820-02016 A2337 Air board with fluctuating voltages

[sekidata]

This 820-02016 board was a test board in a shop and abruptly stopped working during the testing of some bad audio boards. USB-C is at 5V, PPBUS is 12.3V, and PP3V8AON is 3.8V. Many secondary rails are fluctuating between 0 and their expected values, about every 1/2 sec. This includes PP1V25_S2, which makes getting into DFU mode impossible. Any ideas?

 

[2informaticos]

Possibly damage on M1 data lines connected to audio board.
Compare readings in diode mode with good board.

 

[sekidata]

I2C_SPKRAMP_R_CODEC_SCL: 0.32 (good boards: 0.34, 0.33)
I2C_SPKRAMP_R_CODEC_SDA: 0.33 (good boards: 0.33, 0.34)

 

[2informaticos]

I mean all signal/data lines which go to M1; not only I2C bus.

 

[sekidata]

The other data lines look ok:

TOUCHID_PWR_EN: 0.27 (good boards: 0.28, 0.27)
SPKRAMP_INT_L: 0.004 (good boards: 0.005, 0.004)
CODEC_INT_L: 0.32 (good boards: 0.33, 0.33)
CODEC_WAKE_L: 0.38 (good boards: 0.38, 0.38)
TDM_CODEC_D2R: 0.33 (good boards: 0.34, 0.34)
TDM_CODEC_R2D: 0.34 (good boards: 0.35, 0.34)
TDM_CODEC_FSYNC: 0.33 (good boards: 0.34, 0.34)
TDM_CODEC_BCLK: 0.33 (good boards: 0.35, 0.34)
TOUCHID_INT: 0.33 (good boards: 0.33, 0.33)
CODEC_RESET_L: 0.32 (good boards: 0.35, 0.33)
SPI_1V8_TOUCHID_MOSI: 0.60 (good boards: 0.58, 0.59)

The only difference in any JE900 pins I could find was the power rail PP1V8_AON:

PPBUS_AON: 0.50 (good boards: 0.48, 0.47)
PP1V8_AON: 0.21 (good boards: 0.48, 0.53)
PP3V8_AON_VDDMAIN: 0.17 (good boards: 0.17, 0.18)
PP1V8_AWAKE: 0.34 (good boards: 0.35, 0.34)
PP1V25_AWAKE_IO: 0.27 (good boards: 0.25, 0.26 )

 

[2informaticos]

"SPKRAMP_INT_L: 0.004 (good boards: 0.005, 0.004)"
I'm pretty sure isn't true.
Be sure CE903 is not corroded.

 

[sekidata]

Interesting. I found an error in the board view file. CE903 should be flipped around: on all three boards, ground is on the left, not on the right. Explains the low diode readings.

Actual readings are SPKRAMP_INT_L: 0.32 (good boards: 0.0.33, 0.32)

 

[2informaticos]

Compare readings for audio boards too.
This way you may find an info about which line may be damaged.

 

[sekidata]

I'm trying to understand the fluctuating power rail issue. The wildly fluctuating voltages are all buck converter outputs of the PMUs U7700 and U8100. The PMU power input is steady at 3.8V. The LDOs (PP1V5_VLDOINT_MPMU, CAP_PPSPMU_VLDO9, PP1V5_VLDOINT_SPMU) are steady. What tells the PMUs to rapidly turn on and off?

 

[2informaticos]

Don't make confusion between inputs and outputs; you've mentioned only one LDO (from SPMU).
LDO stands for Low Drop Output (voltage).

You can find MPMU LDOs at page 34 (only 4 used) and page 29 for SPMU (6 used).

If you get pulse on the PMUs coils, then LDOs should be good; check them, just in case.
Probably an overcurrent is detected on a secondary power rail.

 

[sekidata]

Thanks, you're right, I got confused by the "LDO" in the rail name. However, note that the schematic does say that PP1V5_VLDOINT_SPMU is an output.

Here are my complete output voltages for U7700 and U8100:

SPMU:
PP0V72_S2_VDD_LOW: fluctuating 0-0.7V
PP1V2_AWAKE_PLL: fluctuating 0-1.2V
CAP_PPSPMU_VLDO9: 1.8V
PP0V855_S2SW_CIO: fluctuating 0-0.8V
PP0V805_S1_VDD_FIXED: fluctuating 0-0.8V
PP1V2_S2_CIO: fluctuating 0-1.2V
PP5V_BSTLQ_VOUT_SPMU: 5V
PP1V5_VLDOINT_SPMU: 1.5V

MPMU:
PP1V2_S2: fluctuating 0-1.2V
PP3V3_S2_UPC: fluctuating 0-3.3V
PP1V8_AON_MPMU: 1.8V
MPMU_VREF_ADC: 0V

 

[sekidata]

If there is an overcurrent situation, how would you find the culprit?

 

[2informaticos]

You should check every secondary power rail.

"MPMU_VREF_ADC: 0V"
Strange, check in diode mode to ground.

 

[sekidata]

MPMU_VREF_ADC shows 0.58 (300k on Ohmmeter), which is exactly the same as on a good board. Perhaps the MPMU U8100 is bad?

 

[2informaticos]

On the working board, what is the voltage?

 

[sekidata]

1.5V

 

[2informaticos]

Then should have bad U8100.

 

[sekidata]

Donor board is the only source available, correct?

 

[2informaticos]

PMUs are interchangable between 14" and 16" M1 machines.

 

[sekidata]

I replaced U8100 from an identical Air 2020 donor board. However, the situation is unchanged, with rapid voltage fluctuations on all PMU outputs. MPMU_VREF_ADC is still 0V.