PSU Repair: A Case Study

Preliminary Testing: PS_ON# Behavior

We already know that the supply has horrible deviations on the 5VSB output, so the next thing to look at would be correlation between the abnormal 5VSB output and turn-on failure.

This first slide shows what happens at turn-on with the old 5VSB caps and no loading on 5VSB. The 5VSB output looks like junk throughout, but its DC value remains close to 5V and the main outputs look perfectly fine. While the overshoots aren't pretty, they are still within the 10% turn-on tolerance. In the remaining four slides, I added a 1200µF cap connected to the 5VSB pin on the ATX connector.

With the addition of that cap and no further changes, things go down the drain. Although the high-frequency content on the 5VSB line was greatly reduced, I now appear to have a problem with the 5VSB flyback operating irregularly, pulsing at about 500Hz. After 70ms though, the unit decides to shut down.

This time, I added the shunt voltage regulator from the previous page on the 5VSB output and adjusted the set-point voltage until I got 100mA through it before signaling PS_ON#. Now the flyback is pulsing at 600Hz and the main outputs are looking much livelier. They still shut down near the 82ms mark and do not look like anything you would want near your components.

Dialing the load up to 200mA produces these traces and the first quasi-successful power-up: the 5VSB flyback is operating at 700Hz, the main rails manage to reach their respective nominal voltages and stay on. However, they do miserably fail the “smooth ramp” and 20ms maximum rise time requirements from Intel's 2013 PSU design guide. Results are also highly dependent on warm-up and cool-down times between runs.

Stepping things up to 250mA starts looking good: the 5VSB flyback still has some minor hiccups just before main outputs come to life but it is now operating steady through the whole ramp, the main output ramps are much smoother and take just over 10ms.

At 300mA of dummy loading, the main output ramps look as good as they are going to get.

What do all of these waveforms have in common? They all show a dip on the 5VSB output just before the main outputs start coming to life, and during that dip, the 5VSB output behaves itself better than usual.

What preliminary conclusions can we draw from these observations? The strong correlation between how steady the 5VSB flyback is and how smooth the main output ramps are tells me the main switcher must either be bootstrapping from or entirely powered by an auxiliary output on the 5VSB flyback, and irregular flyback operation is preventing the main switcher from receiving steady enough power to operate normally. If I am correct about it, I should be able to loosen this dependency by putting a larger capacitor in the controller's power supply. If the controller's bootstrap happens to be tied into the 5VSB feedback circuit on the primary side, this could also explain the non-loaded voltage increase to 6V with either fresh caps or external caps. The main converter being powered by an auxiliary winding on the 5VSB supply would also explain the dip just before outputs start ramping up.

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  • Nuckles_56
    An interesting read, it was interesting following the process you used to troubleshoot the problem
  • 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 :)
  • 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.
  • 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.
  • 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.
  • Mr A
    Daniel, I know next to nothing about electronics, and yet I could not stop reading this article. Fascinating! Thanks very much!
  • 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.
  • nukemaster
    Good read.

    Thanks for taking the time to document this repair.
  • 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.
  • 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.
  • Daniel Sauvageau
    211508 said:
    Disclaimers won't help.

    They may not help the people who choose to ignore them but they help reduce the likelihood of legal issues if something blows up in their face or worse after they do so.

    2008047 said:
    Daniel, I know next to nothing about electronics, and yet I could not stop reading this article. Fascinating! Thanks very much!

    You're welcome. I try to put enough technical details in there to keep technically minded people interested but not so much as to make it inaccessible to more casual readers.

    723938 said:
    Interesting article. I would have simply replaced the entire unit. 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.

    And a new PSU was the fix I initially implemented in my PCs too, though mainly because I could not be bothered to investigate at the time since I lacked the tools necessary to do so properly and with reasonable confidence.

    262841 said:
    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.

    The wires on the IEC plug filter board are likely inserted by hand. Without a few more units to compare it against, it is entirely possible that my inverted wiring was a one-off or otherwise uncommon manual assembly mistake. It likely happens to all other manufacturers too, the question being how often it gets through QC unchecked.
  • Daniel Sauvageau
    1526053 said:
    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.

    No need for miracles here: most spectacular catastrophic PSU failures occur on the primary side which is isolated from the outputs by the transformer. If the BJT/FET switching becomes too slowly due to weak gate drive, the junction or channel may exceed its SOA and blow up - I was surprised this had not happen to my SL300 here. If the switch stays on for too long or the transformer has a flux imbalance that causes it to move up its hysteresis curve, the transformer core eventually saturates and the low impedance destroys the switch. Either way, the amount of energy transferred to the secondary side from the fault should be negligible compared to the total capacity of output caps.

    The scary PSU failures are those involving the feedback circuits: if these fail open, you may end up with the primary side operating at its maximum duty cycle and outputting the maximum voltage the main outputs are capable of based on the transformer's turn ratio. This is basically what happened with the 5VSB output here: the auxiliary output was failing and caused the primary side of the 5VSB feedback loop powered by that auxiliary output to misbehave, allowing the 5VSB output to surge to about 8V not counting noise.

    I would be far more concerned with silent failures (like the 5VSB here) than spectacular ones.
  • chazking260
    Nice to revisit the technician aspect of troubleshooting. It's a dying art and cost prohibitive except in a lab environment. Great Problem reporting!
  • razor512
    I was able to repair a $20 power supply by replacing the dead caps, with some industrial 135C rated caps that I salvaged from some industrial equipment. (the caps likely cost far more than the power supply, but the repair worked.

    For PSUs I generally do not repair them beyond capacitors, unless it is something that is custom and cannot easily be replaced.

    After a repair, I always test power supplies with dummy loads. (mainly car headlights). (pack of 2, 55 watt headlights are generally around $7-8
  • nukemaster
    Hey I was just looking over this again in Spartan/Edge with reader mode and notice those "rubycon"caps are also fakes.

    They say rulycon

    Should have had the logo like this :)


    Instead it is.
  • shrapnel_indie
    120VAC is enough to kill some people, or at least interfere with their hearts enough to cause medical/physical issues. Some may just feel a pulsing "bite." Not worth being careless (or even intentionally) to find out which one you are. 240VAC will kill. I should be more precise though: The associated current does the damages, and each human body's electrical resistance can vary among others and/or with conditions.

    I do agree that neutral (white) and hot/live (black) wires get reversed quite often enough that it could be a problem. It's why decent outlet testers will cross-check the connections between hot, neutral, and ground.


    Quote:
    All capacitors have a tolerance rating that can range from -20% to as high as +80% for aluminium electrolytic’s affecting its actual or real value.
    - Electronic Tutorials, Capacitor Characteristics, http://www.electronics-tutorials.ws/capacitor/cap_3.html

    Sometimes caps do have other tolerance ratings, but they are usually marked when they do. The 600uF cap was more than likely within tolerance (min 544uF) while the 34uF cap was definitely not in tolerance and was bad. The 47uF cap measuring 112pF (min 37.6uF) was bad. The 22uF cap measuring 18uF was within tolerance. (min 17.6uF) (Mins given at -20% tolerance)

    Changing a voltage filter cap to a higher value has no harm other than it can have a higher "charging" current as it charges to the full voltage available to it. Changing it to a lower value will increase noise and ripple in the output voltage. Changing other caps, such as in timing circuits or frequency dividers with other values can change timings and potentially cause problems. It's usually not an issue (other than cost) to use a cap with a higher voltage rating than what is needed. It is important to observe polarity on polarized caps unless you want it to go BANG! Design, age of the design, and the materials called for by the design will affect size, leakage currents, and ESR of the caps, especially electrolytic caps.
  • Daniel Sauvageau
    35532 said:
    Hey I was just looking over this again in Spartan/Edge with reader mode and notice those "rubycon"caps are also fakes.

    In my old junk box which I trashed a few years ago, I also had "Lubycon", "Samxung" and a few other similar spoof-sounding names. The first time I came across a "Fuhjyyu" capacitor, I thought it was a spoof attempt on Fujitsu's name.

    Since many asian languages lack phonetics to clearly disambiguate those spelling variations, I bet many products ended up with these devices in them simply due to confusion or creative interpretation in the supply chain.
  • Crashman
    330834 said:
    I do agree that neutral (white) and hot/live (black) wires get reversed quite often enough that it could be a problem. It's why decent outlet testers will cross-check the connections between hot, neutral, and ground.
    I blame the difference between color codes of AC and DC circuits. Most DC circuits are "Negative Ground" and black is the "Negative" wire, so it gets connected to the chassis. Power supplies are both AC and DC, so the white wire is ground on one side and black is the ground on the other side, if you connected black to black you'd energize the case.

    I tried explaining this to an ELECTRICAL ENGINEER who couldn't figure out how to wire up a motorcycle. Seriously, he kept saying stuff about the stupid motorcycle company hooking up all the hot wires to ground and he was trying to fix it (he was actually converting it to positive ground and couldn't figure that out). The thing is, this guy designed control systems for manufacturing and it was all AC and wiring diagrams in his mind.
  • Daniel Sauvageau
    330834 said:
    Sometimes caps do have other tolerance ratings, but they are usually marked when they do.

    Technically, all components have tolerances on all parameters worth listing. Capacitors' nominal capacitance just happens to be notorious for having some of the widest tolerances in all of electrical engineering.

    In the case of the cap that still read 600µF on the multimeter, it turned out to have about 4 ohms of ESR in my step test when I revisited it. The reason it let so much noise through despite having a "good" capacitance is the ESR: the capacitor is no good at suppressing ripples at 10+ kHz if there is a 4 ohm ESR in front of it. If there is 1A of ripple current from the flyback, you get 4V worth of ripple instead of the expected 0.1V.
  • pjmelect
    I liked this article, It is important that the replacement capacitors are 105C type, as the normal 85C type capacitors would not last very long. I often repair power supplies and I use a ESR meter to test in circuit all of the electrolytic capacitors in the power supply which only takes a few minutes.
  • nukemaster
    Is low ESR caps wasted on normal 60hz power filtering(diode rectifier after a low voltage AC transformer)?
  • Someone Somewhere
    8708 said:
    330834 said:
    I do agree that neutral (white) and hot/live (black) wires get reversed quite often enough that it could be a problem. It's why decent outlet testers will cross-check the connections between hot, neutral, and ground.
    I blame the difference between color codes of AC and DC circuits. Most DC circuits are "Negative Ground" and black is the "Negative" wire, so it gets connected to the chassis. Power supplies are both AC and DC, so the white wire is ground on one side and black is the ground on the other side, if you connected black to black you'd energize the case. I tried explaining this to an ELECTRICAL ENGINEER who couldn't figure out how to wire up a motorcycle. Seriously, he kept saying stuff about the stupid motorcycle company hooking up all the hot wires to ground and he was trying to fix it (he was actually converting it to positive ground and couldn't figure that out). The thing is, this guy designed control systems for manufacturing and it was all AC and wiring diagrams in his mind.


    Oh... wow.

    I didn't realise the US used black for phase and white for neutral - I'd seen the wire colouring in the odd power cable etc. that comes with US gear, but...

    We use black for neutral here down under. Red (or yellow (was white until a decade or so ago, but UV turns everything white) or blue if three phase) is phase. Many appliances are built to European standards with brown phase and blue neutral.
  • Shankovich
    These articles, this is why I always love Tom's
  • Shankovich
    These articles, this is why I always love Tom's