820-00840 with shorted PPBUS_G3H

sekidata

Member
I’m new to the forum but have some limited experience with simple MacBook logic board repairs.

I’m working on a A1708 board (820-00840) that presented with PPBUS_G3H shorted to ground. No obvious liquid exposure. I removed F7000, injected 2 V at PPBUS_G3H, which heated up the CPU power MOSFET Q7960. I removed Q7960 and localized the short to PPVCCIO_S0G (< 1 Ω). I tried injecting 1.2 V at PPVCCIO_S0G, but nothing seems to get warm after a few minutes (10 A power supply). How can I definitively say the CPU is shorted?
 

2informaticos

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

U7800 can be damaged too.
Lift L7960 and check where the short remains.

Do NOT inject again in the same manner.
You MUST discard first a shorted high-side MOSFET from secondary power supplies.
We talked about this so many times on the forum.
Even 2V could damage the CPU.
 

sekidata

Member
Thank you. After lifting L7960 I get a 0.2 Ohm short to ground on PPVCCIO_S0G. Notably, I also get 1 Ohm short on PPVCCPCH_S5G. Safe to assume that this CPU is toast?
 

sekidata

Member
For next time, what's the resistance to ground I should expect for these CPU/PCH power rails (PPVCCIO_S0G, PPVCCPCH_S5G, PP1V2_S3, etc)? Are these all low-resistance (<100 Ohm) power rails?
 

2informaticos

Administrator
Staff member
Take notes from repaired boards.
Is the safest way to make your personal database.
Powerfull CPUs have very low values, 1-2 ohm...
 
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