It's hard to believe that a switching power supply introduced 50Hz ripple. Either the P/S is defective, or something else is going on.
Check the power supply with just a load resistor that draws as much current as your circuit. Measure with the scope the ripple across that resistor. It should be at the P/S switching frequency, in the kHz range, but less than 1%.
If that checks OK, then verify your circuit, especially the grounds. My feel is that something is missing a ground, perhaps even the point where you ground your scope probe.
Check the probe ground lead. It happened to me many times to find it was broken.