Macbook Pro 820-00840 have image no backlight

[Stevechurch]

So it is not shorted.

Check the voltage to ground of:

ISNS_LCDBKLT_P
ISNS_LCDBKLT_N

There should be little or no voltage drop across these 2 test points else the boost circuit will be turned OFF due to an excessive current draw.

Confirm the resistance to ground of PPVOUT_S0_LCDBKLT. No power when checking resistance.

If the resistance is not low, continue...

Remove R8442. Solder a wire from R8442 pcb pad # 2 = EN pin on the backlight IC to a 3v3 rail on the logic board.

This is to manually enable the backlight IC with a logic '1'. This bypass should then provide for a voltage divider to lower the gate voltage @ Q8400 mosfet to allow for the 12V to enter the boost circuit.

Does this enable the backlight for your board?

I'm following this thread:
https://www.badcaps.net/forum/showthread.php?p=1224969
I too have no backlight and followed the thread up to
ISNS_LCDBKLT_P = 12.6v
ISNS_LCDBKLT_N = 0.7v

could you advise please

kind regards and thanks in advance

Steve

[mon2]

Re: macbook pro 820-00840 have image no backlight

Remove all power. Meter in resistance mode.

Measure the resistance to ground of:

ISNS_LCDBKLT_N

[Stevechurch]

Re: macbook pro 820-00840 have image no backlight

Remove all power. Meter in resistance mode.

Measure the resistance to ground of:

ISNS_LCDBKLT_N

ISNS_LCDBKLT_N = 1.8k ohms

[mon2]

Re: Macbook Pro 820-00840 have image no backlight

To confirm, you have an image? After all, image is everything. This model is linked to flex gate. Tested with a known good display?

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

yes tested with known good display i can see login on screen with no backlight

PPVOUT_SO_LCD = 0.03v

[mon2]

Re: Macbook Pro 820-00840 have image no backlight

Let us continue in this thread.

You do have an image, just no backlight?

Check the EN pin voltage to ground of the backlight IC. This to confirm that the backlight is being requested to turn on. Also this signal is linked to the lid open line that is based on the hall effect sensor in your trackpad.

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

Ppvout_s0_lcdbklt = 0.9v

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

Let us continue in this thread.

You do have an image, just no backlight?

Check the EN pin voltage to ground of the backlight IC. This to confirm that the backlight is being requested to turn on. Also this signal is linked to the lid open line that is based on the hall effect sensor in your trackpad.

would that be on R5102 ? IPD_LID_OPEN_R?

sorry to ask but im leaning here

[mon2]

Re: Macbook Pro 820-00840 have image no backlight

See attached.

1) Measure the voltage to ground of the EN signal.

Measure the voltage to ground @ EDP_BKLT_EN

and also @ BKLT_EN_R (R8442, pin #2) - this is the voltage seen by the IC pin 17.

2) Next, remove all power and no battery. Meter in lowest resistance scale.

Meter probe on F8400 (fuse); other meter probe @ PPVIN_S0SW_LCDBKLT_R = Q8400 (pin # 4 is ok).

This resistance check is to confirm that you have a low enough resistance across the current sense resistor @ R8400.

The current sense amplifier @ U5450 is checking the voltage going into the current sense resistor (R8400) and then checking the voltage leaving the same and based on the voltage drop, will disable the backlight IC.

[mon2]

Re: Macbook Pro 820-00840 have image no backlight

For now - just review the backlight EN signal. If this voltage is high enough, then the LID OPEN becomes a no care since the logic board (trackpad hall effect) sensors are able to detect that the lid is open.

Hall effect is a solid state reed switch that with a magnet can tell if the lid is closed or vice versa. These sensors are inside the trackpad and the logic board reads 2 of these sensors and creates a final signal (after the dual input OR gate) = LID_OPEN. Based on this signal, the backlight is enabled or not. That is, no point to enable the backlight if the lid is closed.

Update:

ISNS_LCDBKLT_P = 12.6v
ISNS_LCDBKLT_N = 0.7v

This voltage drop implies you have a defective current sense resistor @ R8400 (rare) or there is a trace break on the current sense trace ISNS_LCDBKLT_N.

With no power to the board, meter in resistance mode. Measure the resistance from ISNS_LCDBKLT_N from R8400 to U5450, pin # 5. Double check this measurement.

If there is a trace break, the voltage leaving R8400 will be missing and then the circuit believes there is a hefty load on the rail so shut the backlight off.

This is the reason for the very low current sense resistor @ R8400 = 25 milli-ohms. There should be a very very small voltage drop across R8400. Certainly not a 12 volt drop.

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

See attached.

1) Measure the voltage to ground of the EN signal.

Measure the voltage to ground @ EDP_BKLT_EN

and also @ BKLT_EN_R (R8442, pin #2) - this is the voltage seen by the IC pin 17.

2) Next, remove all power and no battery. Meter in lowest resistance scale.

Meter probe on F8400 (fuse); other meter probe @ PPVIN_S0SW_LCDBKLT_R = Q8400 (pin # 4 is ok).

This resistance check is to confirm that you have a low enough resistance across the current sense resistor @ R8400.

The current sense amplifier @ U5450 is checking the voltage going into the current sense resistor (R8400) and then checking the voltage leaving the same and based on the voltage drop, will disable the backlight IC.

EDP_BKLT_EN = 0.2v
EDP_BKLT_R = 0.15v
F8400 and Q8400 = 0.10 ohm

thanks for your understanding and patience

[mon2]

Re: Macbook Pro 820-00840 have image no backlight

See above. Need to rake my Margaret (de)Thatcher lawn clippings before it rains. Will check back in a bit. Check the above updated posts. Suspecting a trace break in the current sense leg.

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

See above. Need to rake my Margaret (de)Thatcher lawn clippings before it rains. Will check back in a bit. Check the above updated posts. Suspecting a trace break in the current sense leg.

hope you dodge the rain !

please see photo ..U5450 is missing from this board

[mon2]

Re: Macbook Pro 820-00840 have image no backlight

Yes. Just missed it. Raking dead grass is as exciting as counting cars.

The missing part on your board sounded familiar...

https://www.badcaps.net/forum/showthread.php?p=1070107

So it is ok that you are missing U5450.

However, not ok that 12v enter this circuit @ R8400 but drops to 0 volts upon leaving.

With no power -> check the resistance across ISNS_LCDBKLT_P & ISNS_LCDBKLT_N. It should be ~0 ohms.

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

Yes. Just missed it. Raking dead grass is as exciting as counting cars.

The missing part on your board sounded familiar...

https://www.badcaps.net/forum/showthread.php?p=1070107

So it is ok that you are missing U5450.

However, not ok that 12v enter this circuit @ R8400 but drops to 0 volts upon leaving.

With no power -> check the resistance across ISNS_LCDBKLT_P & ISNS_LCDBKLT_N. It should be ~0 ohms.

glad missing chip is ok

ISNS_LCDBKLT_P & ISNS_LCDBKLT_N = 0.1 ohm

sorry belay that reply...i will recheck

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

ISNS_LCDBKLT_P & ISNS_LCDBKLT_N = 0.2 ohm

[mon2]

Re: Macbook Pro 820-00840 have image no backlight

This is a very good reading for R8400.

Can you confirm where you are measuring the ISNS_LCDBKLT_N to ground voltage ?

See attached. You should be able to measure the same very low resistance between the 2 points noted by the arrows. No power on resistance checks.

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

i have just gone back and checked voltage to ground 3 times on U8400 for both
ISNS_LCDBKLT_N and ISNS_LCDBKLT_P i am getting low readings on two tries then i got 12.6v on both, i then checked on the pad 3 and 5 of U5450 three time and had the same thing , two in mV and one at 12.6v
not sure if its power cycling but it powers up at 20v on the usbc charging ports (not sure if i used the same usbc conector but all times it was the same side)

[mon2]

Re: Macbook Pro 820-00840 have image no backlight

Check if voltage of PPBUS_G3H to ground is stable @ F7000. Meter in DC volts mode (20V or higher is ok).

Red meter probe @ F7000, pin #1 (producer side = ISL side that creates this power rail); black meter probe to ground.

Then check with the red meter probe @ F7000, pin #2 (consumer side) & ground.

What is the voltage measurement? Is it stable?

[Stevechurch]

Re: Macbook Pro 820-00840 have image no backlight

Check if voltage of PPBUS_G3H to ground is stable @ F7000. Meter in DC volts mode (20V or higher is ok).

Red meter probe @ F7000, pin #1 (producer side = ISL side that creates this power rail); black meter probe to ground.

Then check with the red meter probe @ F7000, pin #2 (consumer side) & ground.

What is the voltage measurement? Is it stable?

Both had the same reading of 12.6V