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820-2936, Weird SMC Issue? No Green Light

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  • 820-2936, Weird SMC Issue? No Green Light

    Got an 820-2936 board in that had no green light and ppbus_g3h measured out at 12.26v--"great--the SMC". I checked SMC_RESET_L and it's high, so I pulled the SMC. No liquid or corrosion or anything, so I figured it just died. Replaced the SMC and same behavior--no green light, ppbus_g3h is 12.26v and no shorts detected. I checked SMC_RESET_L again to be sure, and it's still present/steady, so I replaced U5110 to be safe. Still same behavior. I replaced U7000 because I've seen some odd behavior related to it failing before as well, but no dice--same behavior. Thinking maybe I grabbed a bad SMC, I swapped it a 2nd time. Same issue. Out of frustration, I replaced the SMC a third time (totally different chip pulled from parts boards both times I replaced it), and still the same behavior. I can't figure out what would be pulling ppbus_g3h down (no short) and preventing green light.

    Any ideas? I thought for sure this was a bad SMC, but after replacing it 3 times and it not resolving the issue, there's clearly something else going on here. SMC_BC_ACOK at U7000 is 3.3v. No short on 3v42_g3h, and it's present/steady at 3.43v. This board had no liquid.

    All help/direction appreciated!

  • #2
    Sounds like a bunch of bad SMCs,

    Just to be safe check R4999. Any power on pin 2 ?

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    • #3
      R4999 is good, yep. Curious that 3 SMCs in a row are bad in the exact same manner...I might try replacing the SMC one more time, but I'm leaning strongly against the SMC being the issue right now, but stranger things have happened. Thanks, Gurmon!

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      • #4
        SMC not running. Could be the SMC_RESET line is not pulled low and then high after stable power. Short SMC_RESET manually to GND after applying power to initiate a "clean" reset. Could also be a Xtal issue or caps on the Xtal lines.

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