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Old 05-13-2019, 10:12 AM   #1
deanradish
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Default Toshiba Satalite C850 lack screen after LCD replacement

Hi,
I've fitted a new LCD to my Toshiba Satalite Pro C850-1K4 laptop, but made the school boy error of not removing the battery.
The new LCD is blank when the laptop is powered up.
After refitting the old LCD, this is also now blank.
The motherboard to LCD cable looks okay, no broken connection.
There is output from the HDMI socket to an external screen, and if I look closely at the LCD under a bright light then I see a faint image, so my conclusion is that there is a fuse or similar component on the motherboard that is preventing power to the LCD backlight.
I've attached an image of the LCD connector on the motherboard. Can anybody help me identify the faulty component, or offer any advice on how to fix the blank screen?
Thanks
Dean
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Old 05-13-2019, 11:35 PM   #2
MHTSOS
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Default Re: Toshiba Satalite C850 lack screen after LCD replacement

Is the LCD Led backlight or ccfl?
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Old 05-14-2019, 12:46 PM   #3
deanradish
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Default Re: Toshiba Satalite C850 lack screen after LCD replacement

Hi, it's an LED, model number ltn156at24-501

thanks
Dean
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Old 05-14-2019, 02:34 PM   #4
diif
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Default Re: Toshiba Satalite C850 lack screen after LCD replacement

I've repaired a couple of Toshiba laptops that had the same thing happen to them.
It was an inductor near that connector that had blown. Check resistance of the dark grey components that look like capacitors around that area.
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Old 05-14-2019, 04:03 PM   #5
deanradish
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Default Re: Toshiba Satalite C850 lack screen after LCD replacement

Thanks diif. I'll try that.
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Old 06-14-2019, 04:45 PM   #6
deanradish
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Default Re: Toshiba Satalite C850 lack screen after LCD replacement

In case this is of benefit to anyone else with a similar problem.

I found the schematic and board view files for the motherboard which I've attached.

The connector to the LCD is called the LVDS connector, part CN3000 on the schematic and boardview. I was confused at first because the LCD has a 40 pin connector on the back of it and the schematic shows 30 pins for the LVDS connector, but on inspection only about 15 of these are used by the LCD.

I couldn't find an inductor near the LVDS connector but thats likely to be because of my inexperience. Shame, because I would like to have tested it at least.

The schematics confirmed that there is no fuse protecting this part of the motherboard unlike on some laptops.

I tested the voltages on the pins and the pin that controls the brightness of the LCD (pin 20, INV_PWM_3_R) had between 0.5 and 0.9 V, everything else looked ok. This pin needs a bigger voltage for the LCD to begin to brighten.

On the back of the LCD I connected a wire from the 3V input to the INV_PWM pin, as I'd seen this hack in a YouTube vid (https://www.youtube.com/watch?v=zBB1L5F-VZg), and the LCD lit up at full brightness. This is a solution if you don't mind not being able to control the brightness.

Appreciate the help of people on this forum
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Old 06-14-2019, 05:15 PM   #7
diif
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Default Re: Toshiba Satalite C850 lack screen after LCD replacement

Whilst there is no fuse fitted, something has acted like one. Did you check the resistors and capacitors connected to INV_PWM ? C3006, R3009, R3013, R3014.
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Old 06-22-2019, 05:41 PM   #8
raider988
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Default Re: Toshiba Satalite C850 lack screen after LCD replacement

I'd check also R3035, it might be pulling the line to ground
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