Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, bear with us; re-confirm and post back with the complete model number of the Sony.
The described symptom and displayed message almost always point to an internal hardware issue particularly if "...disconnected speakers , disconnecting from mains..." have been tried.
Are you then considering a DIY (do-it-yourself) repair approach? If yes, do you have access to and familiar with a multi-meter and even a soldering iron?
Still, I did a preliminary review of the repair/Service Manual and it confirms the likelihood of an internal hardware issue.
Options then are:
Please post back how you would want to approach this and/or should additional information be needed.
Yes to my understanding (not unless you specifically selected to subscribe).
Good luck and thank you too for the opportunity to be able to share the information.