FAQ
TL;DR: Symptom: ACT and PWR stay solid; in one case, 4 of 8 boards lacked the 1.2 V core rail; “4 have 3.3 and 1.8 voltages but the voltage from U16 … is missing.” [Elektroda, Maciej_Cichy, post #16742985]
Why it matters: This FAQ helps repairers quickly triage non‑booting Raspberry Pi boards with steady LEDs and seemingly correct rails.
Quick‑Facts
- Typical failing rail: 1.2 V core from U16 (core regulator) absent while 3.3 V and 1.8 V present. [Elektroda, Maciej_Cichy, post #16742985]
- Solid ACT+PWR with no boot was observed even when “converters provide the correct voltages.” [Elektroda, Maciej_Cichy, post #16742904]
- DMM on SD CLK can falsely show DC; it’s just the clock waveform. [Elektroda, zster, post #16743062]
- “Measuring voltages on the card interface makes no sense” without a scope or logic analyzer. [Elektroda, 2675900, post #16743600]
- ESD or GPIO overvoltage are credible root causes in damaged boards. [Elektroda, lukashb, post #16742972]
Quick Facts
- Typical failing rail: 1.2 V core from U16 (core regulator) absent while 3.3 V and 1.8 V present. [Elektroda, Maciej_Cichy, post #16742985]
- Solid ACT+PWR with no boot was observed even when “converters provide the correct voltages.” [Elektroda, Maciej_Cichy, post #16742904]
- DMM on SD CLK can falsely show DC; it’s just the clock waveform. [Elektroda, zster, post #16743062]
- “Measuring voltages on the card interface makes no sense” without a scope or logic analyzer. [Elektroda, 2675900, post #16743600]
- ESD or GPIO overvoltage are credible root causes in damaged boards. [Elektroda, lukashb, post #16742972]
What does it mean when ACT and PWR LEDs are solid but the Raspberry Pi won’t boot?
Users reported steady ACT and PWR LEDs with no boot even when regulators output expected voltages. This points to deeper faults than basic 5 V/3.3 V presence, such as missing core rail, SD interface issues, or SoC damage. Confirm all rails, especially the 1.2 V core, then check boot signals with a logic analyzer. Avoid assuming that correct converter outputs guarantee a healthy board. This symptom appeared across several damaged units in the thread and requires signal‑level diagnosis beyond a multimeter. [Elektroda, Maciej_Cichy, post #16742904]
What is the U16 chip on a Raspberry Pi in this context?
In this case, U16 is described as a controlled core power regulator for the SoC. When U16 fails or its output is absent, the 1.2 V core rail is missing, preventing boot even if 3.3 V and 1.8 V are present. Inspect U16 for physical damage or short circuits and verify its output under load. A missing core rail aligns with the steady‑LED, no‑boot behavior reported. [Elektroda, Maciej_Cichy, post #16743010]
How many boards showed each fault in the case study?
From eight damaged boards: one recovered and worked; three had all voltages yet did not start; one showed a short at U16; four had 3.3 V and 1.8 V but lacked the ~1.2 V core rail. This breakdown highlights that power‑good LEDs and nominal rails don’t ensure a bootable system. Use it as a triage map before rework. [Elektroda, Maciej_Cichy, post #16742985]
Could ESD or GPIO overvoltage cause this failure mode?
Yes. A respondent asked about ESD or exceeded input pin voltage as plausible damage paths. Such events can kill the SoC, PMIC, or regulators while leaving some rails seemingly correct. If boards come from unknown history, treat ESD and pin abuse as prime suspects during diagnostics and when deciding whether to invest in rework. [Elektroda, lukashb, post #16742972]
I read 3.3 V on the SD CLK pin—what’s really happening?
A DMM averages the SD clock and can display a misleading DC voltage. That reading is not a stable 3.3 V supply; it’s the clock waveform. Use an oscilloscope or logic analyzer to confirm clock presence and integrity. “A regular voltmeter will show DC voltage,” masking true signal behavior. [Elektroda, zster, post #16743062]
Does probing the SD card interface with a multimeter help?
No. “Measuring voltages on the card interface makes no sense” without time‑domain tools. The interface carries fast digital signals; a DMM cannot reveal if clocks start, if data toggles, or if levels meet thresholds. Use a scope or a logic analyzer to see activity and timing during boot. [Elektroda, 2675900, post #16743600]
How should I use a logic analyzer here?
First, check whether SD and core clocks start after power‑up. Then, trace signals step by step using the SoC documentation and schematics to localize the halt. As one expert put it, “That’s not Arduino”—expect disciplined signal‑level debugging rather than cookbook steps. [Elektroda, 2675900, post #16743885]
ACT LED blinks four times—what does that indicate in this thread?
One board “is close to firing,” then the ACT LED flashes four times, described as an SD read error. Swap SD cards, reimage known‑good firmware, and inspect the SD socket and traces. If the pattern repeats on known‑good media, suspect SoC, SDIO lines, or core power instability. [Elektroda, Maciej_Cichy, post #16743010]
Can replacing U16 bring a dead Raspberry Pi back to life?
A respondent noted a chipped U16 and suggested replacement might revive the board. However, removal and reballing or fine‑pitch soldering are non‑trivial. Evaluate your rework capability and the board’s overall condition before attempting. If the SoC is also damaged, replacement won’t help. [Elektroda, lukashb, post #16742995]
Why doesn’t pin “4” on the SD header show 3.3 V on a working unit?
The poster later measured a functional board and did not see 3.3 V on the referenced pin. This suggests pinout confusion or that the measured node wasn’t a power pin. Verify your exact board revision and follow known‑good pin maps before concluding a rail is missing. [Elektroda, Maciej_Cichy, post #16743315]
What’s the fastest 3‑step triage for steady ACT+PWR and no boot?
- Verify all rails under load: 5 V, 3.3 V, 1.8 V, and ~1.2 V core from U16.
- Inspect U16 for chips/shorts; confirm the 1.2 V rail isn’t missing.
- Check SD activity with a logic analyzer; don’t rely on a DMM.
[Elektroda, Maciej_Cichy, post #16742985]
What is a logic analyzer, and why use it over a DMM here?
A logic analyzer samples digital lines to reveal timing and state transitions. It shows whether SD clocks and data toggle during boot, which a DMM cannot. The thread’s guidance is clear: use a scope or logic analyzer to see if clocks start before deeper fault isolation. [Elektroda, 2675900, post #16743600]
