If a circuit board has no expected supply voltage, begin at the source and move toward the load. Random measurements often create confusion because they do not show where the power path is interrupted.
Confirm the input first
Verify the external adapter, connector, fuse and input protection stage. Check both the voltage level and polarity. A correct adapter voltage does not guarantee that the voltage reaches the board after the connector or protection circuitry.
Follow the rail stage by stage
Identify regulators, DC-DC converters, inductors, protection MOSFETs and zero-ohm links. Measure the input and output of each stage. The point where a valid voltage becomes missing or abnormal is a strong clue.
Check for a short before applying more power
With power removed, measure resistance to ground on the missing rail. A shorted load can force a regulator into current limit or shutdown and make the output appear completely dead.
Check enable and control signals
Many regulators do not start simply because input voltage is present. Enable, power-good, reset and sequencing signals may be required. Consult the component datasheet when the control logic is unclear.
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