ATX PSU: rails, ripple, and false “dead motherboard” calls

Power-supply sanity checks before condemning a desktop board (NLCRC rewrite of common shop practice). Basics - Paperclip / PS_ON test with a load — many modern PSUs need a load to stay on. - Measure 12 V, 5 V, 3.3 V under a real load (not floating). - Ripple on 12 V under GPU load causes random reboots that look like “bad RAM”. When to swap - Coil whine alone is not proof of failure. - Burning smell, bulging primary caps, or rails collapsing under load → replace/repair the PSU before deep board work. Safety - Mains-side work is lethal — if you are not trained for primary-side SMPS, replace the unit. Note PSU model, measured rails, and whether a donor PSU changes the symptom.