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820-3115b Fan spins no chime no image

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  • 820-3115b Fan spins no chime no image

    When I first got this Mac it sometimes had a chime and came on but it would cycle to black screen and them would display login screen. I shined a light on screen sometimes it would just be backlight not coming on, but the next time it would be no display at all. I didn't do anything to it beside testing a few points. It got worse where now it doesn't chime or display at all. Board looks clean but only little spot of corosion I could see was pin 1 on U7400 and u2600. Don't usually work on mac's and not a lot of time so thought I would see what you pro's thought. Thanks

  • #2
    Clean that corrosion points.
    Replace U2600, to discard any internal issue.
    Also replace R7402/03 and check their traces to U7400.
    Don't forget to disconnect battery, when work on the board!

    Post some voltages when the board doesn't start.
    Also try to start in SMC bypass mode.

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    • #3
      Ok so I found that the PP1V05_S0 Rail is shorted to ground. It was measuring .9 volts and fan would still come on. But while testing now it is .00 volts and charger light is still green but dim. With the PPVCORE_S0 rail shorted is it safe to put power supply on 1 volt and try to find short or is there risk to the cpu.? PPBUS_G3H is 12.58 V, PP5V_S0 is 5.07v Thanks I know this board isn't really worth the trouble but it is to me for learning. Thanks Also how do you start in SMC bypass mode. I know I need to fix the short first but....

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      • #4
        Post resistance to ground on VCORE in ohm mode, not diode mode.
        An i7 has a normal value close to 1-2 ohms...

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        • #5
          In ohms mode measures 2.0 ohm on PPVCORE_S0_CPU where resistor R1360 would be but its a No Stuff pad.

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          • #6
            I had .9 volts at for the cpu vcore. now I get .0 volts at L7510 and L7520. Wondering how the phases work on this circuit. At pin 1 on the Q7510 I get 12.57V which is PPVIN_S0_CPUIMVP Rail which is a G3 Hot rail. So what is best way to trouble shoot this circuit. Now sure how the phases work as far as testing them.

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            • #7
              When all OK, VCORE usually drops from 1.1-15V to 0.75-0.85V.
              So 0.9V was probably good there.
              What happened in the meantime, to got now 0V?

              "Also replace R7402/03 and check their traces to U7400."
              No reply about this...

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              • #8
                sorry I checked their traces and continuity to U7400 and resistors measured good. I don't have any replacement parts on hand so trying to just troubleshoot and order parts. So when I first got my hands on it. The computer would chime and come on but would go from start screen to black screen. Then after having on longer it got worse and wouldn't chime. So Im guessing the longer it is on whatever was partially failing when all the way out. there is some sings of water around Q7510 and Q7520. But not much. I tested u7400 and pin 47 should be the enable pin and and it has 3.3 volts on it. and pin# 46 VCC has 5 Volts on it and pin 29 VDDA and pin 19 VDDB has 5 Volts So it has power and is being told to turn on. But Im not sure which pin to test that are to tell Q7510 and Q7520 to make the 1.05 rail. That is if I have all this right. Thanks again.

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                • #9
                  Seems that you have corrosion on CPU area, maybe underneath BGA.
                  You can try to heat CPU and PCH with good flux; no more than 200 degrees.
                  Can also be a corroded resistor, or trace around U7400.
                  Also check all SPI Bus Series Termination resistors.
                  You can even try new BIOS with clean ME.

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                  • #10
                    Thanks will try. And what do you mean by new BIOS with clean ME?

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                    • #11
                      I mean clean ME region from your BIOS; using Medusa programmer, if available...

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