T420 backlight

[csakovacs]

Hello,

If I use my t420 with AC power, the background lighting does not work.
However, if I don't use the ac power supply and it only runs from the battery, it works fine (there is a backlight).

I don't think there could be any problem with the F7 fuse! Where should I look for the error, or has anyone encountered such an error phenomenon and what was the solution?

Thanks

Csaba

[csakovacs]

Any idea what could be causing it?

[kfzhu1229]

Hello,

If I use my t420 with AC power, the background lighting does not work.
However, if I don't use the ac power supply and it only runs from the battery, it works fine (there is a backlight).

I don't think there could be any problem with the F7 fuse! Where should I look for the error, or has anyone encountered such an error phenomenon and what was the solution?

Measure the voltage on that fuse and against the ground. In normal operation the voltage should change from battery voltage (10-12.5V) to charger voltage (19V) when you plug in the charger. if the voltages do exactly lay as anticipated and the screen still has that problem, I do believe your LCD's backlight driver IC is faulty and the LCD needs to be replaced

[csakovacs]

Hello,

If I use my t420 with AC power, the background lighting does not work.
However, if I don't use the ac power supply and it only runs from the battery, it works fine (there is a backlight).

I don't think there could be any problem with the F7 fuse! Where should I look for the error, or has anyone encountered such an error phenomenon and what was the solution?

Measure the voltage on that fuse and against the ground. In normal operation the voltage should change from battery voltage (10-12.5V) to charger voltage (19V) when you plug in the charger. if the voltages do exactly lay as anticipated and the screen still has that problem, I do believe your LCD's backlight driver IC is faulty and the LCD needs to be replaced

The display receives the 19V voltage after the LVDS cable. If it starts from a battery, it's ok, but when you put it on the charger, the backlight goes off immediately! If I unplug the charger afterwards, the background doesn't come back either, only when I turn the machine off and on!

[kfzhu1229]

The display receives the 19V voltage after the LVDS cable. If it starts from a battery, it's ok, but when you put it on the charger, the backlight goes off immediately! If I unplug the charger afterwards, the background doesn't come back either, only when I turn the machine off and on!

Then the last things to check is that at the display panel's PCB, there are 2 test points labelled as PWM and BL_ON (backlight on). See if they each have voltage, the BL_ON should be 3.3 while the PWM can be anything above zero. If they keep receiving voltage along with the 19V backlight power even after plugging in the charger, than all points to the backlight driver circuit on the LCD itself being faulty and I don't think you'll have the ability to repair that so the LCD has to go.