Thinkpads Forum

Hello,
a few days ago I accidentally fried the backlight circuit on my W540 by not noticing the battery was still in while un-plugging the LCD panel connector directly at the panel. At first I thought the whole panel was dead, but then noticed that only the backlight did not work. So I disassembled the W540 to a point where I could do some measurements on the backlight circuit near the 30-pin eDP connector. It turned out that the backlight fuse F11 (0603 3A 32V) had blown leaving the eDP connector without backlight power. I measured the connector side of the fuse for shorts to ground but did not find anything, which hopefully means that just the panel connector shorted upon un-plugging which blew the fuse and everything else is OK. The other side of the fuse, connected to an HP8K22 power mosfet on the other side of the board, also shows no short to ground. When powering the W540 up, I get a voltage reading there of 19V to 20V, which seems to be the supply voltage of the power supply. However, at this point I kind of expected 12 volts for powering the backlight. Also I get the 20 volts as soon as the power supply is plugged in and not only after if I push the power button. The panel specifications at panellook state a default voltage of 12V and a maximum of 21V for the panel. Does anyone know if the 20V reading at this point is expected and the panel is powered on only via a signal on the eDP enable pin? At the moment, I am kind of hesitant just replacing the defective fuse (most likely a Panasonic ERBSE3R00U) with a ERBRE3R00V. I don't want to damage the panel if it is still OK. Does anyone know if the schematics of the W540 are available somewhere? Searched the net but could not find anything.

Kind regards

In those days they more likely used a 0402 ERBRD3R00X (3A/32V/Fast).
I've come across quite a few mobos with blown backlight fuse.
Never measured the voltage on them, just replaced or piggyback-soldered another fuse on top of the old one.
This picture of a T440p shows the above 0402 backlight fuse F3: https://i.imgur.com/9F6hJbw.jpg
It has enough room to replace with a larger 0603 or even 1206 size fuse (or piggyback-solder one).
W540 dates from the same period (and possibly manufacturer) so it might look very similar.

Thank you for taking the time to read and reply. I found clues that the fuse might be an 0402 on the web first too, but on closer inspection I had strong doubts. So I tried to measure the fuse with the help of a caliper. Hard to get in but you can simply cut a small strip of paper, match the size with that of the SMD part and then measure the width of the paper strip. The results matched perfectly with 0603. Here is an image of the fuses near the eDP connector. F11 is on the leftmost side and F4 on the bottom measured OK. I think, the two capacitors above F11 might be 0402. In your image, the size of the fuse seems to be smaller compared to mine when using the eDP pins as a reference.

https://www.defi1.de/files/F11.jpg

Just replaced the F11 fuse with a 0603 Panasonic ERBRE3R00V quick blow 3A 32VDC. Works like a charm. However, the new fuse is black instead of white.

As long as the fuse has the same specifics, you're grand.