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T420 backlight

T400/T410/T420 and T500/T510/T520 Series
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csakovacs
Posts: 3
Joined: Thu May 02, 2024 4:32 am
Location: Budapest, Hungary

T420 backlight

#1 Post by csakovacs » Thu May 02, 2024 4:37 am

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
Posts: 3
Joined: Thu May 02, 2024 4:32 am
Location: Budapest, Hungary

Re: T420 backlight

#2 Post by csakovacs » Tue May 14, 2024 7:39 am

Any idea what could be causing it?

kfzhu1229
Senior ThinkPadder
Senior ThinkPadder
Posts: 2549
Joined: Tue Sep 22, 2015 10:59 pm
Location: Toronto, Canada

Re: T420 backlight

#3 Post by kfzhu1229 » Wed May 15, 2024 2:17 pm

csakovacs wrote:
Thu May 02, 2024 4:37 am
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
Dell Lat CP MMX-233 64mb 40gb W2k
600 PII-266 416mb 40gb WXP
T23 PIII 1.13ghz 1gb W7
Precision M4300 X9000 8gb 160gb WUXGA Ultrasharp fp W10
T530i 15.6" i7 16gb fp W10
UXGA:
A30p PIII 1.2 1gb W7 (IDTech)
T43p 2.26 2gb fp W10 (Sharp)
Lat C840 P4-2.5 2gb 60gb W7 (Ultrasharp)

csakovacs
Posts: 3
Joined: Thu May 02, 2024 4:32 am
Location: Budapest, Hungary

Re: T420 backlight

#4 Post by csakovacs » Thu May 16, 2024 11:37 am

kfzhu1229 wrote:
Wed May 15, 2024 2:17 pm
csakovacs wrote:
Thu May 02, 2024 4:37 am
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
Senior ThinkPadder
Senior ThinkPadder
Posts: 2549
Joined: Tue Sep 22, 2015 10:59 pm
Location: Toronto, Canada

Re: T420 backlight

#5 Post by kfzhu1229 » Thu May 16, 2024 1:17 pm

csakovacs wrote:
Thu May 16, 2024 11:37 am
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.
Dell Lat CP MMX-233 64mb 40gb W2k
600 PII-266 416mb 40gb WXP
T23 PIII 1.13ghz 1gb W7
Precision M4300 X9000 8gb 160gb WUXGA Ultrasharp fp W10
T530i 15.6" i7 16gb fp W10
UXGA:
A30p PIII 1.2 1gb W7 (IDTech)
T43p 2.26 2gb fp W10 (Sharp)
Lat C840 P4-2.5 2gb 60gb W7 (Ultrasharp)

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