820-00850-A without startup (A1989 MacBook Pro 2019)

[sekidata]

This 820-00850 board shows no discernible liquid damage but has an unknown history. The trackpad clicks, fans spin, but nothing else happens. DFU Restore was successful, with Apple logo and progress bar on target machine and padlock on host machine.

I have PPBUS 12.6V, PP3V3_G3H, PP5V_G3S, all AWAKE rails, and the SSD rails. The big coils have similar resistances to ground as on another board. The CPU area gets warm but there is no chime.

Anything else to check before I blame this on a bad U0500?

 

[2informaticos]

Machine gets stuck into progress bar?
Can't enter to recovery mode?

 

[sekidata]

There is no chime, no progress bar, no screen backlight, no response to startup keys.

 

[sekidata]

Upon further inspection it looks like this board is cycling: every few seconds I get these rails for several seconds, then back down to 0V: PPVCC_S0_CPU, PPVCCSA_S0_CPU, PPVCCIO_S0_CPU, along with cycling fans.

 

[2informaticos]

"DFU Restore was successful, with Apple logo and progress bar on target machine"
???

 

[sekidata]

Weirdly, that's correct. Sometimes, the DFU Restore fails, but usually I get video on the target machine (see below) and padlock on the host. Still, the machine reverts to cycling instead of chiming and progressing with the startup.

 

[2informaticos]

Does it stuck in progress bar, or restart before end of it?
Force the machine off and check if starts pressing COMMAND + R.

 

[sekidata]

As I said before, there is no chime, no progress bar, no screen backlight, no response to any startup keys. Just the fans and CPU voltages cycling on and off. I still suspect a short somewhere but haven't found it yet.

 

[2informaticos]

Very unlikely to have a short, once you get image after DFU completion.

 

[sekidata]

During the cycling, PPBUS goes from 12.6 to 12.3 V and so forth. Does this implicate the SMC (i.e. T2 chip)?

 

[2informaticos]

Try reflashing Soc ROM...

 

[sekidata]

Do you have a BIN file?

 

[2informaticos]

A1989 T2 Unlock Data.BIN

drive.google.com

Don't forget to make backup first...

 

[sekidata]

No change after a reflash of the SoC ROM. Still shows video during DFU Restore but never makes it to POST.

 

[sekidata]

Any other ideas?

 

[2informaticos]

Disconnect anything from MLB.
Check if voltages still pulsing.

 

[sekidata]

Since the ROM reflashing, the pulsing has changed to a more steady, stagnating state. The CPU rails are at their pre-POST levels and remain there, without the fluctuations usually seen around the chime time (VCore is a steady 0.9 V, for instance). Stays like this for a minute or two, then turns off.

Same behavior when I unplug everything from the board.

 

[2informaticos]

Probably CPU issue...

 

[sekidata]

Yup, good bye CPU.