MacBook Air 820-01521 Backlight

[eli302]

Hey I bought a MacBook Air to attempt to fix. The board has liquid damage near the top of the board near the LCD connector but there's no shorts in the area. Near the U8400 chip there's no shorts but a open line on LCD backlight enable. I'm stumped at this point looking at the schematics I'm not really sure where I need to go from here. Is the open line a sign of a completely blown component on the rail, is U8400 bad? If someone can give me any help it'll be much appreciated.

[mon2]

Re: MacBook Air 820-01521 Backlight

Check voltage on F8400, pin #1 with meter. Meter in DC volts (20V scale is ok for this test).

Check the voltage on pin # 2 with meter. If no voltage on pin #2 then fuse @ F8400 is blown and needs to be replaced.

Arrow.com has these but you can also source from Digikey / Mouser.

Next, check the presence of the backlight voltage for the LCD display. This rail WILL be higher than 20V as this circuit is a BOOST circuit to light up the backlight LED matrix for the panel.

To test, configure your meter to be 50V or higher (DC scale). Then red meter probe on L8420, pin # 2.

Measure and post the voltage reading.

Finally, confirm this same voltage is reaching the LCD physical connector. Observe the LEFT / RIGHT pin numbering sequence. You will be probing pin 43 OR any of the yellow highlighted locations which are the same net.

Recommend to probe at the position with the arrow since these parts are TINY. Be sure that you do not short to any other part else can face permanent damage. Best to select the cluster shown. If you do not have them already, consider to invest into needle like meter probes from Aliexpress or similar.

[eli302]

Re: MacBook Air 820-01521 Backlight

Where do I inject voltage? I originally did at C8468 but current was never drawn. I got that point to inject the voltage from repair.wiki and I know current blocks DC so I didn't believe it would work as a testing site but it's my first board by myself so I tried it.

[mon2]

Re: MacBook Air 820-01521 Backlight

There is no injecting of voltages for these tests. Injecting a voltage before performing other tests is very dangerous to the logic board. Respectively, voltage injection should be a last resort and it is primarily to locate a shorted component.

Missing a backlight does not necessarily mean that you have a shorted part. Rather, could be a missing part or a damaged part that is halting the boost regulator (takes in a low voltage and boosts it to a higher value). The backlight is a cluster of white LEDs that demands the backlight voltage to be 25-40V so that each LED can be lit up at the proper intensity.

Perform the requested tests and post your results.

For the requested tests, meter in DC volts mode. Red meter lead on the component to test; Black meter lead to ground (metal shield is ok).

[eli302]

Re: MacBook Air 820-01521 Backlight

How do I test the fuse without injecting voltage when F8400 is on the other side of the board when the bottom case is removed? (Don't mind my probes my red is ground because my ground probe has a needle tip).

[eli302]

Re: MacBook Air 820-01521 Backlight

PPVOUT_S0_LCDBKLT_F is reading 0V.

[eli302]

Re: MacBook Air 820-01521 Backlight

Ppvin_sw_lcdbklt_sw = 0v

[eli302]

Re: MacBook Air 820-01521 Backlight

So in diode mode when I have the display plugged in there is a short to ground on PPVOUT_LCDBKLT_F. When the display is unplugged it's an open line.

[rotceh_dnih]

Re: MacBook Air 820-01521 Backlight

I had this the other week on a a1466 , pins 3-4 were short on the lcd connector its self and only found it after blowing the fuse twice and being 100% there wasn't a short after replacing it , it was blowing only when i put it in the deck lmfao.

[eli302]

Re: MacBook Air 820-01521 Backlight

So go ahead and order and replace the connector and fuse?

[rotceh_dnih]

Re: MacBook Air 820-01521 Backlight

Not yet Id wait for someone more advanced "mon2" to help with correct readings for your model I'm kinda new at this aswell but it does look like your fuse is blown. i posted to prompt you to test the lcd cable connector for a short as well just to rule that out as it can troll you like it did me after rebuilding the circuit twice

[eli302]

Re: MacBook Air 820-01521 Backlight

Test each pin on the connector?

[rotceh_dnih]

Re: MacBook Air 820-01521 Backlight

no just the backlight pin, the one on the lcd its self that plug's into pin 43 of J8500. as this may be the cause of the blown fuse etc..

PPVOUT_S0_LCDBKLT_F is reading 0V.

If this reading is really from F8400 PIN 2 then your fuse is blown and needs replacement. thats the main input and if its boots you have _g3h

remember Im new aswell so you should just be waiting right now.

[mon2]

Re: MacBook Air 820-01521 Backlight

Ok.

1) Fuse F8400 is required to be working to support the backlight rail for the LCD display panel. You have confirmed with the DC volts mode, that pin #2 is @ 0V.

So this means either F8400 is blown from an excessive current draw that tripped this one-time fuse @ F8400 location

OR

The voltage incoming to F8400 @ Pin #1 is also 0 volts.

2) With the meter in DC volts mode (20V scale is OK. Red meter probe on pin #1 of F8400 = PPBUS_G3H & Black meter probe to ground.

What is the voltage reading ?

You should see ~8V6 / ~8v2 on this rail for the MacBook AIR models. The MacBook Pro models are higher for this rail.

Reference:
https://www.logi.wiki/index.php?title=PPBus_G3H

If not, then PPBUS_G3H is not working and needs to be fixed before we review the backlight fault.

3) To review the PPBUS_G3H (if this rail is not working properly), measure and post the PPDCIN_G3H voltage value.

This is the voltage that is the INPUT to the switching regulator @ U7000 (ISL9240).

Your MacBook AIR is a Type C design. The external power adapter communicates with the logic board using the USB C 'ACE' controllers from TI. The 2 parties (adapter & controller) communicate with each other to exchange power delivery contracts. A contract is xx volts & yy Amps can be supplied by the adapter.

The 'ACE' controller will receive the contract list and decide on the best choice to power and charge this laptop. Often it is 20V @ yy Amps.

If you are not seeing the 20V contract being selected, then we have to continue to go upstream to locate that fault.

Do you have a USB Type C power meter ? If yes, place between the external power adapter (Apple is best but others will work - we use the LVSUN we purchased in HK at the trade fairs for $20 USD = works very well and will order more of them).

What voltage & current draw do you see with this logic board when 'ON' ?

[eli302]

Re: MacBook Air 820-01521 Backlight

The voltage and current draw is 20.1V and 1.47A.

[eli302]

Re: MacBook Air 820-01521 Backlight

Also I didn’t read PPVOUT_S0_LCDBKLT_F from F4800 is read it from the other side of the board. Could the reading at the fuse be different? Also I don’t know how to test the fuse without using my bench since the fuse is on the other side of the board when testing using AC power. When I did have the board completely out though the fuse had beeped in diode mode so can assume it’s still good from this and it’s not the issue or do I need the exact resistance to determine?

[mon2]

Re: MacBook Air 820-01521 Backlight

1) The 20V reading and current draw looks healthy.

2) Confirm the voltage on PPBUS_G3H. Locate any place you can monitor this rail and post the results. Once this rail is confirmed to be ok, you will need to make sure this same voltage value reaches the Fuse F8400. After this is confirmed, the fuse must allow for the voltage to pass through onto the next side of the fuse.

Search out the youtube videos on the teardown of how to remove this logic board and consider to do the same. The USB paddleboard should also be possible to remove but confirm with the videos.

It will make diagnostic much easier to remove the logic board out of the housing. If not comfortable with this idea, only then use alternate test points but they may not show the full view of the issue. That is, to properly diagnose the backlight area, you will need access to that side of the logic board.

We use masking tape to remove and fix the removed screw near the hole (but still leave it on the logic board with tape) from which the screw was removed. This system works out well when rebuilding the logic board back into the housing. Just be sure to use the masking tape as an insulator so that no parts / traces are shorted from the metallic screw.

Based on the current draw, believe that PPBUS_G3H voltage will be fine.

2) Also interested in the voltage reading of EDP_BKLT_EN. This is the ENABLE logic level that requests the backlight voltage to turn ON. With the proper input voltage from PPBUS_G3H & EDP_BKLT_EN at the logic high, the backlight should turn ON and supply 25-40V (approx) for the LED backlight.

[eli302]

Re: MacBook Air 820-01521 Backlight

F8400 is reading 12.6V on both sides.

[mon2]

Re: MacBook Air 820-01521 Backlight

voltage reading of EDP_BKLT_EN ?

[eli302]

Re: MacBook Air 820-01521 Backlight

EDP_BKLT_EN is reading 0V. So the enable isn't working.