Gentlemen,
The link below shows a scope trace of the output of a linear PSU (voltage
4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in- circuit under load from the boards it supplies. Normally I would assume a filter capacitor to be at fault here, but they all check out fine so something else is causing this ripple. Note there is a characteristic 'knee' on the high peaks and I'm thinking this must be indicative of *something* trouble is, I don't know what. If anyone recognises this waveshape and knows what causes it, that'd be "awesome" - as our American friends describe everything. Check it out: