820-3462 no start, power cycles every 5s

alex12347425

New member
Hi smart guys

Recently I spilled beer on my old macbook and now want to fix it. I am a hardware designer and dont do repairs, so no experience in troubleshooting. Please, help me with it.

So A1425 13 inch Macbook Pro Retina. It was on when I spilled bear and it immediately shut itself off. Took me about an hour to get to home and teardown. I was surprised how much of green corrosion grew for such short time, most of corrosion was around INA current sensors and mosfets at top part of pcb (where air vents), some around components around smc but not much. After cleaning pcb visually all is ok and not much damage. I found a broken connection on one of INA, but no changes after fixing connection. I checked U5410 and U5400 where were most of corrosion and I see with osciloscope that they works fine. I also checked a few voltage sense circuits to make sure their signals goes all the way to smc.

No boot.
Green light on charger than switches to amber(no battery connected)

PPBUS_G3H - 12.56V
SMC_RESET_L - 3.09V (when I measure it in diferent parts of pcb I see slightly diferent voltage)
PP3V42_G3H - 3.4V
PPV5_S5, PP3V3_S5 and other S5 and G3Hs are good and present.
All others S0..S4 powerails are off but jumps to designed voltage every 5s for a 200ms pulse.
I did not spot any damage component or short.
For me it feels like SMC tries to go to S0 state, but senses wrong voltage/current and shuts everything off, than cycle repeats in 5 seconds. Or it is indeed has a short circuit on one of sub rails.

Any ideas what to check?

Thanks in advance.
 

alex12347425

New member
Hm, after some more playing I found that U7400 is not starting. It is strange as I think I saw vcore before. Now it is just off. MAX15119GTM receives power and EN pin but does not trigger mosfets and does not output PGOOD. That I think the reason it stays in power cycle.
 

2informaticos

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

To discard SMC sensors problem, try starting in SMC bypass mode.
Test the board alone on the desk, nothing connected (apart of charger).
 

alex12347425

New member
I worked with it on desk when nothing is connected (only charger).
1 time board started (fans starts to spin), but after reconnecting the charger, it is same again.
It is first time I hear about SMC bypass, what is the right procedure? Should I connect keyboard and keep power button for 5-10seconds? The thing is that I am not sure keyboard is still alive.
 

alex12347425

New member
Hm. Thanks for the suggestion. I tried to connect charger while holding power button and now it behaves differently. It still does not work, but fans spins much longer and now I see activity from U7400, so seems my suggestion that U7400 is bad was wrong.
But it still does power cycles every 5s
 
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alex12347425

New member
Checked with oscilloscope, it is not longer. Same 200ms. I think fans just starts with max speed during SMC bypass and that gave me wrong impression that it works longer.
I checked PPVCORE_S0_CPU with oscilloscope, voltages goes to 1.2v for 50ms than slowly discharges back to 0.
Are you sure about page 43 & 44? In schematics that I have there is no page 43, and 44 has peripheral connectors.
 
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