820-3302 2012 iMac no power

prodigy83

Member
This iMac was powering on at first but experiencing the fan running at full speed, even when shut off. I would unplug the power cable, then plug back in without turning the iMac back on and the fan would spin at full speed again. The client took the iMac home over the weekend, but it powered off on her, and now there is no power.
Here is what I found. Power supply is good (had known good power supply I swapped).

Back to the motherboard:
I used a thermal camera and found U7100 appeared to be shorting, so I removed this chip and still had shorts on the following pins:
4,3,21,20,48,46,42,32.
Because where these appear to go, REG_CPUCORE_VSEN being one, does this mean the CPU is fried?
Thank you!
 

prodigy83

Member
Update, I did not realize the CPU was removable but when I took the cpu out, the short is gone, but I tried sticking in another LGA1155 intel processor I had in stock, and the short returned with this one too. I am thinking the processor is just completing the circuit, but I have no idea where to go from here.
 

2informaticos

Administrator
Staff member
Ivy Bridge CPU has very los resistance on its core; usually 1-3 ohm, depending of type (i7-i3).

Did you remove CMOS coil?
Any LED comes on?
Q7870 and Q7950 are the first suspects.
 

prodigy83

Member
No shorts on Q7870 or Q7950.
Only 1 LED light comes on (the one at the bottom).
I did remove the CMOS battery but not sure which one is the COIL?
I do have a short on Q8961 pins 7 and 8. When I search this in the schematic though, it finds nothing. I am seeing this using the boardview.
 

prodigy83

Member
U8000 keeps lighting up when I inject voltage in to C8962 pin 1. I keep finding shorts that correspond to U8000.
R8910 pin 2 also has a short.
 

2informaticos

Administrator
Staff member
Sorry about "coil"; read "cell" instead.

If LED1 stays on, Q7870 is good.
Nobody said to check for short on it, nor at Q7950.
These MOSFETs suffer by leakage usually.
Trigger SMC_ONOFF_L and check for pulse at 12V_S0_FET.

"I do have a short on Q8961 pins 7 and 8."
Maybe, but GPU core has also low resistance to ground; compare with similar board.
 
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