LA-J281P Rev 1 Backlight Circuit

[m4a2t0t]

Backlight is out and there is no power to the LCD connector. I did locate a bad fuse (FZ2) on the bottom but I dont know the proper value. If anyone has a board viewer file or a schematic that would be helpful too.

Thanks

[mon2]

Re: LA-J281P Rev 1 Backlight Circuit

I think we can learn from other similar Dell schematics.

See attached.

Backlight fuse = 0603 smd size @ 1.5A

P/N 1.5A_65V_T0603FF1500TM

Datasheet:

https://cdn.badcaps-static.com/pdfs/...4d5a5e0b62.pdf

Can be replaced with many others.

[m4a2t0t]

Re: LA-J281P Rev 1 Backlight Circuit

Should I have 19ish volts on the input of qv1? If im getting the pin numbers correct pin 3 has 8.6V which is the same as the output voltage (1,2,5,6) and 1.2v on pin 4

[mon2]

Re: LA-J281P Rev 1 Backlight Circuit

Yes. Something is wrong here.

You should have ~19v on the source pin #4.

Mosfet is P-channel so the gate pin #3 must be lower than the voltage being passed between the source and drain pins to enable.

Once the conditions are met, drain pins 1-2-5-6 should be ~19v.

Remove all power. Meter in resistance mode.

Measure the resistance across the mosfet pins:

Source & drain
Source & gate
Gate & drain

Post your measurements. We are checking for a low resistance which can confirm the mosfet is defective.

Confirm that your laptop has ~19v to ground present at the dcin connector. Continue to check for the same voltage near the lcd backlight circuit. Measure with care as the power is enabled. This voltage must be available at this mosfet which acts like a switch. So if gate voltage is lower than 19v, the EC is requesting to enable the backlight. Then if the fuse is ok, the display assembly will receive this power rail.