The following probably account for 95% or more of the common SMPS ailments:
Problem: Supply appears dead, fuse blown.
Cause: Shorted switchmode power transistor and other semiconductors, open fusable resistors, other bad parts. Note: actual cause of failure may be power surge/brownout/lightning strikes, random failure, or primary side electrolytic capacitor(s) with greatly reduced capacity or entirely open – test them before powering up the repaired unit.
Problem: Supply dead, fuse not blown.
Cause: Bad startup circuit (open startup resistors), open fusable resistors (due to shorted semiconductors), bad controller components.
Problem: One or more outputs out of tolerance or with excessive ripple at the line frequency (50/60 Hz) or twice the line frequency (100/120 Hz)
Cause: Dried up main filter capacitor(s) on rectified AC input.
Problem: One or more outputs out of tolerance or with excessive ripple at the switching frequency (10s of kHz typical)
Cause: Dried up or leaky filter capacitors on affected outputs.
Problem: Audible whine with low voltage on one or more outputs
Cause: Shorted semiconductors, faulty regulator circuitry resulting in overvoltage crowbar kicking in, faulty overvoltage sensing circuit or SCR, faulty controller.
Problem: Periodic power cycling, tweet-tweet, flub-flub, blinking power light.
Cause: shorted semiconductors, faulty over voltage or over current sensing components, bad controller. These symptoms are often very evident on many eMachines stock PSU’s, and other cheaply made supplies.
In all cases, bad solder connections are a possibility as well since there are usually large components in these supplies and soldering to their pins may not always be perfect. An excessive load can also result in most of these symptoms or may be the original cause of the failure. And don’t overlook the trivial: a line voltage select switch in the wrong position or between positions (possibly by accident when moving the supply, particularly with PCs), or damaged.
Note that a PSU’s maximum output load should not be held for normally more than 60 seconds, as at full load the circuitry will get hot, and can trip fuses. Excessive overloading will shorten the life of the supply.