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820-00875-A - No backlight - A stupid noob error

nameless

nameless

Member
Hey everybody

Nice to be here ))

I made a very stupid error. A customer asked me to replace a cracked screen on A1708 EMC2978 Macbook pro.

So I did )) forgetting to disconnect the battery o_O

And now no backlight on the machine, and i'm sure it works as we can see the image. Same thing with old cracked screen. As I thought the new screen might be wrong or faulty.

Here some voltage details :

On disassabled board / no screen connected :

Power consumption at wall :
7.1 volts / 0.085 amps

PP5V_S0_BKLT / R8444 - 5.11V
U8400 pin 17,18 - 0V (C8442 is absent)
U8400 pin 5 - 5V
C8473 - 0V
Q8400 - pins 3,4 - 13V, pins 1,2,5,6 - 0V

Hope i measured right components, kindly let me know if you need more info.

Best regards
 
2informaticos

2informaticos

Administrator
Staff member
First of all, welcome to the forum.

Stop to diagnose backlight problems (nor video related), without LCD connected.

USB-C ports should switch to 20V, not 7V.
Do you get any voltage at L7210/20?
 
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2informaticos

2informaticos

Administrator
Staff member
Then the board doesn't even boot now.
Turning on and booting, is not the same thing.

Now keep the LCD disconnected, as any other device.
Only need the charger, untill get correct CPU core voltage.
Do you get L7370 and L7730 voltage?

Check for short at C8441; if pin 5 has 5V, pin 18 should be the same.
If not, replace R8445.

BTW, write directly into Reply box, instead of clicking Reply button.
Not a real reason to quote an entire post, visible few centimeters above.
 
nameless

nameless

Member
okok noted )

L7370 - 1.122V on one side and 1.059V on the other
L7730 - 1.06V on both

C8441 - 0V on both sides - no bip to ground (photo)
R8445 - 5.11 V on both sides
 

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2informaticos

2informaticos

Administrator
Staff member
As per schematic, C8441 and R8445 have a common point.
Can you explain the voltage difference?
Check trace continuity.

BTW, stop to check voltage on both pads of a coil.
ALWAYS check the output, where the filter cap is connected.
 
nameless

nameless

Member
hmm weird

Ye you are right I do have same voltages (5.18V) on both C8441 and R8445

Double-checked L7370 and L7730 - voltages are 1.05V and 1.06V

Also was wrong I do have pins 5 and 18 voltages !

those are filter caps right ? (photo)
 

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nameless

nameless

Member
ok invested in some usb-c amp meter ))

The board pulling 20V / 0.41A on both ports and CPU getting hot, so I suppose it boots

The U8400 does have VDDD - 5V but no FB on pin 21
I do get 13V on Q8400 - pin 4 and 12.96V on pin 3

but nothing on Drain pins 1,2,5,6

is the Q8400 dead ?

btw the screen is now attached to mb )

Thanks for your support
 
2informaticos

2informaticos

Administrator
Staff member
BKLT_SD must go down, to open Q8400; don't touch it.

Do you get image for sure?
Is not possible, without L7210/20 voltage...
 
2informaticos

2informaticos

Administrator
Staff member
No image for sure in this case, nor USB activity; because of missing CPU core voltage.
Your board doesn't really boot, just stays on.

L7370 and L7730 voltage still present?
 
nameless

nameless

Member
hmm that would be unfortunate

How could that even be possible as I saw it boot and picture(no backlight)

and now it doesn't even turn on ?

couldn't it be a bad U7100 or U7210 ?

getting all those:

PM_EN_CPUVCC_R = 3.219V
CPU_VIDALERT_L = 111 mV
CPU_VCCST_PWRGD = 0.990V
F7000 = 13V

I couldn't fry cpu just by doing some measurements, could I ?

Also I'm getting exact same voltages as here https://boards.rossmanngroup.com/threads/a1708-dont-turn-on-after-taking-apart-820-00875.56105/

But I couldn't find anything knocked off)))
 

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2informaticos

2informaticos

Administrator
Staff member
CPU core voltage is not controlled just by enable signal.
Part of CPU communicates with PCH through VID bus; both are in the same package on this board.
BIOS is also involved there.

CPU gets more volatges than core.
But is not normal to get hot when heatsink is installed, in such case.

This board should start with 1V, or bit higher on CPU core (L7210/20) and drops to 0.6V after 5-10s, when the board boots.
Do not expect iGPU voltage (L7410/20/30); it only appears under high load graphics.

Test the board alone, no SSD, LCD, TP/KB; nothing else than USB port.
Be sure to make good contacts when measure; you can use C10G0/H1 as test points, instead of L7210/20.

BTW, check L7900 and L8060 voltage too.
 
2informaticos

2informaticos

Administrator
Staff member
Something happened with PCH/CPU.
Inspect under microscope that area (both sides), especially around U7100; does it get pins 41 & 42 voltage?
If cannot find anything, then very likely bad CPU.

You can try known good BIOS, as last step.
 
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