MacBook Air 820-01521 Backlight

[eli302]

Re: MacBook Air 820-01521 Backlight

I believe EDP_BKLT_EN comes from U0500 but I don't think I can test for it as it is a soldered on chip. Or should I try something else?

[mon2]

Re: MacBook Air 820-01521 Backlight

Yes, you can monitor @ R8442. Check both sides of R8442 for the logic level for this pin. Just in case, if R8442 is damaged then the logic level will be present on one side of this 0 ohm resistor but missing on the other side.

What is the voltage reading ?

See attached.

[eli302]

Re: MacBook Air 820-01521 Backlight

Both sides of R8442 are reading 0V.

[mon2]

Re: MacBook Air 820-01521 Backlight

This voltage implies that the backlight has been instructed to remain OFF.

When the lid is open and logic board booted, apply a flashlight on the front face and around the perimeter from front sides. Can you see any of the usual graphics on the display panel?

[eli302]

Re: MacBook Air 820-01521 Backlight

Yes you can see everything with a flashlight.

[mon2]

Re: MacBook Air 820-01521 Backlight

Very good. Remove this resistor (R8442). Keep it in your toolbox for now.

Then meter in resistance mode. Check the resistance to ground for the backlight enable pin. This is the side of the resistor that mates directly to the backlight IC.

Confirm there is no low resistance on this pad.

Then solder a wire from the resistor pad that connects to the backlight IC enable pin. The other side of the wire to +3v3 anywhere on the board.

This is only to test if the backlight circuit works if the enable is manually changed to a logic '1' = +3v3.

[eli302]

Re: MacBook Air 820-01521 Backlight

Bklt_en_r the resistance starts at 1.67 then climbs to 3.3 then drops. This is over about a 15 second time frame.

[eli302]

Re: MacBook Air 820-01521 Backlight

^in 20M setting.

[eli302]

Re: MacBook Air 820-01521 Backlight

After I solder the jumper, completely reassemble and test the screen?

[mon2]

Re: MacBook Air 820-01521 Backlight

Yes. Confirming again that your manual logic '1' state is being applied onto pin #2 of R8442 (NOT pin # 1). Be sure of this as the other logic will be trying to send out a logic '0' and we do not wish to assert our +3v3 onto that pin.

See attached.

Moving forward, the PCH component drives this backlight ENABLE pin.

Given that the backlight ENABLE pin = '0', there may be a sensor issue for the lid open function.

Measure and post the voltage readings for:

IPD_LID_OPEN
LID_OPEN_LEFT
LID_OPEN_RIGHT

Take these measurements with the lid open. If the logic board is out of the enclosure, then the raw board should reflect the same 'open lid' status.

[eli302]

Re: MacBook Air 820-01521 Backlight

No backlight still. The voltage is steady at 20.1V and the current ranges from ~1.5 to 1.77A.

[eli302]

Re: MacBook Air 820-01521 Backlight

PPVOUT_S0_LCDBKLT_F is at 0V

[eli302]

Re: MacBook Air 820-01521 Backlight

I believe the issue is between R8400 and Q8400. Everything but pin 6 on Q8400 is at 0V.

[mon2]

Re: MacBook Air 820-01521 Backlight

Everything but pin 6 on Q8400 is at 0V.

Did you mean pin #4 ?

If you have voltage @ fuse F8400, pin #2 then the voltage should be arriving on pin #4 of Q8400. Please confirm and note the location of pin # 1 of the mosfet.

Post the voltage readings on R8450 (to ground) and R8451 (to ground).

So red meter probe on the resistor (any side is ok) and black meter lead to ground. Meter in DC 20V scale is ok.

We want to be 100% sure that the I2C interface for the backlight (ie. SCL & SDA lines) are not stuck. These resistors should be about +5v on each part. Please confirm.

These 2 pins (SDA & SCL) are used to increase / decrease the LCD brightness as desired by the end user via software or keyboard presses.

Using the next higher voltage scale (>20V DC mode),

measure and post the voltages for ISNS_LCDBKLT_P and ISNS_LCDBKLT_N. The voltage readings should be very close to each other for this test.

After this suspecting that the backlight IC may be faulty.

[eli302]

Re: MacBook Air 820-01521 Backlight

I'm back working on the board.

[eli302]

Re: MacBook Air 820-01521 Backlight

I have to resolder my jumper wire but R8450 is reading 5.13V on both sides.

[eli302]

Re: MacBook Air 820-01521 Backlight

Do I need to resolder the jumper cable to continue testing? Or should I resolder the resistor?

[eli302]

Re: MacBook Air 820-01521 Backlight

So before I took off the jumper R8451 read 5.13V then dropped to 0V after testing. Now it's at 5.13V on both sides consistently.

[eli302]

Re: MacBook Air 820-01521 Backlight

I did some test using repair.wiki as a reference and it's pointing to the CPU being bad. Should I try anything else before ending this repair attempt? Or can I do a CPU replacement with this model and it be reasonable with a hot air and soldering rework station?