More Complementary Curiosity
I remembered to take data dumps from the scope before and after the fix so I could at least compare 5VSB standby power. The 5VSB rail has a 51Ω dummy load built-in, which translates into roughly 0.7W with the original voltage waveform and 0.5W at the nominal 5V output voltage. What did the primary waveforms look like before and after the 5VSB fixes?
The two waveforms look similar except for the magnitude of the current peak. By replacing the dead capacitors and fixing the busted resistor, standby power draw dropped from 7.9W to 1.6W, a nice 80% reduction. I wonder how much worse it may have been before R29 blew up. In any case, it is good to know that my P4 should draw less than 2W while hibernating. I know I could simply disconnect it, but I prefer wasting $0.10/year on standby power than $1/year on dollar-store CR2032 batteries.
While I was at it, I decided to have a quick look at 5VSB efficiency with some load attached.
Were it not for the different title, this last set of "after" waveforms with a 12V 20W halogen lamp connected to the 5VSB output could easily be mistaken for the "before" ones. The bulb was drawing 1.03A at 4.97V on the 5VSB rail for 5.12W output power while I measured 8.11W integral power on the AC side, an efficiency of 63%.
How does that compare to modern low-power AC adapters? Under the old class-III efficiency requirements, a 10W supply similar to the 2A 5VSB rail must have an idle power draw under 0.5W and efficiency under load of 49%. The AR300's two-transistor design falls tragically short of meeting the idle power draw requirement, while efficiency under load easily beats it. Not bad for a 12-year-old design. However, this is nowhere near the newer class-V requirements countries started adopting a few years ago, which drops idle power draw to 0.3W and bumps loaded efficiency to 76% for a 10W supply.