820-00165-A boot issues

[Max7]

Without screen, SSD, flex cable: Permanent green light, full fan speed, no sings of booting, no usb activity
Without screen, SSD but with flex cable: Green/Orange light, full fan speed, apple chime, no usb activity
With screen, no SSD but with flex cable, Green/Orange light, full fan speed, apple chime repeating one after the other as if the board keeps resetting, no usb activity

Board in general very clear with no sings of corrosion, U5600 and R5600 seems to be replaced, so not sure if that area indicates to anything. However all voltage / diode readings at this are are good.

Edit: Boots up further with an SMC bypass, arrives to flashing question mark.

 

[2informaticos]

What flex cable did you reffer?

Fan spins to max if no trackpad board/flex connected.
Same if thermal issue is detected.

Booting in SMC bypass, should be caused by an I/V sensor; check the outputs from pages 41-43.

 

[Max7]

Sorry, I meant the flex cable between DC IN board and the LB

Also I am a bit confused, for example U5490, according to the schematics is a INA211 current sensing amp. But on the board that location is blank (there are pads, but no chip), no signs of it being removed and same on other boards. What am I missing ? Or what should a measure if the chip or resistor is missing ?

What am I looking for page 41-43 ISENSE outputs ? Most of these are showing low voltages around 50-60mV.

 

[2informaticos]

Several Isense chips are not installed.
In such case, corresponding R sense is shorted; nothing to check there.

You must check sensor outputs (if installed), when the machine is turned on in SMC bypass mode.
Start checking SMC_DCIN/PBUS_VSENSE and SMC_DCIN/BMON_ISENSE.

 

[Max7]

SMC_DCIN_VSENSE = 2.7V
SMC_PBUS_VSENSE = 1.3V
SMC_DCIN_ISENSE = 250mV
SMC_BMON_ISENSE = 65mV

Also not sure if this helps, but while it is in restart loop (without SMC bypass) SMC_CPUVR_ADJUST_ISENSE_R was cycling between 0.5V and 3.0V. Perhaps a side effect of restarts but wanted to mention.

 

[2informaticos]

"SMC_CPUVR_ADJUST_ISENSE_R was cycling between 0.5V and 3.0V"
Check SMC_CPU_ISENSE at same time.

You should check all sensors output with machine in SMC bypass.

 

[Max7]

Done, all outputs from pages 41-43 are below, taken during a successful boot using SMC-bypass.

I was sure I was seeing 1V on SMC_CPUVR_ADJUST_ISENSE on the first test (after multiple measurements), however after a reboot it start reporting 350mV which matches a good board.

SMC_HS_COMPUTING_ISENSE : 77-80mV
SMC_OTHER_HI_ISENSE : 21-22mV
SMC_P3V3S0_ISENSE : 32 mV
SMC_CAMERA_ISENSE : 23-24 mV
SMC_BMON_ISENSE : 66-67 mV
SMC_1V2S3_ISENSE : 284 mV
SMC_WLAN_ISENSE : 28-29 mV
SMC_SSD_ISENSE : 1.6-1.7 mV
SMC_LCDBKLT_ISENSE : 31 mV

SMC_PBUS_VSENSE : 1.378 V
SMC_DCIN_VSENSE : 3.426 V
SMC_CPU_VSENSE : 1.8 V
SMC_P1V05S0_VSENSE : 1.06 V
SMC_P1V05S0_ISENSE : 28 mV
SMC_CPU_ISENSE : 282 mV
SMC_CPUDDR_ISENSE : 33 mV
SMC_P3V3S5_ISENSE : 28 mV

SMC_CPUVR_ADJUST_ISENSE : 350 mV
SMC_PANEL_ISENSE : 32 mV
SMC_HS_COMP_ALERT_L : 3.324 V
SMC_CPU_IMON_ISENSE : 99-100 mV
SMC_BMON_COMP_ALERT_L : 3.32 V
SMC_BMON_DISCRETE_ISENSE : 2.2 mV

 

[2informaticos]

Test the machine again with original 45W charger (14.5V).
Or lower the lab PSU (if use one) to mentioned voltage; 20V is not proper voltage for an Air.
In fact, SMC_DCIN_VSENSE reports 20.5V to SMC...

 

[Max7]

It is a 45W branded charger which works fine with other a1466s. Tested using lab PSU at 14.5V as well, same behaviour.
Does SMC_DCIN_VSENSE : 3.426 V mean DCIN is actually 20.5V ? How do we calculate 20.5 from 3.426 ? Just trying to understand.

 

[2informaticos]

Then you have a problem with R5513/14 divider; inspect traces for corrosion.
Test with 33-36K in place of R5513, instead of actual 27K.
Check how changes SMC_DCIN_VSENSE.

 

[Max7]

Will do, what value am I expecting to see on SMC_DCIN_VSENSE ?

 

[2informaticos]

Vout=Vin*Rb/(Ra+Rb) is the classic formula for voltage divider.
Considering Ra=R5513 and Rb=R5514, do I need to mention what is Vin and Vout?

 

[Max7]

SMC_DCIN_VSENSE = 2.76 V, sorry if I mismeasured this one.

 

[2informaticos]

That is totally different thing; it means 16.5V DCIN.

So there must be another problem which prevent SMC to start the board in normal mode.
Check if AVREF_SMC voltage is good (C5125).

 

[Max7]

Thanks, PP3V3_S5_AVREF_SMC is 3.3 V

 

[2informaticos]

Maybe bad SMC, or broken solder/traces.
Reball it first...

 

[Max7]

Thank you, I will try this. Perhaps rather reflowing, rather than reballing as I don't have balls to do it. Pun intended

Quick one, is it possible to run diagnostics via D while still doing SMC bypass? Tried but it seems to be shutting down abruptly if I keep D down. Perhaps SMC is involved with diagnostics ?

 

[2informaticos]

You can reball SMC with solder paste, not balls; many videos available on Internet.

For diagnostic, SMC needs to read all I/V and temp sensors.
Starting in SMC bypass disables sensors reading.

 

[Max7]

Yep, thank you. As you may imagine reflow did not help. I will try reballing next, got paste but no stencil. Trying to find one.

Thanks for all the help so far.