Board A1278 mid 2012, 820-3115, fan spin, no chime

[VinceLx]

Hello,

Here are the symptoms:
- green light on charger ok, current around 0.7A
- fan spins ok
- no chime, no long beep even if no memory
- pulse of red light on mouse when starting, then nothing
- nothing on display

Measures:

ALL_SYS_PWRGD=3.32V
PPBUS_G3H=12.6V
PP3V42_G3H=3.48V
PP3V3_S5=3.33V
PP5V_S3=5.1V
PP5V_S0=5.1V
DPBUS_S5_HS_COMPUTING_ISNS=12.6V

PPVCORE_S0_CPU=1.12V
PPVCORE_S0_AXG=0V

PPVRTC_G3_OUT=3.33V
PP5V_S5=5.1V
PP5V_SUS=5.1V
PP3V3_SUS=3.33V
PP3V3_S3=3.3V
PP3V3_S0=3.3V
PP1V8_S0=1.8V
PP1V5_S3_DDR=1.51V
PP1V5_S3RS0=1.51V
PP1V5_S0=1.51V
PPVTTDDR_S3=0.75V
PP0V75_S0_DDRVTT=0.75V
PPVCCSA_S0-CPU=0.9V
PP1V05_SUS=1.04V
PP1V05_S0=1.05V
PP3V3_ENET=3.29V
PPVP_FW=12.75V
PP3V3_FW_FWPHY=3.31V
PP1V0_FW_FWPHY=0.99V

PP1V05_S0_PCH_VCCADPLL=1.06V

PP1V5_S3_CPU_VCCDQ=1.51V
PP1V05_S0_CPU_VCCPQE=1.06V
PP1V8_S0_CPU_VCCPLL_R=1.8v

All these voltages seem ok, except for PPVCORE_S0_AXG=0V, but for what I see on a working board this voltage does not come on when no video is displayed.

So here I don't know where to look now. Again the BIOS?

 

[2informaticos]

BIOS is the next locig step.
Just in case, check R7403.

When the board boots, you should note VCORE droping to aprox 0.75V
AXG voltage will come on under grapichs load; like playing YouTube.

 

[VinceLx]

R7403 and R7402 were ok.
I swapped the BIOS chip with another I know was ok, and the board started normally, blinking folder! Thanks again
Well noted also, PPVCORE_S0_CPU is 1.12V with the wrong chip (not booting), 0.80V with the good chip and booted
I am curious to know: can you tell which percentage of Macbook you repaired had a BIOS issue? And was it the chip itself, or reflashing a good BIOS was enough?

 

[2informaticos]

You need clean ME region to avoid any future issues.
The BIOS chip from other (same model) board has dirty ME region for your own!

I didn' count BIOS repairs; maybe 15-25%, nothing sure.
Macronix MX25L6405 chip (if I correctly remember the code) is known for internal issue; better to change it.
Never changed the chip in MacBooks, but did it several times in normal motherboards (not Apple).

 

[VinceLx]

15-25% of issues is a lot!
You have just motivated me to learn more about BIOS flashing. Do you recommend any tutorial on this, or a tutorial on ME region cleaning?

 

[2informaticos]

Buy Medusa programmer, to make your job easy on Apple stuff.
And read win-raid.com if you want to learn more about BIOS.
However, all you need, is already posted on our forum.