PSU Repair: A Case Study

When your PSU won't start, most enthusiasts don't go beyond a paperclip or multimeter check. Today, we'll do a comprehensive repair of Antec's old SL300.

Foreword

I have never bothered going beyond re-capping output capacitors when repairing power supplies. Most of the time, I fix hardware because I hate throwing stuff away that should have worked for much longer. But now that I have an oscilloscope, I can poke around places I would not have bothered with before. This repair is one I had been meaning to take another stab at for nearly a year. Why bother with a decade-old power supply? Because I can! Should you use a decade-old power supply? Probably not.

Disclaimer

As usual when fiddling inside line-powered equipment (and especially on the primary side), don't try this yourself unless you're a trained professional. You assume all risks for whatever it is you decide to do.

The Basics

Power supply failures can be a nightmare for a variety of reasons. Chances are that if you used any computer for more than a few years without swapping out its power supply for some reason along the way, then you've experienced the joys of owning a senior or defective unit at least once.

The most common and well-known failure that gets heavily criticized in power supply reviews is low-quality output filter capacitors. Many times, though, what this really points to is under-rated capacitors. The same was true in my LCD repair tear-down back in February. In most cases, the symptoms start with random shutdowns or restarts, random crashes, devices misbehaving, failure to turn on and so on. But in some more extreme cases, they can lead to smoke, fire and exploding components, hence our insistence on staying away from bottom-of-the-barrel power supplies if you want to reduce the risk of blowing up your PC or worse. If you have seen some of the tiered PSU lists out there, some of those tier-five units have a reputation for being outright dangerous.

When a power supply failure is due to dying output caps, the cause is often obvious upon opening the unit: you will see many capacitors with bulged tops, possibly covered in dried electrolyte. These supplies can usually be brought back to a perfectly usable state by re-capping them using appropriately-sized and -rated replacements. Other times, the problems run deeper than a basic cap makeover.

In any case, for most people, power supply troubleshooting often starts and ends with a paperclip test that tells them little beyond whether the supply might be completely dead, not responding to PS_ON# or at least sort-of working. A basic multimeter allows you to check what the 5VSB voltage actually is and what the other rail voltages are if the supply is able to turn on.

Sometimes, you also have catastrophic failures with the associated light show, sounds, smoke and smells. In those cases though, repairs do not make much sense, if they're even possible, due to extensive damage to components and the circuit board itself.

Another limitation of the paperclip test is that the power supply might not behave the same way between in-system and stand-alone due to added board capacitance and load across the rails, so any measurements done in isolation may not be representative of what is causing system issues, further complicating the troubleshooting process by introducing false positives and negatives.

When a paperclip and $8 multimeter are not enough to get to the bottom of the story, people normally call it quits and throw in a new power supply. Fortunately for me, this is just one more excuse to put my oscilloscope to work, hopefully get a usable power supply and write an entertaining story along the way.

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