Switching power supply issue.

Here's a good article about power supply troubleshooting. It doesn't cover the latest models but should be good up to ATX. https://www.repairfaq.org/sam/smpsfaq.htm

Hero999: Not to start an arguement, but I think all Nigel was trying to say is that there is more than one way to create a safe enviroment which depends on the situation. All methods have their advantages and disadvantages.

Anyhow, isn't the point of isolating the PSU to allow you to connect the scope probe's ground to a live portion of the PSU ? Shouldn't this ideally be done with a differential probe ?

Just a though, I'll admit that I have no experiance in this area.