macbook pro 820-00840 have image no backlight

[mon2]

Re: macbook pro 820-00840 have image no backlight

Remove the LCD display and check again.

Specifically, what is the voltage to ground of the gate pin on Q8400 ? This is a P-channel mosfet so the gate voltage must be lower than the voltage being passed between the source / drain pins.

With sufficient voltage drop on the gate pin, you should see a non-zero voltage @ PPVOUT_S0_LCDBKLT.

If necessary, apply a direct ground wire onto the gate pin of Q8400. This will force enable the mosfet and then measure the voltage to ground of PPVOUT_S0_LCDBKLT.

Also, check the voltage to ground of the following points of interest:

BKLT_SCL
BKLT_SDA

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

Remove the LCD display and check again.

Specifically, what is the voltage to ground of the gate pin on Q8400 ? This is a P-channel mosfet so the gate voltage must be lower than the voltage being passed between the source / drain pins.

With sufficient voltage drop on the gate pin, you should see a non-zero voltage @ PPVOUT_S0_LCDBKLT.

If necessary, apply a direct ground wire onto the gate pin of Q8400. This will force enable the mosfet and then measure the voltage to ground of PPVOUT_S0_LCDBKLT.

Also, check the voltage to ground of the following points of interest:

BKLT_SCL
BKLT_SDA

BKLT_SCL 5.1 volt
BKLT_SDA 5.1 volt
so now i going to test with the cable to ground to see what i get
The voltage in gate LCDBKLT_EN_L is 12.60 but when i power on the mac droop to 11.46volt but PPVOUT_S0_LCDBKLT is still 0 volt

[mon2]

Re: macbook pro 820-00840 have image no backlight

The gate pin on the backlight mosfet must be much lower to enable.

You will need to attach a wire from the gate pin to ground so that effectively you will have 0 volts on the gate. Then measure the PPVOUT_S0_LCDBKLT voltage to ground again.

While gate is a LOWER voltage (by a few volts or more than what is on the source / drain pins), this P-channel mosfet is enabled.

The SCL / SDA voltages are a good reading and this means that they are not stuck.

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

The gate pin on the backlight mosfet must be much lower to enable.

You will need to attach a wire from the gate pin to ground so that effectively you will have 0 volts on the gate. Then measure the PPVOUT_S0_LCDBKLT voltage to ground again.

While gate is a LOWER voltage (by a few volts or more than what is on the source / drain pins), this P-channel mosfet is enabled.

The SCL / SDA voltages are a good reading and this means that they are not stuck.

Good morning and sorry for the delay.
i Connect the gate pin to ground and in PPVOUT_S0_LCDBKLT i have 12v now .

[mon2]

Re: macbook pro 820-00840 have image no backlight

Good. This confirms the mosfet is not defective.

Lift up one leg of the diode at the output side of the boost circuit. Then with no power, meter in diode mode, check the diode with your meter.

One probe on one pin. Other probe on other pin.

What is the measurement?

Then swap the meter probes.

Post each measurement.

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

Good. This confirms the mosfet is not defective.

Lift up one leg of the diode at the output side of the boost circuit. Then with no power, meter in diode mode, check the diode with your meter.

One probe on one pin. Other probe on other pin.

What is the measurement?

Then swap the meter probes.

Post each measurement.

Hello Thanks for replay and help me so i test the diode and one side gave me 0.333 and the other side gave me OL reversing the tips clearly and with pin 2 (output pin) disconnected

[mon2]

Re: macbook pro 820-00840 have image no backlight

The diode has tested fine and is not the fault. It can be soldered back.

Is the display now connected?

What are the voltage readings to ground of the backlight led pins on the display connector? (With the LCD connected)

Measure carefully since power is on.

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

The diode has tested fine and is not the fault. It can be soldered back.

Is the display now connected?

What are the voltage readings to ground of the backlight led pins on the display connector? (With the LCD connected)

Measure carefully since power is on.

I keep leaving the ground wire of the Q8400 gate to force it to work?? and I connect the display and check the voltage output in the display connection?

[mon2]

Re: macbook pro 820-00840 have image no backlight

Yes. We want to check the LED voltage with your manual feed of the 12v.

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

Yes. We want to check the LED voltage with your manual feed of the 12v.

Excuse me, I have tested the voltages in the test points that are below the connector. I think that this is what you were referring to and I only get 5 volts. i attach a pictures with the connector and 11 volt

[mon2]

Re: macbook pro 820-00840 have image no backlight

Will check the schematics once at the office.

Test the test pads in the picture you posted.

BSCL, BSDA, etc. Check their voltage to ground.

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

Will check the schematics once at the office.

Test the test pads in the picture you posted.

BSCL, BSDA, etc. Check their voltage to ground.

OK thank so much the repair shop are close the weekend so i see you at monday thank a lot for the help

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

So i have a time to take meditions and in this two test point i have
BSCL - 5v
BSDA - 5v

[mon2]

Re: macbook pro 820-00840 have image no backlight

Ok. Continue to test the other test points that end with SCL and SDA.

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

Ok. Continue to test the other test points that end with SCL and SDA.

Good morning and sorry for the time off i am on holidays, this morning and detect a missing component but i cant find on any schematic o buit map i Attach the pictures.

I think maybe this is the cause for the not backlight

[mon2]

Re: macbook pro 820-00840 have image no backlight

Attached.

Remove all power. Meter in DIODE mode. Short your meter probes. Meter should tone beep.

Connect one meter probe to any of the large caps shown with the arrowhead. These caps are mated with the PPVOUT_S0_LCDBKLT backlight rail.

Other meter probe to pin #43 of the LCD connector @ J8500.

Do you hear a tone beep? If not, that trace with the missing part is broken and needs to be fixed.

Suggest to scrap a bit of black solder mask with a sharp knife to confirm that the large trace around the fan opening in the PCB is truly the same PCB trace leading to pin #43 of the LCD connector. Fairly confident that it is.

Excellent find and observation of this PCB layout!!

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

Attached.

thanks for the picture in the schematic i get do not have the type of component that is why it is seen in the photo, it is a resistor, could you tell me what its values are, thanks

[mon2]

Re: macbook pro 820-00840 have image no backlight

Just updated my last post -see above.

[Sacred0489]

Re: macbook pro 820-00840 have image no backlight

Attached.

Remove all power. Meter in DIODE mode. Short your meter probes. Meter should tone beep.

Connect one meter probe to any of the large caps shown with the arrowhead. These caps are mated with the PPVOUT_S0_LCDBKLT backlight rail.

Other meter probe to pin #43 of the LCD connector @ J8500.

Do you hear a tone beep? If not, that trace with the missing part is broken and needs to be fixed.

Suggest to scrap a bit of black solder mask with a sharp knife to confirm that the large trace around the fan opening in the PCB is truly the same PCB trace leading to pin #43 of the LCD connector. Fairly confident that it is.

Excellent find and observation of this PCB layout!!

from the cap to pin 43 there was no continuity so we scraped away the solder mask and tested the continuity to pin 43 and it had continuity , so i need to know find the value of the resister to replace it .

[mon2]

Re: macbook pro 820-00840 have image no backlight

The schematic does not show this missing part.

From the picture of the boardscan from a paid database, this appears to be a ferrite bead. It is not technically required for the operation so you can just short the pads to allow for the backlight to reach the LCD display.

For initial testing, it is recommended to bridge the pads.

Otherwise, this part looks like an 0603 or 0805 ferrite bead (1A or higher should be ok) - we like to use 600 ohms in our designs but 300 ohms should be ok.

We use the parts from MaxEcho (Taiwan) but many others on the market are also ok to consider if you wish to apply this part.

http://www.maxecho.com.tw/en/product...d__ACMS_series

If possible, review the physical pad sizes on the pcb. Or as a plan B, take a part that is 0603 from any donor board and see if it fits over the pads you have that are missing this component. If it does not, try 0805 (will be a larger part). Then proceed to seek out a ferrite bead to replace this component.

In summary, this part will clean up / filter noise on the LED backlight voltage.