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Old 03-08-2021, 01:25 PM   #1
JenneDezaeger
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Default A1708 intermittent backlight (no flexgate issue)

Hi,

I have a 1708 with a 820-00840-A board.
Sometimes I have backlight, I say sometimes because i haven't figured out the pattern when it works and when it doesn't. It doesn't seem to respond to temperature changes (hot or cold).
I do notice when it works and I unplug it and try again it doesn't work anymore, if I leave it for a while sometimes it starts again.
The reason that there's no backlight is because my BKLT_EN_R is 0V (except when it works off course, than its 3.3V).
I've tried and measured everything I could think off. Does anyone have an idea why my BKLT_EN_R is 0V most of the time except sometimes it isn't

Thanks in advance
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Old 03-08-2021, 01:44 PM   #2
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Default Re: A1708 intermittent backlight (no flexgate issue)

This is under apple recall https://support.apple.com/13-inch-ma...klight-service
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Old 03-08-2021, 02:05 PM   #3
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Default Re: A1708 intermittent backlight (no flexgate issue)

Quote:
Originally Posted by MobileXT View Post
That only for 2016 models,the 820-00840-A is a mid 2017 board as far as i know .
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Old 03-08-2021, 02:23 PM   #4
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Default Re: A1708 intermittent backlight (no flexgate issue)

It's indeed a mid 2017
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Old 03-08-2021, 02:28 PM   #5
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Default Re: A1708 intermittent backlight (no flexgate issue)

Could it be that although the screen flex looks OK, that my BKLT_EN_R is missing because of bad communication between the screen and the board? I haven't got another screen to test with for the moment but maybe I should try that.
An external screen isn't possible without a working internal display as far as I know, so it's hard to tell what is happening, but I've seen it boot into the login screen on 1 occasion.
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Old 03-08-2021, 07:16 PM   #6
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Default Re: A1708 intermittent backlight (no flexgate issue)

2017 are also impacted by Flexgate. Just read the many postings on ifixit website. We did a full display assembly replacement to resolve this issue.

However this unit would always turn on while almost full closure of the unit. Would blank out while opening at 90 degrees.

Your case may be different as the enable pin is low on your unit.

Could your lid sensor be a factor?
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Old 03-09-2021, 02:34 AM   #7
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Default Re: A1708 intermittent backlight (no flexgate issue)

I've measured the lid sensor and it seems fine.
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Old 03-09-2021, 06:13 AM   #8
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Default Re: A1708 intermittent backlight (no flexgate issue)

When the backlight is OFF, are you able to see the screen contents with an external flashlight on the front / edges of the display?

If yes, the display is working so backlight needs the review.

An external monitor should be possible to test. I do not believe the internal screen will prevent this test.

Trace back the backlight enable signal lines for a possible break.
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Old 03-09-2021, 07:36 AM   #9
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Default Re: A1708 intermittent backlight (no flexgate issue)

There are 2 lid sensors with 2 different signals and a 3rd combined signal.
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Old 03-09-2021, 09:19 AM   #10
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Default Re: A1708 intermittent backlight (no flexgate issue)

I've ordered a new screen just to rule it out.
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Old 03-16-2021, 09:11 AM   #11
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Default Re: A1708 intermittent backlight (no flexgate issue)

OK, so I replaced the screen and have the same problem.
It starts and draws around 0.580A @ 20V and I can turn it off by holding the power button.
But still no backlight!
If I connect an external monitor nothing show either and I can't seem to see the logo at startup, so it could be no image.

Last edited by JenneDezaeger; 03-16-2021 at 09:16 AM..
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Old 03-16-2021, 11:00 AM   #12
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Default Re: A1708 intermittent backlight (no flexgate issue)

Apparently when I leave the board for 15-20 min without battery and without power and I then plug it in it starts, if I then disconnect the USB-C and reconnect it doesn't start.
When it works it draws around 450mA, when it doesn't it jumps to 580mA. Sometimes when it starts it stops working and also jumps to 580mA.

Last edited by JenneDezaeger; 03-16-2021 at 11:30 AM..
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Old 03-16-2021, 01:22 PM   #13
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Default Re: A1708 intermittent backlight (no flexgate issue)

When it starts, do you hear the chime? Does the backlight work?

Have you checked for possible bad caps on the logic board? Place the meter in resistance mode, remove the power adapter and the battery to the logic board, then check the resistance on each cap (black on ground; red on the + of the cap under test) - record each resistance.

Some of the CPU core caps will be low resistance but proceed as noted.

Perhaps the power rail is shutting down due to 'excessive current draw' due to a shorted or almost shorted cap. This is common to see.
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Old 03-16-2021, 01:32 PM   #14
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Default Re: A1708 intermittent backlight (no flexgate issue)

The speakers are not connected so i suppose the chime is playing if I get the logo, but that is only after I leave the board for a while, and as I said, if it starts it blacks out after a couple of minutes and then it doesn't do anything. I see the current go up to 580mA and I can shut it down with the powerbutton, but no backlight but also no image.
But when it works, everything works also the backlight.
Any pointers on which caps to start with, because otherwise I would be busy for quite a while.
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Old 03-17-2021, 01:48 AM   #15
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Default Re: A1708 intermittent backlight (no flexgate issue)

If I measure on the "big" caps like C1163 you said they would be low resistance, but I get 2.4Ohms. Is that normal?
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Old 03-17-2021, 04:10 AM   #16
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Default Re: A1708 intermittent backlight (no flexgate issue)

OK so I discovered if i spray the CPU heatsink with freeze spray it turns on.
But sometimes with artifacts on the screen.

Last edited by JenneDezaeger; 03-17-2021 at 04:24 AM..
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Old 03-22-2021, 07:15 AM   #17
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Default Re: A1708 intermittent backlight (no flexgate issue)

I reflowed every power IC en it works
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Old 03-22-2021, 08:32 AM   #18
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Default Re: A1708 intermittent backlight (no flexgate issue)

Well done!!
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Old 02-05-2022, 03:24 PM   #19
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Default Re: A1708 intermittent backlight (no flexgate issue)

Quote:
Originally Posted by JenneDezaeger View Post
I reflowed every power IC en it works
wish to know what ic to rework. i dont want to touch every ic
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