Macbook 820-3115 missing 3V42_G3H

[Nuld_]

Hello there,

So i have a A1278 13" (820-3115-B) at first i had a problem with U7200 which was getting very hot, I noticed shorts on the nearby capacitors.
I´ve swapped U7200 and all shorts are gone but sadly I accidently forgot that the Magsafe Connector was still attached.
Now I`m struggeling with PP3V42, there´s no light on the Magsafe Connector anymore and U6990 is behaving weird according to the readings.
I´ve already replaced U6990 twice, no shorts or damaged components are found.


U6990
1. 1.2
2 3.68
3. 15.11
4. 16.61
5. 0
6. 0
7. 16.61
8. 3.68
9. 0
10. 0

[mon2]

Re: Macbook 820-3115 missing 3V42_G3H

Resistance to GND on L6995?

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

It´s around 3.1 ohms

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

It´s around 3.1 ohms

[mon2]

Re: Macbook 820-3115 missing 3V42_G3H

That is a very low resistance to ground. There is a short somewhere.

Remove this inductor. Careful to not lose or burn the part in the process. Use lots of flux, low air pressure so other parts do not go flying. If you have it, apply kapton tape to prevent other parts from the heat near this inductor.

After this inductor is removed, check the resistance to ground again at the pads on the pcb where this part was soldered.

Which pad measures the low resistance to ground?

Is it the pad that is linked to the 3v42 regulator (producer side) ?

Or is it the pad that is linked to the downstream (consumer side) ?

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

It´s definitely the producer side which is 3.2 ohms.
Consumer side is climbing above 16Mohms.
Pin 9 of U6990 is now definitely shorting to ground.

[mon2]

Re: Macbook 820-3115 missing 3V42_G3H

Excellent. Using great care, reflow the buck regulator IC and use tweezers to judge it slightly when the flux melts the solder. Be patient. Once the solder is in liquid state, the IC can swim around and the tweezers will help to align the pads. Once you nudge the IC at any 2 corners, the chip will bounce away and return back to the pads like a magnet. This process helps to remove shorts under the chip that we cannot see. Louis calls it 'make it dance' technique.

After you are done, check the resistance again to ground. It must not be this low.

Also be very very careful to not lose the resistors on the FB pin. These 2 resistors define the downstream 3v42 rail. If these parts are lost, the downstream rail can receive the wrong voltage and may cause damage.

Use low air pressure so that other parts do not get lost near this IC. Better if you protect the nearby parts with kapton tape or aluminum foil.

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

So, i removed the IC again, checked on Pin 9 and the short is still there,
i guess C6994 or C6999 are faulty

[mon2]

Re: Macbook 820-3115 missing 3V42_G3H

Wait. I may have my bearings in reverse.

Which side is with the short ? If it is on the left side (of the schematic / coil) = producer side. The side that is creating this power rail.

The right side (of the schematic / coil) = consumer side = downstream.

Please clarify.

See attached so we can be on the same page.

Is it 'A' or 'B' with the low resistance to ground ?

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

Low resistance was on Pin1. It´s B

[mon2]

Re: Macbook 820-3115 missing 3V42_G3H

Ok. That is the downstream side and this rail travels all over the board.

Do you have an adjustable power supply?

Start with removing C6999 and check the resistance again to ground. Perhaps we will get lucky.

If the resistance is still low, then a power supply will help to locate the shorted part - likely a capacitor.

If you are confident there is no solder short - probably is not the case since you have some resistance and is not 0 ohms, then we can try with voltage injection.

The method is to configure your power supply to 0v8 (our choice of voltage) @ 2A / 3A setting. Then red probe on the pad that is with the low resistance and ground to the logic board.

You want to keep the injection voltage LOW so that in the case the same rail is shorted to a CPU power rail, the CPU will not get killed due to a high voltage injection.

You will not power the board but use this adjustable power supply to locate the faulty component.

This injection will smoke out the bad cap and you can check with a thermal camera or using IPA poured over the parts. Often, it is a tantalum (black in color) cap that gets shorted over time. As a rule, we do not use tantalum caps in our very high volume of our PCB builds. The ceramics cost more but we have products that are running 24/7 for 10++ years to date without a failure. Tantalums just suck.

PS: Watch this video to understand the idea on the voltage injection:

https://youtu.be/EWuzmIz7hOE

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

Tried voltage injection with the IPA-method, it seems that U7941 is heating up

[mon2]

Re: Macbook 820-3115 missing 3V42_G3H

Ok. Flux and carefully remove this part. Allow it to cool down and then check if the low resistance is indeed resolved. If it is, perhaps there was corrosion here and you just need to reflow the part with flux.

Be patient with flux & heating since this part has a metal belly that is soldered. Will take some time to remove the part.

Make a note of the pin # 1 orientation for the IC - perhaps a dot on the top side of the component. Take pics of the present component on the board.

This part may be defective if it continues to heat up or causes the low resistance to ground and will need to be replaced.

In general, Silego parts are programmed parts but this specific component may be a fixed function device. That is, if you just replace it, it should work.

https://www.aliexpress.com/item/1005...98971802%21sea

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

Removed the IC and the short was gone, checked the component but it doesn´t seem to be defective. Reseated the IC and the short comes back up

[mon2]

Re: Macbook 820-3115 missing 3V42_G3H

What is the resistance on the logic board without this part ?

You can also check the resistance between pin # 1 and the metal belly on the removed part.

Pin # 1 = Vdd (power pin that we are measuring for the short) & metal belly = ground on the part. Believe that you will observe a low resistance here. Confirm it.

The measurements appear to confirm that the component is internally defective unless you can see some short on the PCB traces / pads where this component was soldered.

Clean up the area with IPA and a q-tip to get a better view. It is a fail for Apple / Foxconn to use black solder mask - makes tracing the PCB tracks a real joy.

You can source a replacement part from Aliexpress or a donor board from Ebay, etc. - buy a defective cheap board for the spare parts. Just a suggestion.

PS: Need to do some fence repairs (not my choice) so will check back later today. I think that you have made some good progress with the logic board.

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

Without the IC Pin1 to Ground measures around 135kohms.
It doesn´t seem that any trace is shorted or defective.

Good luck on the fence repair thanks a lot

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

On the IC itself, it measures O.L between Pin 1 and Pin 9
but Pin 1 to Pin 5 is about 3 ohms.

[mon2]

Re: Macbook 820-3115 missing 3V42_G3H

Bingo. This part is defective with such a low resistance. It must be replaced.

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

Ordered another defective 820-3115 logic board for spare parts, hopefully this one will have a good IC. Should be here in 2-3 Days ����

[Nuld_]

Re: Macbook 820-3115 missing 3V42_G3H

So now I´ve replaced the IC with another one, Pin 1 is something around 125kohms and the short is gone. Is there anything else i should check for? Espacially on U7200?