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820-3115 only starting without screen attached

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  • 820-3115 only starting without screen attached

    This 820-3115 had waterdamage and wont start. I cleaned that and now it only starts if there is no monitor attached to it (internal and external). It i plug any monitor in fan just twitch once every time i press the power button. Without a screen it chimes but turns off after ~20 seconds. It beeps without ram and also stays on beeping forewer without ram (not turning off like it does with ram - i tested every combination of the two sticks in both slots and also one of mine - everytime with the same result.). If i "trick" it by starting it and instantly plugging in a external monitor after it chimed it shows a white screen but turns off same way as if there is no screen at all. I tested a known good LCD and it still wont start with the LCD connected. I measured all Pins of J9000 in diode mode and compared it betreen screen plugged in, screen unplugged and a working board (with screen plugged in and also unplugged) and i cant find any differences. So i am stuck.

  • #2
    Check if R7403 OK?

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    • #3
      R7403 = 181,7 kOhm (should be 182 kOhm), there was no Corrosion in that area.
      R7402 is also exact 150,0 kOhm as it should be
      Last edited by Fizzl92; 04-24-2019, 11:21 AM.

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      • #4
        Seems thatb turns off only when AXG voltage is enabled.
        Check for corroded traces, or components (resistor maybe) on CPU area...
        Also try to start in SMC bypass mode...

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        • #5
          There is not really any corrosion on the cpu Side. It was just a little bit near U7200 and J4510 aswell as j3502
          SMC Bypass did not help aswell

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          • #6
            Your symptom points to AXG issue.
            If fails to start even with (only) external monitor...

            Try new BIOS, just in case.
            You can also try removing Q3880; only internal LCD available then.
            Last edited by 2informaticos; 04-24-2019, 05:50 PM.

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            • #7
              I have no board with bios on it here today. IĀ“ill swap bios an monday (saying the original one).
              Could i also remove one of these instead of Q3880: R3881 or Q3805 or D3895? This should also stop thunderbold from working and would be easier to replace...

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              • #8
                R3881, or Q3805 disable TBT too.

                Do not swap the BIOS chip, reflash with new clean ME file.
                I suppose can be found on the forum...

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                • #9
                  As i want to disable thunderbold i want to check the voltage that U3890 prodces before i remove R3881 and after to make sure TBT is disabled. I measured without LCD and there are just 2,5 V at D3890 (both sides - diode mode measurement > 0,5 V at both sides - no short). That should be 12.6 V (if not boosting) or 0 V if i am right. Also i would expect it to be 0 V with R3881 removed. With R3881 back in place i get 12,6 V at the top of the voltage divider, 8,4 V at TBT_div_L and ~6 V at Pin 2 of R3881 (i expected that to be 0 V). So i came to Q3805 and the enable is 0 V. To be sure i am not out of mind here i removed Q3880 (as you suggested) and i still get 2,5 V at PPVIN_SW_T29BST and i cant understand where that voltage is coming from... But also with Q3881 removed board wont start with lcd plugged in...
                  Also i wrote down all parts that had corrosion on them as i got the board. Heres the list maybe that helps.


                  U3890
                  Q5480
                  U4900
                  R2570
                  D9498
                  U7542
                  U4582
                  U5701
                  U7200


                  U3890 is one of them. The corrosion was in the area of pins 1-10... R3889 and the traces to U3890 are fine.

                  Last edited by Fizzl92; 04-26-2019, 09:14 AM.

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                  • #10
                    I tested with a clean bios today but still the same behaviour. I am not sure but as far as i understand it couod be a TBT issue as the Voltage at D3895 is not correct. Should i try a new U3890?

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                    • #11
                      15V_T29 is enabled only if an external device is detected on the thunderbolt port.

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                      • #12
                        Even if i start it and plug in the external monitor while its on i get only 2.5 V at D3895 but it shows a questionmark folder. So thunderbold is working i guess. Is there anything else i can check in a case like this?

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                        • #13
                          Which pin of D3895?
                          High voltage should appear on K.
                          But you should get 5V on A, if Q3880 is enabled...

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                          • #14
                            I get on both sides 2.5 V at D3895. TBT_A_HV_EN is not there but at the same time i can see a question mark folder on external monitor plugged into J9400

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                            • #15
                              Did you reflash BIOS?

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