High-value resistors (often in the kilo-ohm range) provide the initial "kick" to the PWM controller. If these go open-circuit, the board will stay dead.

Remember that the "Hot" side (primary) and "Cold" side (secondary) have different ground planes. Never connect your oscilloscope probe ground to the primary side unless using an isolation transformer. Safety Warning

Look for bulging tops. Even if they look fine, they can have high ESR (Equivalent Series Resistance). Common culprits on this board are the output filter caps.

If the fuse is blown, always check the bridge rectifier and the main switching MOSFET for shorts before replacing it. Tips for Reading the Schematic

If the MOSFET is shorted, there is a high probability the PWM chip is also fried.

Are you currently troubleshooting a like a blinking power LED or a completely dead unit?

If you are using the schematic to troubleshoot a "no power" or "flickering" issue, focus on these components:

The optocoupler (usually a 4-pin IC) bridges the hot and cold sides. It provides feedback to the controller to regulate output voltage. If the output voltage is "pumping" (cycling up and down), the feedback loop is often to blame.