Lose back light after 3 secs everytime

First it's basically the same tv but not, it a e420i-boamd I have replaced both boards “io and power supply" hopping for a quick fix. Then I went on to testing led voltage and all 6 hit 70 ish vdc for a few seconds then cut back to 40 ish vdc. Still have picture and sound on all boards and in different configuration. Basically do I have a cathode problem so it can't see voltage and cuts back to standby or do I need more from what ever tells the back lights to stay up and going. Trying to do research on the dim voltage coming from the io board but not having much luck. Any thing that would help would be great. Thanks guys