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M2 A2681 Air 820-02536 with USB-C liquid damage

S

sekidata

Member
This is my first board with the M2 architecture. This 2022 Air arrived with liquid-damaged USB-C port connectors and a few small spots of corrosion on the logic board. After cleaning, I found a short on LP800 and removal of the backlight controller UP800 removed the short. Now the board sits at 5.2V 0.03A USB-C with PPBUS 12.3V for plugging into USB-C ports 1 and 5. Port 0 produces PPBUS = 0V.

PPBUS_AON: 12.3V (0.35 diode, >10 MΩ)
PP3V8_AON: 0V (0.17 diode, 2 MΩ)
PP3V3_AON: 0V (0.51 diode, 2 MΩ)
PP3V3_S2: 0V (0.47 diode, 5 MΩ)
PP5V_S2: 0V (0.04 diode, 43 Ω)

Any words of advice as I continue troubleshooting?
 
Last edited:
2informaticos

2informaticos

Administrator
Staff member
"PP5V_S2: 0V (0.04 diode, 43 Ω)"
That's a big problem; find the cuprit.
Voltage injection method should help, if you have a good IR camera.
 
S

sekidata

Member
I found the culprit (it was UP700) and removing it removed the short.

Now, however, I'm afraid to put LC320 back into the PP5V_S2 circuit because the step-down converter produces 12V (measured at P5VS2_SW) instead of 5.2V. I already replaced UC300 but the output is still nowhere near 5V.
 
2informaticos

2informaticos

Administrator
Staff member
You can't check 5V_S2 without LC320!!!
Learn how a buck converter works.

Be sure all resistors and traces from feedback network (RC322-5) are good.
Also pay atention to R/CC332.
I recommend to change UC300 too.
 
S

sekidata

Member
To clarify, I did have LC320 connected, but off the board. I also connected the FB and BIAS line. But you're right: there was a damaged resistor in the feedback resistor divider. The circuit is correctly producing 5.2V now.
 
S

sekidata

Member
It's clear that I have one bad CD3217 on the board. Questions:
1. Can I replace a single CD3217 from a donor board, or do I need to swap all three?
2. Can the donor board be from a related model such as the 15" Air (A2941)?
 
S

sekidata

Member
Thank you. I also seem to have an issue on the PP3V8_AON line. The rail shows 3.8V for a moment, then drops to 0V, and continues cycling. Compared to a good board I get about half of the diode mode reading on this rail, but no short. I removed the 3 sense resistors R5800/5820/5840 and I get steady 3.8, 2.8, and 3.0V readings. Some circuit powered by PP3V8 is drawing too much current, correct?
 
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