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Missing all power rails except PP3V3_G3H, SMC_RESET_L present 820-00875-01

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NovemberTurtle

New member
This A1708 had the logic board replaced in 2017 (USBC port problems, SSD not replaced), and a couple months later, it won?t turn on. Stayed on the shelf for a while, until I started to watch Youtube repair videos. I thought it must have been the TI USBC mux, so I got a USBC power meter, but that showed 5V -> 20V transition, and 0A current draw.

Further diagnostics showed a good PP3V3_G3H and PPDCIN_G3H_CHGR, but everything else was missing (PP1V8_S3, PP1V2_S3, PP3V3_S3, PPBUS_G3H- well, no battery connected.) SMC was suspect, but SMC_RESET_L is present (3.4V).

I doubt that all the rails are short to ground, so is the SMC suspect? How does one tell if the SMC is active and doing its job/jobs?

Any next step debug suggestions much appreciated.

No water damage, clean board.
 
2informaticos

2informaticos

Administrator
Staff member
If you get 20V from charger, SMC does something.
That can't assure is good 100%, but is not the first suspect.

Do you get steady 20V at PPDCIN_G3H, but nothing at PPBUS_G3H?
Post resistance to ground at F7000; diode mode, red probe to ground.
 
2informaticos

2informaticos

Administrator
Staff member
Check which side of F7000 remains 032 value; don't write again that end "V".
Then check if any big coil get similar value to ground.
Finally check for pure short between that coil and PPBUS_G3H...
 
N

NovemberTurtle

New member
The short was on the PPBUS_G3H_CPU branch, I isolated that by measuring the current sensing resistor's voltage drops of various branches. The CPU branch had a 3.3mV drop over 3mOhm, so that's 1A at 1.5V injection into PPBUS_3GH. I tried to find a hot spot all over the board, but nothing. The only thing heating up is the CPU, and not all 3 dies (shinning pieces of silicon), just the center (larger) CPU die.

Can I assume this is a CPU replacement?
 
N

NovemberTurtle

New member
Thanks for the diagnosis. What is the CPU doing with a connection to PPBUS_G3H? There is 18 volts on that bus, and cpu voltage levels should be >~1. Does anyone replace CPU or buy a new logic board? Apple wants $450, that's before they know I messed with it.
 
2informaticos

2informaticos

Administrator
Staff member
"What is the CPU doing with a connection to PPBUS_G3H?"
Nothing; you just don't read my posts...

Remove CPU power switches (U7210/20, U7410/20/30), one by one, and you'll see there will be no connection from CPU to PPBUS_G3H anymore...
 
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