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820-3115 Green Light, No Power, VCORE Shorted.

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SMMRepair

Member
Working on an 820-3115 board, and PPVCORE_S0_CPU is shorted to ground. I can tell the board has been worked on by Apple (black sticker, neat work, so I presume it was an Apple refurb) in the VCORE/U7400 area. Additionally, probe point 1329 is light-green from previous liquid. Board is clean otherwise. I've never dealt with a shorted VCORE before, but based on information from Louis' videos, it looks like the next step would be to replace Q7510, Q7520, Q7550 and U7400 all at once. I want to confirm that this is the proper next step before proceeding.

CPU gets warm (as normal--not hot, so I don't think it's getting a straight 12.55v), and PCH gets appropriately warm as well. I have green light and a slight fan twitch, but no vcore voltage (all other S0 rails are present).

Clean board, no sign of damage or liquid. Thanks!
 
S

SMMRepair

Member
Hey duke, it's a dead short via diode (.000) and 1.3ohms resistance to ground. A good board seems to be 3.2ohm.
 
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dukefawks

Administrator
Make sure the board you are comparing has the same CPU. CPU does not short for no reason so either that 1.3 is normal or something happened.
Also Apple exchange boards are GARBAGE. They all have had issues and were repaired too. If I see an Apple exchange board that failed it goes on the bottom of the pile and I mean 3 feet under ground.

Compare Vcore and AXG to GND, they are usually very close.
 
S

SMMRepair

Member
Hey Duke, yes the board was worked on in the U7400 area for sure. I can tell they replaced at least 8-10 resistors/caps all near U7400. U7400 itself even looks brand new (perfect edges, no chips, no stains from dirt/dust, perfectly bright pins, etc--perfect).

And yeah I was using an i7 board to measure; checking an i5 board shows 1.7ohms, so you're right.

How would you recommend proceeding? All other rails present except vcore.
 
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dukefawks

Administrator
Could be anything if ALL_SYS is present. As it was messed with you can not say if it is U7400. U7400 is enabled by communication with the CPU.
Check if ALL_SYS is high and check everything around U7400
 
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