The back light turns on for about 1min and then turns off i allready replaced the dm and power board what could it be

Hi there,
Just wondering if they were new or second hand boards you used. I would be inspecting the inverter circuit for the back-light is causing the problem for you. Short of replacing that also,I would be looking at repairing the area itself. If you have some electronics experience and the tools required...common problems you may find are

• dry solder joints about inductors, power transistors and components that run hot. The heating and cooling associated with running then turning off produces dry solder joints over time.
• replace electrolytic capacitors in the inverter stage. They can also over time dry up and their effectiveness be reduced.

The fact that it operates for 1 min makes sound like a heat related problem. There are methods of determining problem areas using freeze spray to cool individual components to determine if they are the offending part. Be careful to avoid moisture build up condensed from the air when using this method. Gentle prodding of the individual parts and spots on the PCB with a plastic tool can also help to locate any dry solder joints. Hard to tell you exactly where to look, but with perseverance, you may get to the bottom of it.
regards robotek