Hi, thanks for the response.
Yes correct there are two 5V's. There's 12V, -12V, -5V, 3.3V also.
The 5V in question is indeed the aux and should always be on (so long as there's mains power, obviously). The rest of the lines come on when the PSU is 'switched on', and by that I mean I ground the appropriate pin on one of the connectors.
A few photos. The 5V line in question is to the bottom right of the 2nd stage board. I've highlighted with a red arrow in one of the pics. Those two capacitors just above the red wires are the ones I've pulled and tested (for capacitance).
On the suspect PSU, I see 5V on the aux line when it's unconnected but it drops to 3.5V when it's connected to the load. On the known good PSU, I see 5V on the same line when unconnected and also 5V when connected to load.
With respect to ripple, are you asking if I see any AC on that 5V line? If so, then "possibly". On the known good PSU, I see flat 0VAC on the aux but unreliable results on the suspect PSU. It sort of flips around a bit then settles on 0VAC.