820-3115 fan spin but no boot

[Fizzl92]

Hello,
this 3115 looks perfectly fine. Probably never disassembeled... It spins the fan if plugged in but thats it. No cime, No USB, no beep without ram. Its missing PPVCORE_SO_CPU, PP0V75_SO_DDRVTT and PPVCORE_SO_AXG. I followed that path and came to PLT_RESET_L missing. CPU_IMVP seems fine and CPU_VCORE has ~5 Ohms resistance to GND. Is that to low pointing to the CPU? I replaced U7400 and U2650 but no change. Any ideas what could cause this? Also i an not 100% sure how the power up sequence works here and what needs to be present first.

 

[2informaticos]

PLT_RESET_L is the last signal in power on sequence.
Do you get ALL_SYS_PWRGD?

Ivy Bridge CPU has a very low resistance to ground on core power rail.
Aprox 1 ohm for an i7.

 

[Fizzl92]

Thank you. ALL_SYS_PWRGD measures steady 3.308V at U2650.

 

[2informaticos]

Check basic voltages of U7400; pins 46, 19, 29, 47, 1, 48.
Be sure R7402/03 have correct ohm values and good solders.

 

[Fizzl92]

U7400 (measured at IC pin):
Pin 1: 0.533V
Pin 19: 5.06V
Pin 29: 5.06V
Pin 46: 5.01V
Pin 47: 3.306V
Pin 48: 0.645V

R7402 149,5 kOhm
R7403 182 kOhm
I resoldered both.

 

[2informaticos]

Just in case, try known good BIOS.

 

[Fizzl92]

I took a Bios chip from another 3115, flashed a new File with a clean ME on it but still no change.

 

[2informaticos]

Probably bad CPU, or PCH.
You can try heating them, to burn eventually accumulated crap; use quality flux.
Unless you find some bad resistor, or broken trace on CPU area.

 

[Fizzl92]

Thank you. I inspected that area already closely and havent found any issues. Also i had it in the UC after i cant fix it on my own. Ill try heating both - nothing to loose here.