may help people in future in case this is a common fault.