Repairing a 450 watt switching power supply is a dangerous business. I remember a few years ago after exploding the ground clip on a Tek oscilloscope, I decided to approach it in a safe manner. This time I’m using my DSOX3024 and I didn’t take any chances. If you’re testing the soft start circuit, use an isolation transformer. With respect to the AC mains, the PWM ground reference could have about 320 volts or more. While connecting the probe, make sure that the power supply capacitors are discharged. Ten minutes ago I didn’t do that and because I touched something I shouldn’t, I have a small electrical burn on my pinkie. The stored energy was 285v at 122 joules.
To bring up the power supply slowly I used a Variac in front of the isolation transformer. The scope was connected to the PWM ground and I used an external power source to power the PWM chip. The external supply and scope were referenced to the same ground. With this setup I was able to move the scope ground, with out worring about an arc flash, to the secondary and back to the primary side.