820-00840-A no backlight image present

[Francisco.s]

Hi, this mobo has not enought backlight voltage. (only 13 volts)

The boost converter does not work, there are no pulses at drain of Q8401.

BKLT_EN_R voltage is present, (3 volts) and there are BKLT_SDA and BKLT_SCL pulses.

SMC reset works well with its keyboard.

I tried PRAM reset, but can not make it work. Same with external keyboard.

I have no adapter for external display. I have ordered one so I will try PRAM reset via OSX terminal.

Can someone point me what to do next?

[mon2]

Re: 820-00840-A no backlight image present

hi. start with booting up the computer and shine a flashlight (cell phone will do) on the backside of the Apple logo on your display.

You can also attempt to shine the flashlight from an angle on the front side.

Move around this flashlight - do you see any of the usual screen icons / desktop stuff?

If yes, then the LCD display is working but as you have noted, the backlight is not operational.

Please confirm this first and post your comments.

Next, power down and place your meter in resistance mode.

Apply the black (ground) lead to ground (shield is ok).

Red led onto Cathode of D8410. You will be performing resistance checks with the computer always OFF / NO POWER / NO BATTERY.

What is the reading? Trying to confirm if the resistance is ok on this rail or if too low then you may have a short circuit inside one of the caps.

If the resistance is ok then the issue may be U8400 is defective.

The LP8548B1SQ-04 @ U8400 appears to be common across assorted macbook models so you may be able to pull it off a donor board for testing.

Please post your updates.

[Francisco.s]

Re: 820-00840-A no backlight image present

hi. start with booting up the computer and shine a flashlight

LCD works OK, I can see the logo when booting.

Red led onto Cathode of D8410

There is no short circuit, diode mode value is 1.137

you may have a short circuit inside one of the caps

No short, there are 12v at PPVOUT_S0_LCDBKLT. (13 when boots)

the issue may be U8400 is defective

I think you are wright, probably U8400 is defective, but I don't know if there is something else that could be damaged, as the flex cable.

Is there a flexgate issue in this model?

Thanks for your help.

[mon2]

Re: 820-00840-A no backlight image present

On D8410, check the resistance to ground (not diode value).

Do this with no power to the logic board and your meter in resistance mode. We are checking for a bad capacitor here.

Please post the resistance value.

Not flexgate issue yet. Flexgate is with all voltages are ok including the backlight is at a high reading of 25++ volts but the flex cable cracks and this backlight voltage does not reach the display assembly. The high voltage is used to light up a batch of white LEDs.

[Francisco.s]

Re: 820-00840-A no backlight image present

On D8410, check the resistance to ground (not diode value).

The resistance value are a few mega ohms. (without battery nor charger)

D8410 is also good (13 volts at anode and 12.7 at cathode)

Time to replace U8400, I think.

[mon2]

Re: 820-00840-A no backlight image present

Yes. Starting to agree on the same conclusion.

You may be able to source this component off another donor board or order from China very soon. I believe some are returning back to work on Monday but all will face a shipping delay due to back log of orders.

Maybe a local shop has these available for you?

Please continue to post your updates for future readers.

[mon2]

Re: 820-00840-A no backlight image present

The backlight is a custom ic #LP8548 from Texas instruments (former National Semiconductor). This device is controlled over I2C bus (SDA and SCL).

It is possible that I2C bus is not operational. Check for any possible liquid damage related to the I2C lines.

If it looks ok just replace this ic and remove the display before powering up.

Looking forward to your progress report.

Then check if this voltage rail is high as it should be. Once this is confirmed, then power down and attach the display to confirm the repair was successful.

[Francisco.s]

Re: 820-00840-A no backlight image present

I would get a fully working Macbook with same motherboard.

I will try to do comparative measurements and post results.

It is possible that I2C bus is not operational

There are good pulses in both lines. (pin 15 &16 ic U8400)

Thanks.

[Francisco.s]

Re: 820-00840-A no backlight image present

Time to replace U8400, I think.

Hi, I've replaced U8400 and boost converter doesn't work.

I've also replaced flex edp video cable and doesn't work either.

The replaced chip and edp video cable are working OK in the other laptop.

Help, I'm running out of ideas.

Will try with other bios, just in case......

[mon2]

Re: 820-00840-A no backlight image present

Remove the video cable.

Power on - what is the backlight voltage?

[mon2]

Re: 820-00840-A no backlight image present

Post the readings for the attached points (Voltage mode on your meter).

[Francisco.s]

Re: 820-00840-A no backlight image present

Remove the video cable.

Power on - what is the backlight voltage?

There are 11.4 volts, with and without video cable.

The second motherboard has no voltage without video cable.

I'm thinking that Q8400 must be in short-circuit.

I will test it and post readings.

Thanks.

[Francisco.s]

Re: 820-00840-A no backlight image present

I'm thinking that Q8400 must be in short-circuit.

After replace Q8400 the problem is solved.

All of its pins had the same voltage, so BKLT_SD signal could no control PPVIN_S0SW_LCDBKLT.

I don't know how the controller chip is "programmed', but I think it didn't like to sense LCDBKLT_FET_DRV all the time, so it never generate LCDBKLT_SW with this anomalous fedback sensed voltage.

Thanks for your help.

[mon2]

Re: 820-00840-A no backlight image present

Awesome! Thanks for the update.