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Can I ask about an Alienware in here?

daxxone

daxxone

Member
M18x R2 that isn't turning on the LCD backlight. Tried switching GPUs, LCDs, LVDS cables. Found schematics with a search of 'Compal LA-8321P' and found page 24-25 cover LCD baclight, LVDS connector. This doesn't have signs of damage. System functions normal besides this one problem. Any help would be great. What do you need me to test?
 
2informaticos

2informaticos

Administrator
Staff member
Do you get image, only missing backlight?
Backlight enable and adjust (PWM) come out from U7 multiplexer.
Check some voltages for U7, like pins 16, 7, 9, 2, 14.
Also check +INVPWR_B+ presence at JLVDS.
Need to check signals and voltages with LCD connected, of course.

Some components, marked with @, are optional.
Depending of which ones you have on the board, you should follow corresponding signals and voltages...
 
daxxone

daxxone

Member
Thanks for the quick response. I really didn't expect that.

I do get an image on screen, just no backlight.

U7 testing with LCD connected and system on, image on screen, no backlight:
Piin 16: 3.329v
Pin 7: 1.984v (that is not what I expected here)
Pin 9: 3.329v
Pin 2: nothing (not what I expected here either)
Pin 14: 3.377v
JLVDS INV PWR_B+: .066V

Q45 (power is low on my multi-meter, I'll test again with a new battery in it.
1, 2, 5, 6 give .05 to .075 to .089, the longer it is on the higher it goes.
3, and 4 give 19.56v ( I am going to get a new battery for my fluke, and then re-test this. It reads crazy high with low battery. I hit between two pins and had backlight for that split second,4 and 5. So does that point to this P MOSFET not working right? Or do I need to look deeper?
 
daxxone

daxxone

Member
Testing Q45 again.
Pin 1: 0.05 climes to 1.15
Pin 2: 0.05 climes to 1.15
Pin 3: 19.00
Pin 4: 19.56
Pin 5: 0.05 climes to 1.15
Pin 6: 0.05 climes to 1.15

R423:
Pin 1: 19.00v
Pin 2: 0.0v

Q44: (Data sheet has different pin numbers than the laptop schematic so I just labeled the source, gate, and drain)
Pin G: 3.3v
Pin S: 0.0
Pin D: 0.0
 
2informaticos

2informaticos

Administrator
Staff member
First check resistance to ground on +INVPWR_B+ (LCD connected).
If short, or low values, solve that.

Check for short/low value G-S on Q45.
If yes, replace Q45.
If not, replace R423.
My bet is bad Q45; could be caused by LCD or video cable.
 
daxxone

daxxone

Member
Diode mode test +INVPWR_B+ (LCD connected) gave me nothing.

Gate to source has low resistance at .758
R423 is at 48k
I tested this same P MOSFET on a scrap Alienware 17 Ranger I have ( used the same circuit design according to the schematics for it 'compal LA-9331P' ) Gate to source resistance should give no response in diode mode?
I'm still pretty green when it comes to circuit toubleshooting, but if I've understood this right so far, I can swap this Q45 tonight and have this board ready to go back into this system.

Thanks for taking time to help me with this.
 
B

Biggtechman

Member
Had this exact same issue on an LA-8321P also, this thread helped me fix it! Replacment also taken from a scrap Alienware 17 Ranger. Thanks. :)
 
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