Macbook Pro A1990 (820-01814) no backlight display

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

you are right... actually R8442 has 0 resistance...

I measured relative to the ground, by analogy with your previous attachments (when we measured R9833)

ps dead end again, I already hoped that we found a solution

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

Ok. Then this resistor is fine as long as 3v3 is reaching the side close to the backlight IC. It is not defective.

From the first post, this ic was replaced? Suggest to flux the part and apply hot air with low air pressure to allow the solder to melt and swim into place on the smd pads.

Louis suggests to make the part dance by nudging it slightly with tweezers while the solder is in melted state. The ic will move away slightly and then bounce back. This will remove any possible shorts under the IC. He has videos on this process.

The backlight is being enabled so it should turn on the above mosfet by pulling the gate voltage lower. The issue could be just with the soldering of the replacement part.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

what else can i measure to make sure the problem is really in the U8400?

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

Do confirm the soldering of the replacement part.

Then if still not enabling, try, a piece of wire with one end to ground. Other side of the wire to the GATE pin of the above mosfet mated to the backlight IC. This will force the above mosfet to turn on. Does this allow for any change in the backlight?

The backlight IC should do all of the above on its own to boost the led voltage so this trick may not work the same.

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

The mosfet is not enabling so it is back to the backlight IC.

Test the mosfet as well,

Remove all power. Meter in resistance mode.

Measure resistance of:

Source & drain
Source & gate
Gate & drain

Post each resistance check. You are checking across the mosfet pins.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

I confirm, U8400 was replace

thanks for the help, tomorrow we will try to test the U8400 soldering, I will report the results later

upd

Can the L8410 affect the backlight? even before the replacement of the U8400, it attracted my attention, tell me how can I check the L8410?

upd 2

is there any difference in LP8548B1SQ-03?

before replacement, I had an IC marked 48B1-03, I ordered an IC for my model, but the new one was marked 48B1-04, does it matter?

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

and yet, if the problem is really in the U8400, can the webcam not work?

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

I measured the resistance of the L8410 relative to ground, and got ~0.7 ohm, given that the circuit should already be ~0.47, I get about 0.23, which is close to the value of the inductor
so this item is probably correct.

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

Can you post a pic of your meter setting being used for this measurement?

Maybe the meter is in DIODE mode ? It is important to be in resistance mode and then check again, the resistance to ground of L8410.

One meter probe on either side of L8410 -> other meter probe to ground (shield is ok).

What is the resistance reading in ohms ?

If it is as you have posted, then there is a short on this rail. It should not be so low.

For such cases, lift up one leg of L8410 and then test the resistance to ground again. Check one side of L8410 PCB pad -> what is the resistance to ground?

Then check the other side of L8410 PCB pad -> what is the reading ?

By following this process, you can determine which block of components may be shorted. Also, for this test, REMOVE the LCD display to rule out that the LCD panel back light is shorted.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

I measured relative to the ground without power and display, the motherboard was removed from the case, the black probe to the ground, red to pin 1, then the same thing but to the second pin

[mon2]

Re: Macbook Pro A1990 (820-01814) no backlight display

Ok. Then you definitely have a shorted component on this leg of the backlight power rail.

With flux and hot air, lift up one leg of this coil. After the leg is lifted, measure the resistance to ground again.

Which side of the coil is showing the short ? Post each measurement.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

now I can't solder the U8400 backlight chip and the L8410 inductor, because my friend with the equipment went on vacation, I hope I will solve this problem soon, as soon as I start soldering, I will report the results after.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

Hello World! I think I need my own soldering dryer, now I don't have time to deal with my macbook, my friend who owned all the necessary equipment quit the service and now I have no one to turn to for help with soldering. Now I am developing a website for my client, I work from an old laptop on Windows, the sooner I hand over the work, the sooner I can buy myself soldering equipment, including a microscope. Please don't close the thread, I'll be back later with news. Happy new year everyone btw 🎉

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

Hello world! I'm back, I finally got my soldering station and now I can learn the basics. Unfortunately, I have not been able to purchase a microscope yet, so I can only warm up the U8400 from the current tasks, as well as replace the L8410, I will do this one of these days and share the results.

[XVLZ]

Re: Macbook Pro A1990 (820-01814) no backlight display

Hello world! Sorry for my absence, I was practicing with my new soldering station. While I'm waiting for the new LP8548 chips, I want to show you what I found on my board. To your attention C8461, on which I found cracks, although there is no short circuit when checking with a tester, I do not know. And also C8441, which seems to be raised and I'm not sure about this element (when measured at 1 pin, the tester shows 5V, if my memory serves me). I am also attaching a photo of the LP8548(U8400) landing site, I have a question about what I circled in red, shouldn't there be another pin there? By the way, should I clean the contact pad with a braid? (I added tin to the LP8548 landing pins, but I'm not sure if this is also necessary in the middle, circled in green)


P.S. I really don't have a microscope, so I use an iPhone lens for pre and post work inspections -_-

[ugamazing]

Re: Macbook Pro A1990 (820-01814) no backlight display

That cap is 100% toast. Those caps love to fail, and sometimes they still work off/on until they fail completely. Replace that cap and your issue will almost surely be resolved.