Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

Where an SMPS doesn't start and it isn't obvious why, it might help to drive the chopper from an external signal source to see what then works. The only time this is really practical is where a single transistor or MOSFET is used - generating a push-pull waveform probably isn't worth it.

This will always be a risky procedure both for you and the power supply. The switching frequency is likely unknown but for these tests you can assume it is in the 10 to kHz range. You can reduce the risk somewhat (to the supply at least) by using a series light bulb load and/or running on reduced line voltage. The most important thing to avoid is putting in an excessively long drive pulse which will result in the high frequency transformer saturating, huge amounts of current, and likely a dead transistor and possibly other parts if there is nothing to limit the current. If you have the option, start with a narrow pulse waveform to minimize on-time and don't push your luck! :)

Similarly, where a power supply attempts to start but cycles or shuts down, consider powering the controller chip from a separate supply to eliminate any issues of the transformer derived voltage that normally runs it after startup.