Hi John, your description of the problem is too vague: after what time does it switch off, does it go completely dead or is the on/off LED rapidly flashing, etc.
Also, without the type number of the TV mentioned, there is not much I can do to try and help you.
There are lots of websites out there that have detailed repair advice for various Philips TVs. Just google the type number of your TV and the word "repair", and you'll find what you need. It worked for me.
Good luck.
SJ, February 2009
