Philips Magnavox 37" LCD Flat Screen 37MF331D Have replaced power board,main board,hit power a faint light on whole screen for about 2 seconds the nothing.

Fault with the main power supply regulator section. Either it will be on a separate board, or on the part of its main board. Check and replace damaged component/s at its main power regulator [SMPS] section circuit. Contact any service technician. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electronicshelponline.blogspot.com/

check the power supply board for swollen filter capacitors

For magnavox/Philips model #37mf331d/37 part# is 313815864961.

You have a bad Capacitor or two!

There are two places to look for the problems, the backlight inverter board and the power supply. Look at the power supply first since it is easier to get to, look for burnt parts, bad capacitors, etc.
http://en.wikipedia.org/wiki/Capacitor_plague
Please post back what you find.

Replace cap location 2074 which is 25 volt 2200Mf and cap location 2081 which is 35 volt 1200Mf. You can visually see these when you remove the back...Good Luck! PS...be sure they are rated at 105 degrees!!!!

The problem can be in the backlight inverter board (hard to get to), or the main power supply module may not be working. Look at the power supply first since it is easy to get to, look for burnt parts, bad capacitors.
http://en.wikipedia.org/wiki/Capacitor_plague
Please

I had the same problem with a Magnavox LCD TV. It would not turn on and the light on the external PSU flashed. I found a replacement 12V 5A power supply on EBAY. It brought the set back to life.

Your set either has a bad SSB (small signal board) or needs a software upgrade. Try the upgrade first by going to http://www.m4c.magnavox.com/cgi-bin/dcbint/cpindex.pl?scy=US&slg=AEN&cat=FLAT_TV_CA&sct=LCD_LARGE_30_40_SU&grp=TV_GR&session=20090830171041_4.131.87.247&ctn=37MF331D/37&mid=Link_Software&hlt=Link_Software
Download the latest software release for your set then follow install instructions that are contained in the user manual for your set.

There are two capacitors in the internal power supply regulator board that have been the main culprit of this particular issue. You can identify the bads ones by the slight buldge at the top of the capacitors.