Macbook pro a1286 no backlight

[Instafixx]

Hello,
I just received a Macbook A1286 from a client with no backlight. The display graphics are visible as seen with a torch. The laptop boots up and chimes.

The customer says that he had taken it to a shop for software update and post software update the screen went blank.

There is no sign of corrosion or tampering inside the board.

Reading on F9700 is 12.2 V.

Any help would be appreciated

[mon2]

Re: Macbook pro a1286 no backlight

What is the 820-xxxx logic board number?

Link or post the schematic if on this forum.

Assuming the fuse is the backlight fuse, the boost voltage is not being created. Respectively, the boost regulator has an enable pin. Locate it and check the voltage (with care) to confirm the backlight is being enabled.

Do you read this voltage on both sides of the fuse?

Post the reading. Also note that the backlight will only enable if the display is being detected. So, with caution, inspect the LCD connector on the logic board for any damages.

[Instafixx]

Re: Macbook pro a1286 no backlight

Apologies for not posting the Board Number its 820-3330.
LCD Connector checked- seems ok.

Thanks for pointing it out. The Fuse has Voltage only on Pin #1 and not on Pin#2. Working on it and will update soon

[mon2]

Re: Macbook pro a1286 no backlight

Before replacing the fuse, do measure the resistance to ground on the no voltage side. With no power to the board.

The fuse blew for a reason. Do not bridge the fuse as it is too risky.

[Instafixx]

Re: Macbook pro a1286 no backlight

Before replacing the fuse, do measure the resistance to ground on the no voltage side. With no power to the board.

The fuse blew for a reason. Do not bridge the fuse as it is too risky.

Removed the Fuse and check resistance on #1.

The reading 450 and on Pin#2 its 850.

[mon2]

Re: Macbook pro a1286 no backlight

Can you confirm the units of the measurements ?

Do you mean 450 ohms and 850 ohms ?

What is the meter scale or can you post a pic of the meter face during these tests?

The resistance may be ok and not too low.

Do you have spare fuse from a donor board perhaps ? The rating should be similar so that it does not allow too much current to pass through, if there is a fault.

[Instafixx]

Re: Macbook pro a1286 no backlight

Yes i mean 450 Ohms.

I installed another fuse from the donor board but same result.

There is no voltage on point #3 on Q9706 (LCDBKLT_EN_DIV). Do you think that might be the problem?

[mon2]

Re: Macbook pro a1286 no backlight

See attached. Post the voltage readings for each.

The mosfet @ PQ9706 = P-channel mosfet. This needs to be ON for the backlight to work.

For the P-channel mosfet to work, the GATE voltage needs to be LOWER than the voltage it is passing through source / drain pins.

For the GATE to be lower, the resistor based voltage divider on the gate pin must be present.

That is, BOTH mosfets inside of Q9707 must be ON. Only if BOTH of these mosfets are ON, then the resistor based voltage divider will be created - that is the bottom resistor will mate to ground.

Then if you review a calculator (online), the 12 volts will enter the TOP resistor and the lower resistor will pull down this voltage and in the end, the GATE will be a lower voltage to enable the top mosfet.

So, we need to know the gate voltage of each Q9707 mosfets to see if they are being enabled or not.

Update - @ 850 ohms to ground on fuse pin # 2 - someone is leaking. Post your voltage measurements.

[Instafixx]

Re: Macbook pro a1286 no backlight

See attached. Post the voltage readings for each.

The mosfet @ PQ9706 = P-channel mosfet. This needs to be ON for the backlight to work.

For the P-channel mosfet to work, the GATE voltage needs to be LOWER than the voltage it is passing through source / drain pins.

For the GATE to be lower, the resistor based voltage divider on the gate pin must be present.

That is, BOTH mosfets inside of Q9707 must be ON. Only if BOTH of these mosfets are ON, then the resistor based voltage divider will be created - that is the bottom resistor will mate to ground.

Then if you review a calculator (online), the 12 volts will enter the TOP resistor and the lower resistor will pull down this voltage and in the end, the GATE will be a lower voltage to enable the top mosfet.

So, we need to know the gate voltage of each Q9707 mosfets to see if they are being enabled or not.

Update - @ 850 ohms to ground on fuse pin # 2 - someone is leaking. Post your voltage measurements.

Voltage at Q9707

#1 0.5V
#2 3.35V
#3 10.89V
#4 0.4V
#5 1.7V
#6 0.6V


Please guide.

[mon2]

Re: Macbook pro a1286 no backlight

1) confirm the voltage on both sides of fuse F9700 again.

2) remove all power. Measure the resistance from F9700 to pin #3 of the mosfet Q9707.

Post the measurements.

[Instafixx]

Re: Macbook pro a1286 no backlight

1) confirm the voltage on both sides of fuse F9700 again.

2) remove all power. Measure the resistance from F9700 to pin #3 of the mosfet Q9707.

Post the measurements.

Voltage on Fuse F9700 (Fuse was Changed)
Pin#1 12.56V
Pin #2 12.56V

Resistance from Fuse to Pin#3 Q9707 is .448.

We had to move up the meter to 2M from 2k as it was showing 1 as reading. { Picture Attached}

[mon2]

Re: Macbook pro a1286 no backlight

Resistance to pin #3 of the mosfet is fine.

With a piece of wire, connect pin # 3 of the mosfet to ground. This bypass will act the same as the 2 x mosfets when they are enabled. That is, the lower resistor will be mated with ground and then the top mosfet should have a lower voltage at the gate to enable the backlight.

It is a quick test method.

[Instafixx]

Re: Macbook pro a1286 no backlight

Resistance to pin #3 of the mosfet is fine.

With a piece of wire, connect pin # 3 of the mosfet to ground. This bypass will act the same as the 2 x mosfets when they are enabled. That is, the lower resistor will be mated with ground and then the top mosfet should have a lower voltage at the gate to enable the backlight.

It is a quick test method.

Hello, i did that but no still light on display.
Please check out the image of the wire.

[mon2]

Re: Macbook pro a1286 no backlight

Measure the voltage on each pin of Q9706.

With this short, pin #4 should be a lower voltage and should turn Q9706 ON.

Post the measurements.

[Instafixx]

Re: Macbook pro a1286 no backlight

Measure the voltage on each pin of Q9706.

With this short, pin #4 should be a lower voltage and should turn Q9706 ON.

Post the measurements.

All Pins are at 12.57V
Pin #3 4.373

[dellxps15]

Re: Macbook pro a1286 no backlight

measure value of that resistor .... looks like it's burned.

[Instafixx]

Re: Macbook pro a1286 no backlight

measure value of that resistor .... looks like it's burned.

Hello Resistor R9741 as marked in the Picture

Voltages
#1 0V
#2 0.01V

Resistance (With No Power and Meter Se at 2K)

#1 0 ohms
#2 .504 i.e 505 ohms

[mon2]

Re: Macbook pro a1286 no backlight

R9741 = 10k in the schematic. From the LP8550 datasheet this value may not be critical to the operation but the logic board does not match the schematic value.

Now that the top mosfet is passing through the required voltage, the LP8550 will function provided there are signals on:

I2C_BKL_1_SCL
I2C_BKL_1_SDA

PPBUS_SW_LCDBKLT_PWR

LCD_BKLT_PWM

Measure the voltage of each and post the readings.

[dellxps15]

Re: Macbook pro a1286 no backlight

i mean resistor value from pin 1 to pin 2 (to see if its just looking burned or it is burned actually.

[mon2]

Re: Macbook pro a1286 no backlight

Attached.