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Old 05-05-2020, 06:38 AM   #1
BoardNoob
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Exclamation Macbook 820-3787 Backlight Not Working

Hello All,

I'm quite new to this kind of this, Im just a lowly Process Engineer, so I am hoping you all can help me.

I have been googling and researching for a day, and have discovered the relevant information I need to troubleshoot my 820-3787 motherboard backlight.

So, here is some information which I hope you all can use to point me int he direction of a solution.

When I check the back light capacitors, I get the following resistence:

C7719/ 7716/ 7717/ 7715/ 7718 - 167 kilohms without screen connected. 83 kilohms with screen connected.

Resistance to ground of Pin 1 on the LCD connector is 167 kilohms, but drops to 200 when the screen is connected.

Resistance to ground of Pin 32 on the LCD connector is very low at 0.05 ohms.

Resistance across L8300 is 2 kilohms. This also shows some continuity when black probe to earth and red probe on one end.

Resistance across C8301 is 39 kilohms.

Chip U2800 has a water indicator on it that is half red. And there is evidence of corrosion around the bottom of the LCD connector and L8300.

It did have a meeting with water immediately prior to failing.

My question is, which of the above should I replace (and where do I buy these parts?)


Edit: Removed commas on numbers.

Last edited by BoardNoob; 05-05-2020 at 08:02 AM..
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Old 05-05-2020, 06:55 AM   #2
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Default Re: Macbook 820-3787 Backlight Not Working

Here are some images of the area
Attached Images
File Type: jpg 1.jpg (202.5 KB, 17 views)
File Type: jpg 2.jpg (147.2 KB, 20 views)
File Type: jpg 3.jpg (154.4 KB, 15 views)

Last edited by SMDFlea; 05-05-2020 at 07:01 AM..
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Old 05-05-2020, 07:02 AM   #3
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Default Re: Macbook 820-3787 Backlight Not Working

You can post images here if you use the Go Advanced button, then Manage attachments .
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Old 05-05-2020, 07:05 AM   #4
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Default Re: Macbook 820-3787 Backlight Not Working

Schematic + boardview - https://www.badcaps.net/forum/showthread.php?t=62268
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Old 05-05-2020, 07:07 AM   #5
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Default Re: Macbook 820-3787 Backlight Not Working

Quote:
Originally Posted by SMDFlea View Post
Hi,

Yes, I have all those schematics and board views. I'm hoping for some input no interpreting the data I have.

Apologies, electrical is a weak area for me.
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Old 05-05-2020, 07:48 AM   #6
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Default Re: Macbook 820-3787 Backlight Not Working

Is it no image, no backlight or both?
Given the state of the board I assume this doesn't give an image either, but this is very important since no image and no backlight are 2 different issues that have almost completely separate circuits. Also backlight usually depends on image, but not the opposite.

The inductor L8300 with liquid damage near the LCD connector seems to be damaged so it'll probably have to be replaced. The measurements also seem to point to a failure around L8300, so first thing I'd remove it, clean the dirt on the board and then measure resistance to ground on both pads.

Also please report the resistance readings with unit prefixes like 1kiloohm rather than 1 000 ohms. A lot of countries use the comma as a decimal separator, and sometimes the dot as thousand separator so it can be quite confusing. (even more when some numbers have a separator and others don't)
Here I assume that 167,000 Omhs means 167 kiloohms, please confirm.
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Old 05-05-2020, 08:01 AM   #7
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Default Re: Macbook 820-3787 Backlight Not Working

There is an image. When I sign a torch behind the apple logo on the laptop, I can see a dialogue box open.

I will replace L8300.

Do you think the connector is bad, or do you think it is finding a short through the L8300?

Apologies for the use of a comma on numbers, this is a habit of mine. You are correct that when I use a comma, I am separating thousands and not decimals. So yes, the resistance on the capacitors is 167 kilohms. I will edit the original post.
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Old 05-05-2020, 08:58 AM   #8
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Default Re: Macbook 820-3787 Backlight Not Working

Quote:
Originally Posted by BoardNoob View Post
There is an image. When I sign a torch behind the apple logo on the laptop, I can see a dialogue box open.

I will replace L8300.

Do you think the connector is bad, or do you think it is finding a short through the L8300?

Apologies for the use of a comma on numbers, this is a habit of mine. You are correct that when I use a comma, I am separating thousands and not decimals. So yes, the resistance on the capacitors is 167 kilohms. I will edit the original post.
You can't know if the connector is bad without testing it, but judging by the state of the board you should take it off and clean the pads and solder it back anyway before any meaningful testing.
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Old 05-05-2020, 09:45 AM   #9
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Default Re: Macbook 820-3787 Backlight Not Working

Hi,

I just read online that L8300 typically blows if the LCD screen, LCD screen cable, or LCD connector short to ground. The advice is to check pins 28/29/30 of the LCD connector with the LCD plugged in. Low resistance means it would blow again.

I do get low resistance from these pins, so I think I have answered my own question that I should replace the LCD connector.
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Old 05-05-2020, 09:59 AM   #10
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Default Re: Macbook 820-3787 Backlight Not Working

That's a bit confusing because the measurements + the state of L8300 points toward an issue with the LCD panel power rail, which would prevent any image from appearing. (and since the LCD panel is not powered, it cannot be recognized by the machine, and the machine won't turn on backlight either)
The backlight rail (on the other side of the LCD connector on pin 1) doesn't seem to be affected by liquid damage and the measurements sound good.

However as I said you have to take L8300 first as it is visibly damaged and there is corrosion that needs to be cleaned there. Once L8300 is taken off you can at least confirm if the short to ground is on the LCD connector side or on U8300 side.
You can also inspect the connector on the LCD cable to see if there is any damage there.
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