Another One Doesn't Bite The Dust
This repair required a little more time and effort, but it turned out successful. It would have been a few pages shorter if I had stuck to chasing the lack of voltage across the EC4 auxiliary output capacitor before investigating the secondary-side circuit on a remote chance the issue may have been there. Still, I would likely have ended up checking everything out anyway.
What have we learned?
Common troubleshooting wisdom exists for a reason. Start troubleshooting from the source to avoid wasting time chasing wild geese when you don't have to. I only ignored it here because going backwards was going to be more interesting. Knowing which components to pay attention to when repairing devices is nice; knowing why these components need to be paid attention to is nicer.
As a forum member pointed out in comments on my SL300 repair, those two-transistor 5VSB designs are potentially dangerous. When coupled with low-endurance capacitors, they are downright evil as shown in both my SL300 and AR300 repairs. With integrated standby PWM controllers like the TOP252GN costing less than $0.70 in 1000-unit reels, while requiring fewer support components and less board space, the cost savings from using a discrete implementation with a TO-220 main switch is questionable. Increasing pressure from governments and various agencies around the world for reduced standby power usage and higher standby supply efficiency should also help guide two-transistor standby designs on their way to extinction.
Just like the SL300, the AR300 demonstrated how efficient flyback supplies are at killing their output capacitors evenly, albeit with some extreme heat assist in the AR300's case, and this is not limited to two-transistor-type flyback contraptions.
The AR300 also reminds us of why printed circuit boards, electronic component packages and device enclosures are either made of non-flammable materials or contain flame retardants: they may get extremely hot, but do not catch fire easily and should not be able to sustain a flame. So, most failures result in little more than an unpleasant smell announcing that some of your equipment went up in smoke.
In retrospect, I am a little surprised this power supply managed to give me at least four solid years while powering my 3GHz P4C and Radeon X700 in such an airflow-challenged enclosure through thousands of hours of moderate to heavy use (for a 300W PSU) and nearly 24/7 power-on time. With that said, I am not pleased with how close this unit may have come to ruining one of my PCs through what I like to call engineered failure--using some of the cheapest parts possible in near-critical locations to barely exceed warranty requirements, effectively guaranteeing high failure rates shortly thereafter.
In any case, the original mostly just-good-enough caps lasted nearly 10 years, and with what little use my P4 has been seeing over the past few years, its reworked PSU should easily outlast the PC's utility.