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820-00850-A Medium Liquid Damage Not Completely Loading OSX

Y

yourmacrepair

New member
Hi Guys, good evening. I got from a customer a A1989 EMC 3214, board 820-00850-A and it suffered medium liquid damage, in my opinion. I replaced some caps and res, some other ones I just reflux and it's turning on now. The problem is it doesn't load the OSX until the end, even from Internal or External drives.

The major problem I have now is, the R4802 lost its point 2, and looks like it's a point that sends energy to a lot of other points over the board. There's a whole in its place and I don't know how to solve this.

I run Apple diagnosis and no-fault was mentioned. The battery is still working good.

Any ideas?

Thanks, Eduardo
 
2informaticos

2informaticos

Administrator
Staff member
Oops, that's true.
I cannot find U4801 too.

As I saw in boardview, R4802 is part of PPBUS_HS_CPU sensor.
Not too many connections as you've mentioned.
Try starting in SMC bypass mode.
 
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2informaticos

2informaticos

Administrator
Staff member
We cannot get all schematics and boardviews from Apple.
There are several revisions and we are lucky if get one.
 
P

piernov

Moderator
Staff member
U4800/U4801 circuit can be found on 820-01700 schematics. It seems like it can throttle hard the CPU/GPU if it detects a voltage drop on PPBUS_G3H (below 7.5V I think).
R4802/R4800 is just a voltage divider from PPBUS_G3H to PBUS_DIVIDER (pin 1 of U4801) so that should be easily repairable.
 
Y

yourmacrepair

New member
2informaticos said:
Did you try to start in SMC bypass mode?
Just to see if loads OSX.
You can also try safe mode.
Click to expand...

I've made the SMC Bypass and the computer loaded the previous User login screen. when I put the login password to proceed, the computer restarts and it's like an ever ending cycle again...
Looks like there's something messed on the T2...

Thoughts?

Thanks again
 
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Y

yourmacrepair

New member
Hi guys... The customer didn't care about the data, so I went to restore mode on the DFU mode... It appears to complete the process. The point is that, when I boot the computer holding option to choose and external drive, I can see my USB drive and, as I click on it to proceed, at same time it goes to the Internet Recovery... It won't be a problem if, when I chose my wifi network, and goes to the password screen to enter it, the mouse starts stepping and it never finishes the connection and do not proceed...

I tried to start from Catalina and BigSur drives I have here.

BTW, I don't fix the issue on the R4802 yet... I don't have another resistor with same specs to put there.

Thanks guys.

Thoughts?
 
Y

yourmacrepair

New member
I'm back to work on this board... I stated that the R4802 lost its point 2 BUT it lost point 1 (PPBUS_HS_CPU). That's why I told you, it had a lot of other points on the board. I run a wire from this pin 1 to the other side of the board and the Internet recovery is now working and the computer is back to life.

Thanks for the help and you can close the thread.
 
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