On 7/9/2017 8:10 AM, Cursitor Doom wrote:
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:


https://www.flickr.com/photos/128859...in/dateposted-
public/

You might get this when you have two caps separated by an inductor.
First cap usually gets more peak charge current than the second one
and develops high ESR first. Even if both caps are good, the inductor
(or resistor) in between can cause this effect.