Released June 2012 / Core i7 processor with Turbo Boost / Up to 1 GB DDR5 Video RAM

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still have no backlight. External display works fine

I am having the same issue... however I replaced my entire display before bothering to check the backlight fuse. After replacing the display, I looked into the backlight fuse and it tested fine but I still have no backlight. External display works fine. No short in the display circuitry either. Kind of lost from here.

Update (12/21/2016)

@oldturkey03 Also, I was looking at it with my phone earlier and noticed that there are some potentially bad pins on the LVDS connector. Could this be causing the backlight issue? When I look at the schematics, the suspicious pins don't seem to be related to backlight..

http://i.imgur.com/O9TbOUi.jpg

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Pins 10,11,12 seem fried..

Answer this question I have this problem too

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@jbwftw what model is your logicboard 820-XXXX and besides buying a new display what have you checked? How did you check for "no short in the display circuitry"?

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@oldturkey03

I have the 820-2936-B

I tested for the short by using a multimeter, with the ground on the exit of the circuit on C9799 and a positive on the ground. Found out that trick from a MBP repair shop's youtube channel.

I also used my multimeter to test the fuse for continuity... it showed the fuse was good, but I have a junky harbor freight mm so I am ordering a new one to retest.

As I replaced the entire display assembly, this means that I have also used new cables and everything else that comes attached to the assembly.

Can the U9701 chip go bad entirely and still allow me to get external video, or is it only responsible for the backlight? I should also note that this motherboard used to not work after a coffee spill, so I was able to get it working again with some alcohol

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@jbwftw not having taken a look at the schematic for your board, I can tell you that it really wont matter. You do have to replace the connector before even thinking of doing any further troubleshooting. That is one of the reasons why I advocate a good visual inspection first. It does not matter which other component you will measure if your connector is gone like that. Personally I think you found your issue

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@oldturkey03 actually scrubbed the connector down and it appears that the connector is fine. Was just melted plastic from the black piece of tape that lays over it.

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@jbwftw thanks for letting us know. Back to the drawing board. Okay so you do have power on the fuse? What voltages do you have on the fuse and on D9701? Yes U9701 can have failed with the same symptoms.

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@oldturkey03 I tested the fuse and had 12.6V on each side of the fuse. Going to find a way to get the board out of the case and still be able to turn it on now so I can test the rest of it.

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Jacob Williams will be eternally grateful.
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