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820-3115 only starting without screen attached

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Fizzl92

Member
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
 
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2informaticos

2informaticos

Administrator
Staff member
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...
 
F

Fizzl92

Member
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
 
2informaticos

2informaticos

Administrator
Staff member
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.
 
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F

Fizzl92

Member
I have no board with bios on it here today. I?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...
 
2informaticos

2informaticos

Administrator
Staff member
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...
 
F

Fizzl92

Member
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.
 
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F

Fizzl92

Member
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?
 
F

Fizzl92

Member
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?
 
F

Fizzl92

Member
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
 
F

Fizzl92

Member
I hope so. If you have one that is 100% sure working and it you would provide that i can try that aswell but as i get the exact same issue i guess thats not a bios thing.
 
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