Just like I did with the SL300, I'll replace the 5VSB output caps and the 100µF auxiliary capacitor to see how much that helps. I am not going to bother with the main output capacitors at this stage, since they are not relevant to the immediate issue of the 5VSB output being wildly out of bounds and the unit not showing any signs of powering up whatsoever when PWR_ON# is shorted to ground. Also, most of the main output caps are inconveniently tucked away under the secondary-side heat sink, so I would rather not bother with the hassle of replacing them until the 5VSB and turn-on issues have been addressed.
The new caps are in place (my usual 1200µF FMs for the 5VSB outputs and a 100µF FC for the auxiliary output). As before, they come nowhere close to fitting on the board. But that's alright since all I need them for is getting the power supply to either work or blow up. If it works, then I can add parts that fit.
Another advantage of capacitors that do not fit, or intentionally leaving long leads on temporary components, is that they make convenient test points to attach oscilloscope or multimeter probes to on the top side, rather than soldering wires on the bottom to expose the points of interest.
Don't pay attention to the Anode, Cathode and VAK traces for now; we will get back to them later. They're visible only because I had no screen captures from the first round with only the 5VSB trace on.
The result? Not much change apart from the “10VSB” output (yellow trace) being cleaner. In the SL300, the dead auxiliary capacitor was causing the flyback supply to fail to read the photocoupler's output, but it doesn't help at all in the AR300, which means there is something else to look at.
What could be the issue? A problem with the 5VSB feedback circuit? A busted feedback photocoupler? Dead feedback filter capacitor on the primary side? Busted auxiliary transformer winding? Remember “point four” from my SL300 investigation plan, which I secretly hoped I would not need to get to? Well, I am not getting away from it this time around; I will need to investigate the whole 5VSB feedback loop just to convince myself that it survived the grossly abnormal conditions it got exposed to.