Is it struggling during this time to come on perhaps you can hear a clicking or sizzling? eventually firing up ok?
I have to tell you I have just finished doing one of these sets and we had to replace the mainboard although originally it looked like a powerboard issue
So its either a powerboard issue (could be bad electrolytic capacitors) or a mainboard problem, if you like you could check the powerboard by physicaly looking for these bad capacitors ,the powerboard will have the powercord going to it
The capacitors look like the 2nd picture perhaps with swollen heads or leaking
Let me know if you need more info
It may not have been apparent to you on the issue of the capacitors, if its the mainboard this will need to be swopped out had the same issue myself however its either powerboard or mainboard I cant be any specific than this for your trouble i'm afraid