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820-3115 Power rail issue

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  • 820-3115 Power rail issue

    so I have an 820-3115 on the bench with a strange issue. PP5V_S5 cycles from 3.3V to 4.5V then to 5v over and over again. Is this caused by something opening and closing? doesn't seem like a short would do this. I'm going to keep looking. Board has no signs of liquid damage or corrosion.

  • #2
    Test the board alone on the desk, no TP/KB or other device connected.
    USB mouse is good enough for test.

    Any pulse at 3V3/5V, or other coils?

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    • #3
      I'll have to check for pulses. I have the board on the bench by itself by the way. does it need RAM in the slots? I have one stick in. What would cause the 5V rail to oscillate like that?

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      • #4
        If SMC detects a short or overcurrent, will switch off and cycles SMC_PM_G2_EN.

        BTW, try to start in SMC bypass mode.

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        • #5
          Ok I will try that, thanks!

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          • #6
            tried SMC bypass. No go, I get PPBUS_G3H, PP3V42, etc. drawing 18mA. But PP5V_S5 is not stable

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            • #7
              "Any pulse at 3V3/5V, or other coils?
              --------------------------
              I'll have to check for pulses."
              ???

              "does it need RAM in the slots?"
              No, for power tests...

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              • #8
                I found that PP5V_S3 is shorted to GND. Know of any common causes?

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                • #9
                  and that rail is pulsing

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                  • #10
                    Is it safe to inject voltage on this line? P5VSUS_EN_L IS bouncing around too. But I suspect everything on PP5V will be

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                    • #11
                      "I found that PP5V_S3 is shorted to GND."
                      Exact ohm value?
                      Always allow 15-20s after power is removed, before measuring in ohm scale (or diode mode).
                      If low enough injecting voltage could help to find the culprit.

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                      • #12
                        0.2 ohms short

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                        • #13
                          Measured by U7200

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                          • #14
                            0.2 ohm at L7260 is really short.
                            But low enough to find the culprit with voltage injection method.
                            Start with 1V and go high a bit, if needed...

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                            • #15
                              Will do. I'll post back with results. Should I remove L7260 to inject? or is there a better place. I am worried the memory slot plastics may melt.

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