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When troubleshooting backlight circuit, the output voltage is important. Different voltages tell you different things. On an air, 39-49v means that the backlight boost circuit is working, but there is no load - aka, it doesn't "See" the light of the screen. Once there is a load on the circuit...
As fuse was burnt, check for short to ground on backlight circuitry.
Also check XW7720 integrity.