820-00928 No post

[tcampbell34]

Got a strange one here. Board is in excellent shape, with no signs of corrosion or damage anywhere. When you try and turn it on, you hear it chime very faintly, and then it just keeps chiming over and over again and never turns on. Nothing on the screen and cap lock does not react. CPU VCORE is stuck at 0.930v at this time, so it appears to not be fully booting. I read that the chime loop can be caused by a bad bios, so I flashed a known good bios with a clean ME to this board. Now when I try to turn it on with just the charger and no battery, it does not chime at all anymore and CPU VCORE is still stuck at 0.930v, but the backlight on the internal screen is on now, but there is no image. If I connect a USB drive to it, the light lights up and flashes, indicating CPU is working, but the keyboard does not work in this state. No reaction from the caps lock key. I then tried to start it SMC bypass mode and it turns on with normal fan spin, not high fan like it normally does in SMC bypass, but now I get a low battery symbol on the screen for a few seconds and then it turns off. CPU VCORE is correct at 0.680v when I do SMC bypass and get the low battery symbol. I can only get that low battery symbol when trying to do an SMC bypass. If I try and connect the charger and the battery, CPU vcore is stuck at 0.930v again and now screen is completely off, no backlight. Do you think its a SMC issue since it seems to try and start is SMC bypass?

 

[2informaticos]

Check if AVREF_SMC is correct.

 

[tcampbell34]

Yes it is correct at 3v

 

[2informaticos]

Check SMC_DCIN/PBUS_VSENSE and SMC_DCIN/BMON_ISENSE levels.

Be sure to use 85W original charger.

 

[tcampbell34]

I am using an original apple 96W charger actually.

PPBUS_S0_VSENSE - 12.56v
SMC_DCIN_VSENSE - 0v
SMC_DCIN_ISENSE - 176mv
SMC_BMON_ISENSE - 5.9mv

 

[2informaticos]

"PPBUS_S0_VSENSE - 12.56v"
I didn't ask that.

Check mentioned sensors level when you start in SMC bypass.

 

[tcampbell34]

Sorry wrong signal. SMC_PBUS_VSENSE is 2.1v both when I try and start normally or in SMC bypass.

Normal boot (Charger only):
SMC_PBUS_VSENSE - 2.1v
SMC_DCIN_VSENSE - 0v
SMC_DCIN_ISENSE - 176mv
SMC_BMON_ISENSE - 5.9mv

SMC Bypass:
SMC_PBUS_VSENSE - 2.1v
SMC_DCIN_VSENSE - 2.65v
SMC_DCIN_ISENSE - 195mv
SMC_BMON_ISENSE - 8.3mv

 

[2informaticos]

Levels look normal.
However, if only starts in SMC bypass, there should be a sensor issue.
When turned on, check all I/V sensors output from pages 50-53.

 

[tcampbell34]

SMC_CPU_HI_ISENSE - 44mv
SMC_CPU_IMON_ISENSE - 18mv
SMC_DDR1V2_ISENSE - 22mv
SMC_CPUDDR_ISENSE - 28mv
SMC_CPUHI_COMP_ALERT_L - 0
SMC_CPU_ISENSE - 6mv
SMC_SSDLIM_ISENSE - 21mv
SMC_CPUGT_IMON_ISENSE - 4mv
SMC_CPUSA_VSENSE - 0.9v
SMC_CPUSA_ISENSE - 17ma
SMC_CPUGT_ISENSE - 20ma

 

[2informaticos]

Not sure about SMC_CPUHI_COMP_ALERT_L, being marked as active low; compare with good board.

Any chance to start in Diagnostic mode?

 

[tcampbell34]

All the parts for SMC_CPUHI_COMP_ALERT_L are no stuff on these boards so 0v seems normal. I checked another board and it also has 0v there.

I cannot get this board to start at all. I tried another charger and did SMC bypass and now I do get high fan spin, so its definitely in SMC bypass, but has only backlight no image and CPU vcore is stuck at 0.9v. If I connect the battery and try to start it, CPU vcore gets stuck at 1.1v now instead of 0.9v and now the screen does not turn on at all.

 

[2informaticos]

If board only turns on in SMC bypass mode, there should be a SMC sensor issue.

 

[tcampbell34]

Well it does not really turn on it SMC bypass mode either, only shows low battery. While testing, I noticed that it will only show me low battery symbol and 0.6v on vcore when trying to start in SMC bypass mode with the charger connected to the UB300/400 side of the board. If I try the same on the other side, it turns on with high fan spin, faint backlight on the screen, but only 0.9v on vcore. I found that PP0V9_TBT_T_SVR was shorted to ground from UB000. I removed UB000 and have left it off the board for now. The short is gone, but still the same thing on this side, where it shows low battery when trying to start in SMC bypass. All other symptoms are the same also. The other weird thing on this board is PPVCCGT_S0_CPU is 0.7v all the time, even on boot. This is not normal, since this voltage only should be enabled under heavy graphic load. It seems like the board thinks its under heavy graphic load immediately on boot, so maybe that could point us in the right direction of what could be the issue?

 

[2informaticos]

Missing UB000 may affect communication with PCH; it may think HDMI (through thunderbolt) monitor is connected.

 

[tcampbell34]

I was getting 0.7v on PPVCCGT_S0_CPU even before removing UB000, but yea I am thinking something else is wrong on this board where it thinks HDMI is connected even when its not. Will try and see if I can find anything else wrong.

 

[2informaticos]

Mmmmhhh, this has dedicated GPU; connecting external monitor, should enable it.
If still get iGPU core voltage, may point to CPU problem...