While a good multimeter's direct current voltage measurement tells us the average value of a signal over some period, it does not tell us what the signal looks like if it has some other stuff on top of it. You can use the alternating current measurement to get a general idea of how much non-DC voltage might be present, but the RMS, crest approximation or other numeric representation still does not really tell you what is really happening, assuming the meter even has the necessary 10-200kHz AC measurement bandwidth necessary to keep up with the frequencies that might be present on switching power supply outputs to represent a reasonable chunk of total noise and transients. The only way to know for certain what is actually happening is to use an oscilloscope.
What does that 5.8V “DC” look like when observed with microsecond-scale horizontal resolution? Surprise! I thought the SL300 was bad with power-up transients up to 16V, but the AR300 beats it hands-down with steady state transients up to 27V. I am surprised my P4's motherboard survived, and I can already predict with fairly high confidence that there will be no surviving capacitors on any of the auxiliary and 5VSB transformer outputs. The massive spikes are a dead giveaway for equally massive equivalent series resistance on flyback output filter capacitors.
After connecting a good capacitor (1200µF, 16V Panasonic FM) across the 5VSB and GND pins on the ATX connector, the switching transients almost completely vanished. However, the DC voltage rose to an uncomfortably high 10V, which was considerably worse than the SL's 6V or so. How can a motherboard survive having 10V on the 5VSB input? Simple: most circuits connected to the 5VSB supply are powered through point-of-load linear or switching regulators, and those regulators are typically built using parts rated for at least 10V on the input side. Far out of spec, but not necessarily catastrophic.
From the looks of it, the 5VSB rise with the external capacitor attached could either be an indication that the AR300's auxiliary supply capacitor(s) might be deader than the SL300's, or that it has other issues as well.
All of this may sound like déjà-vu if you read my first repair, and perhaps you're questioning the usefulness of this story. Well, let me assure you: there would not be much of a story to tell if this repair turned out exactly like the first one. Expect a twist or two.
It's time for a visual inspection to see how many issues, if any, are apparent.