820-00840 no backlight after sleep

[luckydip]

Came in dead, the backlight area and both rows of backlight caps were misaligned and fluxed.Got it working but has a sleep/backlight issue. When you close the lid, display darkens but when you open the lid the backlight doesn't restart - dim display as if no backlight. No other obvious liquid damage.

If you put it to sleep via the Apple menu then it wakes up fine with backlight.

Things I've tried - replaced hall sensor, replaced U5255 and given the SMC a light reflow - still no change. U5255 area resitors all ok. Any other ideas?

thanks

[piernov]

Re: 820-00840 no backlight after sleep

Smc_lid_left, smc_lid_right, lid_open?

[luckydip]

Re: 820-00840 no backlight after sleep

lid open with backlight

Smc_lid_left, smc_lid_right, lid_open all 3.4v

Using a magnet to trigger hall sensor - no backlight

Smc_lid_left, smc_lid_right, lid_open all 3.4v

[luckydip]

Re: 820-00840 no backlight after sleep

Tried the board with a known working display - same result.

Clutching at straws - is it worth changing u8400 and finally the SMC?

[mon2]

Re: 820-00840 no backlight after sleep

What is the voltage reading of EDP_BKLT_EN with the lid open and backlight ON ?

What is the voltage reading of EDP_BKLT_EN with the lid open and backlight is OFF ?

[luckydip]

Re: 820-00840 no backlight after sleep

Thanks for that -
backlight on EDP_BKLT_EN 3.3v
no backlight EDP_BKLT_EN 0v

This signal comes from GFX/PCH, can I use a 3v supply to keep the backlight on all the time so the machine is usable?

[mon2]

Re: 820-00840 no backlight after sleep

This signal comes from GFX/PCH, can I use a 3v supply to keep the backlight on all the time so the machine is usable?

Technically, yes. You could remove resistor @ R8442 (so the SMC does not challenge your next move).

Then solder a wire from R8442 pad # 2 (be sure it is the pad going to the backlight enable pin # 17) and mate this wire to +3v3.

Do note that this means that the backlight will ALWAYS be on when the laptop is powered - including when the lid is closed so this will deplete the battery much faster.

It is a 'hack' but should work. The real question stands as to why this signal (from the SMC) is not sending out the '1' (+3v3) for the enable when it should do so.

Q: What is the voltage level of LID_OPEN when the lid is open ?

What is the voltage level of LID_OPEN when the lid is closed ?

If you see +3v3, when open and 0V when the lid is closed then you could apply the EN of the backlight directly onto the LID_OPEN rail.

The same logic level will then turn OFF the backlight, due to the logic low when the lid is closed.

Post your update.

[luckydip]

Re: 820-00840 no backlight after sleep

Thanks for the pointers. This is begining to feel like a rabbit hole:

Checked LID_OPEN - voltage behaves as expected. Ran a wire to R8442 pin 2 (removed resistor). The voltage on EN is 3.3v when lid is open but still not waking/no backlight. I changed U8400 from a donor board but still the same result.

[mon2]

Re: 820-00840 no backlight after sleep

Given that you have replaced U8400 already, the issue could be elsewhere.

Also when the backlight is working, what is the voltage on R8410 ? This is the resistor that drives the mosfet @ Q8401.

When the backlight is not working, what is the voltage on R8410 ?

Power down, remove battery (if present). Meter in resistance mode.

Check the mosfet @ Q8401 for shorts.

Resistance between source & drain ?

Resistance between source & gate ?

Resistance between gate & drain ?

[luckydip]

Re: 820-00840 no backlight after sleep

what is the voltage on R8410 ? 2.7v

When the backlight is not working, what is the voltage on R8410 ? 0v

No shorts on Q8401

LCDBKLT_FB 50v when wake after sleep (no backlight)

[mon2]

Re: 820-00840 no backlight after sleep

1) Check on the resistor values of R8401 (80k6) & R8402 (63k4).

2) Check that mosfet Q8400 is not shorted.

3) What is the value of LCDBKLT_EN_L when backlight is working & not-working?

4) Monitor and post the OUTPUT voltage of U5450 (pin 6)(current sense for the backlight rail; ISNS_LCDBKLT_N and ISNS_LCDBKLT_P) when backlight is working & not-working.

[luckydip]

Re: 820-00840 no backlight after sleep

R8401 (72k) & R8402 (64k)

Q8400 ok - not shorted

Backlight on

LCDBKLT_EN_L 7.2-8.4v

U5450
4 13v
5 13v
6 1.2-1.3v

Backlight off

LCDBKLT_EN_L 11v

U5450
4 13v
5 13v
6 1.8-2v

There is physically no U5450 on the board (no stuff?)

[mon2]

Re: 820-00840 no backlight after sleep

As far as I can tell, U5450 is required to operate the logic board correctly. Perhaps someone can check as well.

Review the U5450 support parts and region again. This could be the show stopper.

[piernov]

Re: 820-00840 no backlight after sleep

U5450 is no stuff on production, BOM option SNS_I_BLC for development only.