820-3032-A quarter fan spin - not powering on

[dudipupan]

Hello again,

I got another problem with a Macbook Air (820-3032-A).
I get a short fan spin (for ~1second) every 5 seconds - ALL_SYS_PWGD and all S0 rails are pulsing, S5 rails are stable and there is no Voltage on the VCORE rails.
On U2750 i get:
Pin 1: pulsing
Pin 2: pulsing
Pin 4: 0V
Pin 5: 3.3V

The board had some corrosion around C7710 and C7623.

Has anybody an idea what could cause this porblem?

Best regards

 

[dudipupan]

The MBA is also not starting in SMC bypass mode - just short fan spins.

 

[2informaticos]

I suppose you mean 3023, not 3032.

If pins 1 & 2 of u2750 pulse, then pin 4 MUST pulse too.
If not, change U2750.
Check diode mode to ground on its output.

BTW, pin 2 pulses because VCORE pulses too (not missing).

 

[dudipupan]

Thank you for your reply!
I replaced U2750 and now I got a fanspin. But PPVCORE_S0_AXG_R on L7550 is missing.
On Q7550 I got:
Pin 1/2: 8.5V
Pin 3: 0V
Pin 4: 0V
Pin 5: 0V
Pin 6/7/8: 0V

No short to ground on this pins.

But I got PPVCORE_S0_CPU_PH1, so U7400 seems to be powered?!

Thank you.

 

[2informaticos]

AXG refers to Intel graphics, which doesn't require power until high graphics load is detected.

So VCORE is stable now?
Check for USB activity, if no video present on internal LCD.

 

[dudipupan]

Okay, good to know - thank you - learned something new again

Yes my Vcore is stable 1.16V now.
I'm getting no USB activity. Already replaced the BIOS chip with a known working one - same result.
I'm getting fanspin with slowly and steady increasing speed

 

[dudipupan]

So dead CPU? :-(

 

[2informaticos]

No RAM beep error?

 

[dudipupan]

No, nothing.

 

[2informaticos]

Try heating PCH and CPU with good flux.
May have luck to be an issue caused by accumulated crap between BGA balls.

 

[dudipupan]

Unfortunately no luck :-(