w520 very dim screen

Hi guys I have a coworker with a w520 that has a very dim screen. It's not the screen because we have tried it on a w510 spare machine that no one uses here and the screen works on the w510 (are all the screen on the w510 and w520 interchangeable). Weird part is in the office we have maybe 10-15 w520 and last month someone had a very similar problem with a very dim screen and when hooked up to an external monitor via vga or displayport there was no display to the external monitor even when pressing fn+f7. That was under warranty and sent in, this one is out of warranty and I'd like to try and fix it. So I think we can narrow it down to either the lcd cable or the motherboard. Does anyone know what may cause this? I've heard it can be a motherboard fuse on the t40x series, http://support.lenovo.com/en_US/diagnose-and-fix/detail.page?DocID=HT031613

And where might that fuse be on the w520? Thanks

Thanks

 

Let's define "very dim" first:

a) Is the picture on the LCD visible to the naked eye without using the flashlight pointed at it?

b) If you remove the hard drive/SSD does the machine behave in the same (dim LCD) manner?