T420, no backlight and image on LCD, external works OK

[kitor]

I'm looking for T420 and found one in decent condition and equipment for value, but has a problem:
There's no backlight on internal LCD. I asked seller to point flashlight directly to lcd, he says that there's visible difference when laptop is powered on (;cd get's completely black, so it's getting power), but no actual display is visible (from my experience it may be also too weak flashlight ).
He also changed lcd ribbon (hd+, replaced with proper one) and that didn't help.
I read about blown LCD/backlight fuses in many Thinkpad models - can this be a reason of this problem? I found no information about fuses in T420.

[ajkula66]

I read about blown LCD/backlight fuses in many Thinkpad models - can this be a reason of this problem?

Yes. That would be the most likely culprit.

I found no information about fuses in T420.

I'm not quite certain what you mean by this statement, but if I were buying that machine I'd presume that *both* the planar and the LCD were bad, and offer something along the lines of $25...

[kitor]

I can't find any information about locations and purposes of fuses in T420. Price is higher, but still about a half of working unit with same components (ssd, ram, etc) in my country. Anyway - I checked the prices, and even in worst case scenario (need both board and lcd replacement) it's still worth buying...

[RealBlackStuff]

The fuses concerned will all be around the LCD-cable connector on the motherboard.
Haven't seen any schematics for this one yet..., only for T420s, which is slightly different.

[wileE]

T420 schematics say backlight is F7. It is a size 0402, 3A fast.

Same as in T410. Mainboard pics with fuse location here: http://thinkwiki.de/T410_Sicherungen
I can not say if the T420 fuses have the same location, but it is likely to be similar.

[richk]

Another possibility on that model is the Hall sensor (lid switch). If that is the case, you can confirm by disconnecting the LCD cable all the way to the right side. This will also disconnect the microphone, camera, etc.

[kitor]

Ok, topic is dead as I found even better deal (two t420 with missing lcd bezels, ram and hdd, with i5, 1600x900 lcds and good condition 6 cell batteries for less than $150 equiv. total). But it's still a great lesson - I would never thought about eg. Hall sensor.