Apple 820-3115 b not posting - shorted cpu?

[justin35]

I don't know the history of the board... it does look clean, i couldn't find anything then paper towel resitues on a few places...
The board has fan spin
It deeps without ram stick
Pch and cpu get a little too hot...
Measured ppvcore_s0_axg and i get 0.8 Ohms...
On any other board i get 15+ ohms
So i assume is shorted. I took out R7550 to see on which side is the short and is on the cpu side...
also i did power the board without r7550 and a saw a little flas on the mouse diode, so it tried to post but went off.
There are a few caps on that rail on cpu side, and i want to think one is bad and not the cpu...
If i inject 1v @ 0.5 to 1amp on that rail will it fry anything else on that rail?

I know ppvcore_s0_axg is ment to be present only if the internal display is connected, right?

[piernov]

Re: Apple 820-3115 b not posting - shorted cpu?

Yep you're right.

Apparently VCore is not shorted and you have 0.8 ohms on VCCAXG, so I'd say that the CPU itself is not shorted but rather some component on the VCCAXG line.
You can inject 1v, but don't go higher (maybe one or two hundreds millivolts more max) as you may risk killing the CPU.
No real need to limit current there, it will probably not dissipate much power to begin with so the short may not become apparent right away.

[justin35]

Re: Apple 820-3115 b not posting - shorted cpu?

Well...bad news for me
At 1v it takes 0.3-0.4 amps and nothing is getting warm...
At 1.3v it takes 1.1amps and cpu starts to heat on the axg side...
It looks like the cpu is bad

[justin35]

Re: Apple 820-3115 b not posting - shorted cpu?

Is it possible to have spinning fan if missing all_sys_pwrgd? (Different board but i don’t want to open a new thread yet)
Ty!

[justin35]

Re: Apple 820-3115 b not posting - shorted cpu?

Anyone?!? Thanks!!

[justin35]

Re: Apple 820-3115 b not posting - shorted cpu?

Well i can confirm the cpu axg side was shorted...
pulled out cpu short is gone
Measured cpu axg pins to gnd ang get 0.8ohms to 1.5ohms...
Another donor (

[justin35]

Re: Apple 820-3115 b not posting - shorted cpu?

Sorry for reviving this post, but my problem is related...
Maybe some of you know and can help me understand something...
I have multiple 3115 boards that have cpu axg line measuring arround 2 ohms...
All of them are not working by some other faults But i wonder if it worh the effort to fix them and realize the cpu is “shorted”
The questions is:
What is the lowest ohm value measured on cpu axg and the cpu still works?

I only have one working 3115 board and it measures 8.1ohms

Thanks!!!!

[jasko_jacker]

Re: Apple 820-3115 b not posting - shorted cpu?

With multimeter in diode mode, you read 000 ?

[justin35]

Re: Apple 820-3115 b not posting - shorted cpu?

In diode mode reding 0.001

[piernov]

Re: Apple 820-3115 b not posting - shorted cpu?

I don't know the exact figures but above 2 ohms isn't alarming. Exact figure will depends on board, CPU model, temperature and may even vary from board to board.
Usually Ivy Bridges have pretty low resistance to ground compared to other generations.

[justin35]

Re: Apple 820-3115 b not posting - shorted cpu?

Thanks again piernov!!!
You are right! I’ve gor my hands on a 3115 board 2.5 i5 which works and measured cpu acg and it was 2.2ohms !!!