Facing TV Problems? Ask a TV Technician.
Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, the description of "...shows on the screen for about 5-10 seconds and then the screen goes black..." would point to a likely auto-protect mode kicking in.
As a process of elimination, determine if the power supply is fully working as it would be the most likely suspect by checking the presence and consistency of the output voltages. The expected voltages at each connector pin are normally printed on the board itself such as (but not limited to):
Post back results / observations of the above once tested.
Additionally, determine and post back the version of the power supply board; something like a 56.04419.601.
A friendly reminder; voltage measurements by its very nature requires that the unit be plugged in, attempted to be turned ON and therefore LIVE! Please exercise due care and caution.
The backlight would be the least likely suspect; ...if it were faulty, then there should still be audio even if when the "...the screen goes black..."
Are you able to manually turn Off the TV when the "...the screen goes black..." ?
Sleep mode would be triggered by the absence of a usable video signal but not "...5-10 seconds..."; usually much longer.
Can you provide clear picture of the back of the TV showing the different boards? Of particular interest would the main board/shielding
A review please; what is the voltage at the BL_ON & PWR_ON pins during standby? and when attempting to turn on?