Sometimes the additional connection to a scope (with its separate connection to protective earth!) can add a new set of problems. At least, when measuring mains related stuff. Ideal here would be the use of a battery powered scope.


Oh yes, that's true! These SMPS can behave quite terribly. Mains voltage dips can cause spikes and overvoltages at the regulated output voltages!
When measuring the regulated voltages with a scope, the PC's and scope's mains wires should be plugged into the same mains socket, next to another. This will minimize the effect of voltage drops across the protective earth wire between them, eventually caused by motor currents running through a mains filter with Y-caps.

A good idea is also to use a digital voltmeter with peak reading feature. Even if the dips or spikes are too narrow to show a precise result, the meter will at least be able to detect occuring fluctuations in most cases, indicating that there's something wrong.

Kai