macbook pro 820-00840 have image no backlight

[kingskie]

hello,

i am working on macbook pro 820-00840 no backlight.
there is no water damage indication nor corrosion.

measurements:

F8400 = 13V on both sides
PPVIN_S0SW_LCDBKLT = 0V
LCDBKLT_EB_L = 13V
PPVIN_S0SW_LCDBKLT_R = 13V
PPVIN_SW_LCDBKLT_SW = 0.012V
PPVOUT_S0_LCDBKLT = 0V
LCDBKLT_SW = 0.019V
LCDBKLT_FET_DRV_R = 0V
PP5V_S0_BKLT_D = 5.1V
LCDBKLT_FB = 0V

no short to ground on backlight caps
already change U8400, Q8401, D8410 and Q8400 3 times but no progress.
already check the pull up resistor all correct in value.

really confuse now..

any suggestion would be appreciated.
thanks before

[reassor]

Isnt this the one with flex gates?

[kingskie]

Originally Posted by reassor

Isnt this the one with flex gates?

flex gate is ok..already tried with good lcd also

[reassor]

Is there picture? just no backlight?
in older models i saw once cracked coil on backlight line. There was 0ohm on it but it was not working lol - that was some wierd crap.

Will check schematic @ home.

Maybe ill find something.

[kingskie]

Originally Posted by reassor

Is there picture? just no backlight?
in older models i saw once cracked coil on backlight line. There was 0ohm on it but it was not working lol - that was some wierd crap.

Will check schematic @ home.

Maybe ill find something.

yes was apple logo and then login picture but no backlight at all

[mon2]

What is the voltage of BKLT_SD ? (R8402, pin # 2) ?

Q8400 mosfet will TURN ON if the GATE is lower than the voltage it is trying to pass between source & drain.

You are measuring GATE = ~13 volts which is about the same as the PPVIN_S0SW_LCDBKLT_R so the mosfet remains OFF.

To turn ON this mosfet, the voltage @ R8402, pin #2 must be LOW.

Then R8402 & R8401 will create a resistor based voltage divider and the voltage at the gate will be a divided down voltage than PPVIN_S0SW_LCDBKLT_R.

See attached - if all is well, you should see ~5v7 on the gate of Q8400 which will then allow for the backlight voltage to pass through and ideally turn on the backlight.

Also post the measurements for the following signals:

EDP_BKLT_EN
EDP_BKLT_EN_R

BKLT_SCL
BKLT_SDA

I2C_BKLT_SCL
I2C_BKLT_SDA

[kingskie]

Originally Posted by mon2

What is the voltage of BKLT_SD ? (R8402, pin # 2) ?

Q8400 mosfet will TURN ON if the GATE is lower than the voltage it is trying to pass between source & drain.

You are measuring GATE = ~13 volts which is about the same as the PPVIN_S0SW_LCDBKLT_R so the mosfet remains OFF.

To turn ON this mosfet, the voltage @ R8402, pin #2 must be LOW.

Then R8402 & R8401 will create a resistor based voltage divider and the voltage at the gate will be a divided down voltage than PPVIN_S0SW_LCDBKLT_R.

See attached - if all is well, you should see ~5v7 on the gate of Q8400 which will then allow for the backlight voltage to pass through and ideally turn on the backlight.

Also post the measurements for the following signals:

EDP_BKLT_EN
EDP_BKLT_EN_R

BKLT_SCL
BKLT_SDA

I2C_BKLT_SCL
I2C_BKLT_SDA

R8402 pin 2 = 12.84V
R8401 = 80.3K Ohm
R8402 = 63.4K Ohm

EDP_BKLT_EN = 0V
EDP_BKLT_EN_R = 0V

BKLT_SCL = 5.1V
BKLT_SDA = 5.1V

I2C_BKLT_SCL = 5.1V
I2C_BKLT_SDA = 5.1V

should i change the Q8400 again?

[kingskie]

i change again Q8400 but still the same...

[mon2]

What is the voltage of LID_OPEN ?

Is the screen connected?

Are the contacts for the lcd connector and the lcd cable clean?

[kingskie]

Originally Posted by mon2

What is the voltage of LID_OPEN ?

Is the screen connected?

Are the contacts for the lcd connector and the lcd cable clean?

LID_OPEN = 3,39V

yes all test with screen connected and the contacts all clean and no sign of water damage

[kingskie]

just curious, can i get 5v7 on the gate even without backlight ic (U8400)?

[mon2]

Yes. Let us test..remove R8442. Do not lose the part but it is only 0 ohms so a blob of solder can replace it.

Then apply a 3v3 voltage wire onto the C8442 pad (this capacitor is not stuffed). This is the same trace as R8442, pin #2. Locate the 3v3 rail on the board for this signal.

Your board is not enabling the backlight so the enable pin is low. The EN pin must be high to turn on the backlight boost IC.

With this manual enable, does the backlight turn on?

If any doubts or questions, just ask before proceeding.

[kingskie]

Originally Posted by mon2

Yes. Let us test..remove R8442. Do not lose the part but it is only 0 ohms so a blob of solder can replace it.

Then apply a 3v3 voltage wire onto the C8442 pad (this capacitor is not stuffed). This is the same trace as R8442, pin #2. Locate the 3v3 rail on the board for this signal.

Your board is not enabling the backlight so the enable pin is low. The EN pin must be high to turn on the backlight boost IC.

With this manual enable, does the backlight turn on?

If any doubts or questions, just ask before proceeding.

so, without take off u8400,
1. remove R8442
2. inject 3v3 on C8442 pin 1
3. turn on the macbook to see whether the backlight is on or not..

are the steps correct?

[mon2]

Correct.

With the logic '1' on EN of U8440, you are manually turning on the backlight.

What is unknown at this time is at what intensity will the backlight be with this method.

Post your update and also if you are able to control the intensity. Specifically, believe the brightness level of the backlight is communicated over the I2C bus lines.

[kingskie]

Originally Posted by mon2

Correct.

With the logic '1' on EN of U8440, you are manually turning on the backlight.

What is unknown at this time is at what intensity will the backlight be with this method.

Post your update and also if you are able to control the intensity. Specifically, believe the brightness level of the backlight is communicated over the I2C bus lines.

sigh, still no backlight. should i try change U8400 again?

[mon2]

No. Something else is wrong.

With this manual turn on of the IC, what is the gate voltage of the first mosfet?

Confirm the two 0 Ohms resistors for the I2C bus to U8400 are present and not damaged.

Without power, measure the resistance to ground of the backlight caps. What is the resistance reading?

Remove the lcd, measure this point again. What is the resistance?

If you wish, you can force the first mosfet to turn on by a wire from ground to the gate of the backlight mosfet. This is the junction between the 2 resistors. This should allow for the backlight rail to pass the voltage through. Just do not know if the voltage will be boosted at the backlight output as this role is all controlled by the backlight IC. The backlight IC needs the I2C interface to function.

Need to get some beauty rest...will review the forum in the morning.

[kingskie]

Originally Posted by mon2

No. Something else is wrong.

With this manual turn on of the IC, what is the gate voltage of the first mosfet?

Confirm the two 0 Ohms resistors for the I2C bus to U8400 are present and not damaged.

Without power, measure the resistance to ground of the backlight caps. What is the resistance reading?

Remove the lcd, measure this point again. What is the resistance?

If you wish, you can force the first mosfet to turn on by a wire from ground to the gate of the backlight mosfet. This is the junction between the 2 resistors. This should allow for the backlight rail to pass the voltage through. Just do not know if the voltage will be boosted at the backlight output as this role is all controlled by the backlight IC. The backlight IC needs the I2C interface to function.

Need to get some beauty rest...will review the forum in the morning.

R8450 = 0.2 ohm
R8451 = 0.2 ohm
backlight caps resistance to ground = 164K with lcd
without lcd from couple M go down to K and then OL and repeat again couple M and so on.

by force you mean when turn on i have to connect R8402 pin 2 to ground?

[mon2]

No. R8402, pin #1 which is the gate of the mosfet. Make that ground.

We are selecting this point since we do not know if the backlight IC is driving the R8402, pin #2. That is, we do not want to force 2 outputs to challenge each other.

[kingskie]

Originally Posted by mon2

No. R8402, pin #1 which is the gate of the mosfet. Make that ground.

We are selecting this point since we do not know if the backlight IC is driving the R8402, pin #2. That is, we do not want to force 2 outputs to challenge each other.

plugged the adapter to turn on...shorted the R8402 pin 1 but still no sign of backlight..
changed again the U8400 but still the same...
after change U8400 BKLT_EN_R is 3.3V but still no backlight

[mon2]

Refresh the list of voltage measurements

Voltage after fuse ?

Voltage before current sense resistor.

Voltage after current sense resistor.

Gate voltage of mosfet.

Voltage on drain of mosfet.

And so on..