PSU Repair: A Case Study

References
By Daniel Sauvageau
published

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.

Preliminary Testing: The 5VSB Rail

All initial signs pointed towards the power supply's turn-on issues being related to 5VSB, so let's start investigating there.

Luckily for me, it turns out I still had what I believe to be the original caps in my junk box, so I was able to revisit the before and after effects of that 5VSB capacitor swap, seeing what I would have seen if I had an oscilloscope before my modifications.

Image 1 of 2

These screen captures show the 5VSB behavior upon throwing the hard-switch on. The first one is with the original 5VSB capacitors, and the second adds one of those 1200µF Panasonic FM caps I used in my LG repair plugged into the ATX connector. With only the old 680µF Fuhjyyu caps, the 5VSB output settles around 5V, is horribly noisy and shows horrible power-up transients peaking at up to 16V (clearly no good for capacitors rated only for 10V). With the external capacitor, voltage ends up settling around 6.4V, looks a whole lot cleaner but still overshoots to 8.5V. The original 5VSB capacitors are definitely busted if a capacitor connected at the end of the 30” ATX cable can provide this much of an improvement on ripples. The increased voltage from adding an external capacitor was unexpected.

While I was measuring voltage on 5VSB between hard-switch cycles under different loading conditions, I was surprised to see it settle anywhere from 5.4V to 6.4V depending on capacitive loading, power-on/off cycles, current draw and how long I let it warm up before turning it off and back on. This would seem to indicate a possible issue with the feedback loop, such as a flaky component or a cold solder joint. I have already ruled out cold solder joints by touching up the whole 5VSB section after the external cap failed to help, leaving flaky components as the prime suspect.

Before messing with anything more sensitive than output caps, let's see what else can be found first.

Rigging the Game

Since something related to 5VSB voltage appears to be causing issues, I decided to put together a shunt regulator based on a three-terminal reference and a booster transistor. Let the circuit figure out how much current it must draw to keep 5VSB at whatever voltage I want it at.

Image 1 of 2

My simple shunt regulator is slower than I would have liked, with a reaction time of 4µs. Otherwise, it does exactly what I meant for it to do: draw however much current it needs to when 5VSB exceeds the set-point to drag it back down there. It would probably perform better if it was built on a PCB instead of a breadboard and connected directly to the supply PCB instead of 36” leads.

The first slide shows how the 5VSB output reacts to having my 5.05V shunt regulator attached with the old caps. To prevent transients from rising to 16V as they did before, the shunt needs to draw about 4.2A and barely manages to keep them below 8V. With the external cap, transients peak at a still-unreasonable 6.4V and the shunt manages to keep them there by drawing less than 1A.

If I fail to fix the 5VSB stability issue at the source, I could put a 2A fuse in front of my shunt regulator and call that good enough for occasional bench use.

Daniel Sauvageau
Daniel Sauvageau is a Contributing Writer for Tom's Hardware US. He’s known for his feature tear-downs of components and peripherals.
62 Comments Comment from the forums
  • Nuckles_56
    An interesting read, it was interesting following the process you used to troubleshoot the problem
    Reply
  • Crashman
    I used to do this for a living :)

    Don't tell my boss, I've managed to convince him that I'm only an expert at running benchmarks and writing about the results :)
    Reply
  • epsiloneri
    Disclaimers won't help. The people who will likely hurt themselves trying this are the same who lack the reading comprehension and self awareness to understand those disclaimers are directed at them. I admire you courage in publishing this.
    Reply
  • beetlejuicegr
    the truth is the paper clip and multimeter is all i can go in to psus. after all i haven't studied electricity or circuits or whatever.
    However i do hate to throw stuff earlier than it should, like you.
    Reply
  • C12Friedman
    I like this article and I fully agree with the conclusion. I've repaired a few PSU's but, for the most part I scavenge them anymore since I can't put them in a new system (nor would I want to) IMO they aren't really worth anything other than for on a test bench.
    Reply
  • Mr A
    Daniel, I know next to nothing about electronics, and yet I could not stop reading this article. Fascinating! Thanks very much!
    Reply
  • Urzu1000
    This was a great article! It was informative, as well as interesting. Personally, I've only had one PSU fail on me so far. My brother-in-law's self-built computer had a really low-end Thermaltake PSU. 800W Bronze, and oh man, did that thing go out in a blaze of glory. Very loud popping, and smoke, and funny smells. When I ripped it out of the computer, there were burns inside the case. Miraculously, the other components remained unharmed, so I slapped in a new PSU (750W Gold Seasonic) and fired it up.

    Still working good, but I get black soot on my hands every time I open up that case. It's a black case, so it's hard to clean it off properly.
    Reply
  • nukemaster
    Good read.

    Thanks for taking the time to document this repair.
    Reply
  • Interesting article. I would have simply replaced the entire unit. You saw how to fix the failure, but how many units were damaged that you didn't see? A ticking time bomb that will eventually send some spike to your much more valuable hardware than a 10 year old PSU. Wasteful, yes. I get it. I don't like to waste either. And if it's on marginal hardware, fine. But on primary systems I'm not willing to take the risk. I'd rather throw away a 200$ part that has a 0.05$ repair solution, than risk frying 800$+ hardware.
    Reply
  • kalmquist
    "Antec's manufacturer (Channelwell in this case) got the live and neutral wires backwards, which means that in the “off” position, the neutral line gets opened and everything on the primary side becomes live instead of neutral."

    That's really bad--I doubt it is even legal to sell a power supply wired like that. I've never bought a CWT (Channelwell) power supply, and based on this I wouldn't buy one, except perhaps for a high end model where you might gamble that the company would exercise a bit more care.
    Reply