T420s screen backlight problem

[SaskFellow]

So I took my perfectly working 1600x900 LG made TN stock screen out. Put in a B140HAN01.3 matte display... The indicator LEDs, webcam, thinklight, and screen off when closing the lid didn't work.

It all went together great, but there was one problem the backlight never worked. You could see an image on the display with a flashlight but it just wouldn't light up. I unplugged the cable that runs the indicator lights, as it wouldn't shut off the screen with the lid closed before. No change.

I swapped back the original screen, and it was still black.

I changed the power board, the indicator and thinklight LEDs started working but no backlight on either display.

So I tried to find out about the fuses, but there's nothing I can find about the T420s, I found out about the T420. I couldn't find my DMM, but the fuse F7 looks dark, assuming it's the same as the T420.

The seller will replace the screen, but unless I can get the original working or prove that the screen I got blew the fuse, I'd be gambling on sending it back and it testing fine, then I'm out quite a bit on shipping at the least.

I checked the backlight power consumption numbers...

LP140WD2-TLB1 - 4.2W(Max) -> Panel total: 4.81W(Typ) -> Signal: 420mA(Max)

B140HAN01.3 - 3.4W(Max) -> Panel total: 5.4W(Max) -> Signal: 394mA(Max)

So the upgraded panel shouldn't cause an excessive draw. Though I don't now how much power the converter board takes, changing from 2 channel LVDS to 2 lane eDP.

I don't want to assume though, so I'm hoping someone can help me, as this is my university machine and it's important I get it back up and going.

Edit...

After staring at the pictures I took... The fuses F3 and F7 are black, while the other fuses near by are beige. I found out the F7 should be a 0402 format 24V 3A fuse. I'm guessing if F3 is used for just the indicator lights(which work now but didn't) would use a lower amp fuse.

[SaskFellow]

Testing revealed the following:

Fuse F3 = Good

Fuse F7 = Bad

When I tried to bridge the fuse using tweezers I was greeted to sparks and pops. I tried with the meter I dug out after the light show set to measure amperage, and I was greeted to lesser sparks.

So I sent the machine off to the doctors. Hopefully he can find out what's wrong, but right now the preliminary diagnosis I that the backlight unit on the display has issues.

[SaskFellow]

Blew the fuse, got it repaired after finding a gentleman who offered to try and replace the fuse. It wasn't the prettiest job, but it works.

All is working now.

Many thanks to RMSMajetic for his helpful advice.