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Model A1989, EMC 3358. A refresh of the existing design, with updated processor options and a change to the keyboard materials. Available in Silver and Space Gray. Released in May 2019.

133 Questions View all | 37 Parts Shop

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No backlight but 12v on f8400

After changing thermal paste and some clean keyboard with 96% isopropyl, Mac reboots ok. Totally working.

While doing reset smc just for a fresh restart, Macs turns off. After rebooting it, nothing changes.

Took it to a repair man and find out that, if lcd is connected Mac doesn't start completely (20v, green caps lock, apple logo but with led light pointed to screen, sometimes chime, keyboard backlight on, but only "esc"written on Touch Bar, if monitor plugged, nothing happen).

If lcd connector is disconnect!!!, Mac boots normally connected to a monitor and completely works.

No further technician help, so I was going by myself.

Started measuring dc while pc on connected to an external monitor (fully working).

Found 12,6 on side 1 of f8400, and same value on side 2 of it.

Continued checking voltage, but already on c8410 0v, same on al the big caps c846x/c847x (so no boosting up to the 46-55v...)

So I was going to check mosfet q8400 since c8410 was on 0v.

Started checking 6, after 5, after 1 (all at 0v) and once I touched the port 2 it has done a spark.

After that I was going to check port 4 and 3 of the mosfet, but the fuse f8400 is not giving continuity.

Pc still fully working if connected to external monitor.

What can I do?

Could the mosfet be the first cause of all the problems?

If can be useful I've checked r8401 and r8402 and they are as schematic, but I do not know how to continue and what can I do.

Thank you if you've read up to here and have a good day

P.s. I got a usb c multimeter but and would like to help you to know how is doing with the spark mosfet, but with the fuse interrupting the line I think it is not a good idea, or maybe it can?

Should I try to connect the lcd with the backlight line blocked by blown fuse and check if the old behaviors are still there? Or it is unnecessary?

Update (01/13/26)

@oldturkey03

Arghh...

Have I done a big mistake?

One of the two pad of the fuse come out with the solder

…is it solvable?

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Update (01/13/26)

@oldturkey03

Here the piece that came out

Could be a good idea to try to attach it to the old fuse and the try to solder it to the pcb or is it a bad way?

There is sonething easier?

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Update (01/14/26)

@oldturkey03

Thank you as always!

Here you re, that the best I can do with my lens, a microscope is coming…

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Just today if needed, as arrived my dc power supply, if can be usefull;but, as you can imagine I ve never used it

Is it there any kind of measurement I can do without power?

Without the fuse I think I can inspect the backlight line alone?

Answered! View the answer I have this problem too

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@oldturkey03

Here the piece that came out[image|3964323]

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Not possible anymore, I ve loose the micro square…any solution?

Fusing al the 6 points with copper lines and use them as a base for the fuse?

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@bbqlover look like you lost the solder pad. Is there any chance you can post a better picture of what has happened? We can portably run a jumper wire to the pad.

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@bbqlover you said that "Found 12,6 on side 1 of f8400, and same value on side 2 of it." and then you say that "but the fuse f8400 is not giving continuity."

"Should I try to connect the lcd with the backlight line blocked by blown fuse" no. If you have a blown fuse you need to Replace it.

Do you show 12.6V on both sides of the fuse F8400? If you do not, you do not have to test any of the circuitry. You will need to replace the fuse first. As you can see, your question is a bit confusing. You really need to clarify this for us.

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Start moving backwards. You need to measure the voltage on your lcdbklt; what voltage do you get on pin 1 C8500? What voltage do you get on pin 2 on L8410 (or on D8410)? You need to check your BKLT_ENable line on your WLED driver. If your backlight is not enabled, you will not get the boost circuit to turn on. Make sure that you also check SMC_Lid signal . If the lid sensor "thinks" the lid is closed, BKL_EN will always stay at 0V.

What voltage do your get on U8400, which is your LED driver? If it does not get any power, it can't turn on.

Sure it could be the MOSFET , but it could also be many other components in that particular circuitry. I would suspect the driver before I would suspect a MOSFET unless there is any visible damage etc.

Update (01/14/26)

@bbqlover Without the fuse I think I can inspect the backlight line alone? No, you can't.

Is it there any kind of measurement I can do without power? Give this a try and see what it shows you. It might for your backlight driver too. Setting: Multimeter to Diode Mode. Red probe on Ground, Black probe on Pin 43 or 4 of the LCD connector.

0.000 to 0.007: Direct short to ground (usually a bad capacitor, connector pin, or cable).

0.200 to 0.300: Failed LPxxxx driver chip.

0.459 to 0.511: Broken feedback via or corroded feedback ball (Ball A5) on the LPxxxx.

Let us know what values you get.

As for the solder pad, looks like you have some contacts still available. Try to rebuild the pad by tinning it heavily and resolder the fuse. If not, you can run a jumper wire from F7000 to that pad.

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Thank your sir.

Very very very much ;)

Giving clarity to the first question:

-before doing voltage test on the mosfet (only the q8400, and not to the q8401 on the back of the board), I had checked both fuse side to see if it was blown or not.

It was ok, and so I went for the mosfet check, and due to the spark, I 've checked the fuse, and after there spark (on the mosfet body), the fuse was blown.

Does the sparks indicates anything?

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Please consider that I do not still have a donor board so I'm waiting for one to take the fuse and do the test you asked for.

Would you give me any advice on how to proceed? Or I should wait for the fuse? Or jump it?

Now I'm kind of stuck.

I thought that checking the big caps c8460 and seeing there 0v, was enough to think that even on the j8500 pin 1 there will also be 0v but I had not checked (and cannot now due to the fuse blocking the line)but probably it is a bad mine deduction, as you can image I m a newbie;(same for l8410 and d8410)

For measuring c8465, c8410 I pointed the black cable of the multimeter on a ground (screw) and the red on the active side of the caps, while the Mac was on and running;

Should I do the same for those you asked for? Always with Mac running and black on a ground?

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@bbqlover it means you did have power but somehow shorten the circuit. Now you will need to replace the fuse. Then recheck your circuit as I described. There is no reason to try to measure each component until you know what has failed. All of the once you are measuring now will be at 0V since you do not get any power to any component. Replace the fuse instead of bridging it. That can lead to more trouble if there truly is a short on the board.

Once you have the fuse replaced, check from your display connector backwards to your WLED driver etc. you need to make sure that your backlight gets turned on. Originally you have the right voltage on the fuse to power your circuit, once it got shortened out, you can't proceed with testing. Make sure you buy more then 1 fuse :-) just in case.

Yes, to test the voltage, Black on Ground, Red on the contact, multimeter set to somethin like 30-50VDC

No reason to test all the capacitors since that voltage will show up on PPVOUT_S0_LCDBKLT Pin 1 on C8500

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@oldturkey03 If ever you fancy a 'working' vacation to the PH I have a bunch of problematic boards I really need help to fix :D

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@livfe I'll definitely keep that in mind ;-)

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Bbq Lover will be eternally grateful.
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