Bad startup circuit - initial base (gate) drive is often provided by a high value, high power resistor or resistors from the rectified AC voltage. These can simply open for no good reason.
Symptoms: In this case the supply will appear totally dead but all the semiconductors will check out and no fuses will blow. Check the startup resistors with an ohmmeter - power resistors in the AC line input section. WARNING: there will be full voltage on the main filter capacitor(s) - 1X or 2X peak or around 160 or 320 VDC depending on design. Discharge before probing.
Dried up capacitors - either input or output side.
Symptoms: The main filter capacitor may dry up or open and cause the output to be pulsing at 60 (50) or 120 (100) Hz and all kinds of regulation problems. Measure voltage across main filter capacitor(s). If the reading is low and drops to a much lower value or 0 instantly upon pulling the plug, then one of these capacitors may be open or dried up. If you have an oscilloscope, monitor for ripple (use an isolation transformer!!). Excess ripple under moderate load is an indication of a dried up or open capacitor.
In extreme cases, a main filter capacitor with greatly reduce capacity or that is totally open may result in failure of the switchmode transistor and a dead supply that blows fuses or fusable resistors. Therefore, it is always a good idea to test the electrolytic capacitors whenever repairing a SMPS that has blown its switchmode transistor.
Capacitors in the low voltage section may fail causing regulation problems. Sometimes there are slew rate limiting capacitors which feed from the primary output to the regulator controller to limit initial in-rush and overshoot. A failure of one of these may mess up regulation at the very least. For example, excess leakage may reduce the output of the main output (and as a consequence, all the others as well).
Where a controller like a UC3842 is used, a failure of the capacitor on its Vcc pin may result in a aborted startup or cycling behavior as it is starved for juice each time it pulses the switchmode power transistor:
(From: John Hopkins (email@example.com).)
"I have encountered a bad cap (10uf 35v) on the Vcc input of a UC3842 IC in the power supply. Turn unit on, get very short burst of power supply output, then nothing. Every time the 3842 output a pulse, it ran out of VCC. Small part, big problem."