Do We Have A Definitive Fix?
I started out with a whiny power supply that would not reliably turn on, discovered that I could make it become predictable by loading down the 5VSB output and decided to start working backwards from there. In the end, I got it to function properly without crutches using $2 worth of new caps, even if they didn't fit on the board quite right. Of course, that was on top of the $3 worth of capacitors I replaced back in 2009.
Looking back at the results, I believe I can put together the sequence of events that lead to this failure: putting my Core 2-based machine into standby on a regular basis, something I rarely did on my Pentium III, wore down the under-rated 5VSB output caps. As they got worse, ripple on the auxiliary capacitor also increased until it failed while the main outputs were off. Beyond that point, the PWM's capacitor was alone and the power supply continued chugging along until its ESR became too high and the supply would no longer turn on.
Disconnecting the unit from the motherboard removed the load and board capacitors, which provided some clamping for the 5VSB output like my external capacitor did. This allowed surges on the auxiliary output to reach levels high enough to flash over inside the dead auxiliary capacitor, and once the capacitor had warmed up enough from it, contact got made and the power supply would work normally again until the next time it cooled down. Adding load on the 5VSB rail caused the 5VSB's pulse rate to increase. Once the load was sufficient to make the pulse period shorter than the voltage decay time on the PWM's bypass capacitor, the power supply would turn on properly again even when cold.
The increased 5VSB voltage before the auxiliary capacitor replacement was likely caused by the flyback circuitry using its own auxiliary output supply to shut itself off based on the 5VSB feedback. With no power to operate the primary side of the feedback circuitry after the auxiliary voltage dies off from the dead cap not holding a charge, a new flyback cycle starts even if not required and the voltage rises.
Thinking back to the earliest days when this power supply started giving me trouble after my first repair attempt, I vividly remember hearing an irritating whining noise coming out of my speakers with a pitch that shifted in time with the blinking of my PC's standby LED about a year later. I actually ended up buying an EA650 for my Core 2 and put the SL300 in one of my spare PCs until I reused the EA in my i5-based system. I would not be surprised if that happened to be the very first hint of trouble on the 5VSB and auxiliary outputs. As shown in my preliminary tests, even a 50mA change had a significant impact on the flaky 5VSB's behavior. With the fixes, load on 5VSB makes hardly any difference whatsoever anymore.
Throw in a proper $0.50 PCB-mount fuse to replace my thin wire fuse hack, a pair of 8mm capacitors with at least one of them rated for 2A ripple that fit properly for the 5VSB output and perhaps something with the correct lead spacing for the auxiliary cap--that's what I would call a definitive fix. Would I use this supply in a PC again? Sure, but not in anything beyond a spare system until I got around to putting proper parts in and doing some more testing.
Should you attempt to repair decade-old $40 power supplies? Most likely not. Even if you have the knowledge, skills, tools and parts to do it safely and properly, buying a new $40 power supply may still end up cheaper than the repair once you account for your time. Also, the new unit will usually be considerably more efficient and contain only minty-fresh components, complete with manufacturer warranty coverage. It only made sense to me because I could satisfy my own curiosity and do something useful with my oscilloscope. Having spare known-good power supplies with high performance caps never hurts either.
Next in my repair queue: the proprietary form factor supply from my Antec Aria case. It is the worst case I have personally worked with to date, made worse by the Pentium 4 (3GHz Northwood with HT) I put in it and the effectively nonexistent ventilation through it, especially in the processor area. The CPU fan is screaming even at idle when all three covers are on. If you want to read about that, let me know in the comments!
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