For $29/mo, we provide access to advanced level technicians who will answer your questions on any Macbook board related matter to the best of their knowledge promptly & walk you through how to solve your problem so you can deliver a working board to your customer.
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...
boards.rossmanngroup.com
As fuse was burnt, check for short to ground on backlight circuitry.
Also check XW7720 integrity.