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Thread: Dell NPS-250KB power supply woes and question

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    Question Dell NPS-250KB power supply woes and question

    Today my Dell OptiPlex 170L desktop shut off randomly. When turning it back on I was greeted with a message that said something along the lines of "Previous shutdown due to thermal event" and another message below it saying something about an internal fan failure. Upon further investigation I discovered the CPU fan wasn't spinning at all and the CPU heatsink was very hot (so hot I almost burned my fingers on it...) I unplugged and removed the CPU fan, the buzzing sound was still present... and with further troubleshooting, I found out the buzzing sound is definitely coming from the PSU, my guess is that it's not supplying enough power to the motherboard to run the CPU fan.

    Given that my 170L has been my daily driver for at least the past decade, I'd like to have this computer working again as soon as possible. I looked up replacement power supplies and it turns out the NPS-250KB has many different versions or "revisions". The one I pulled out of my computer is the NPS-250KB D REV 06 rated for an input between 100-120V (9.0A) or 200-240V (4.5A) both 50-60Hz, with an output of +5V (22.0A), -12V (1.0A), 250W MAX. +12V (16.0A), +3.3V (18.0A) +5VSB (2.0A) and the maximum combined power on the +5V and +3.3V output is 170W. To avoid confusion I've provided an image of the label on the PSU.
    20200511_181948-scaled.jpg

    There are some differences between the revisions: for example, the NPS-250KB D REV 01 has mostly the same specs, but the +12V output line is rated for 14 amps instead of 16, and on some revisions the wire colors are different (all revisions have the same connectors as far as I can tell.)

    Now to the question: With such suttle differences between the revisions, should I even have to worry about getting an "incompatible" power supply, or would they all work pretty much the same as long as they have the same connectors? Given that high voltage electronics isn't exactly my field of expertise, any advice would be greatly appreciated!

  2. #2

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    CPU fans do indeed die, especially on small form factor Dell units. If there was a voltage problem, knowing Dell equipment like I do, I was Dell certified for years, the power supply wouldn't turn anything on. Replace the CPU fan, you should be good to go. The power supply MIGHT be failing; but like everything else in life, could be tomorrow, or 10 years from now.
    Daniel P. Cayea - The Lyon Mountain Company - Plattsburgh, New York 12901
    Vintage Equipment: IBM 5150 * IBM 5161 * ThinkPad 770ED
    Modern Equipment: MacBook Pro 13 * Alienware M15R3

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    Quote Originally Posted by lyonadmiral View Post
    CPU fans do indeed die, especially on small form factor Dell units. If there was a voltage problem, knowing Dell equipment like I do, I was Dell certified for years, the power supply wouldn't turn anything on. Replace the CPU fan, you should be good to go. The power supply MIGHT be failing; but like everything else in life, could be tomorrow, or 10 years from now.
    The buzzing sound is definitely coming from the PSU (the noise is present with or without the CPU fan installed) which is what led me to believe this is a power supply-related issue. However, given that the only other problem seems to be the CPU fan not spinning, that could be the culprit as well. The power supply doesn't have a dedicated power switch, and I really have no way to test the CPU fan either.

    As you said in your post, would I be better off just replacing the CPU fan instead?

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    Does the PSU also have a fan? If it does, that likely could be what is making the noise.

    If that CPU fan has never been replaced, then yes you should try replacing it. Fans usually don't last very long.

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    Quote Originally Posted by SomeGuy View Post
    Does the PSU also have a fan? If it does, that likely could be what is making the noise.

    If that CPU fan has never been replaced, then yes you should try replacing it. Fans usually don't last very long.
    Yes, the PSU has its own fan. What does the buzzing noise likely indicate?

    As for the CPU fan I can't be 100% sure, but given that it was literally blanketed in dust before I cleaned it, I'm guessing it's never been replaced.

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    Quote Originally Posted by TH2002 View Post
    Yes, the PSU has its own fan. What does the buzzing noise likely indicate?
    Buzzing can either be the fan or coil whine.

    If the fan is buzzing, it usually indicates a bearing failure.

    Coil whine is where the coils on transformers or inductors start to resonate at a frequency humans can hear, this can be caused by a few things like manufacturing defects, improper dampening of the coils or improper design that allows the coils to run at an audible frequency. It can also be caused by capacitor failure, here is an example of a Dell power supply I repaired which had bad line capacitors:

    https://www.youtube.com/watch?v=lgwUadgIM8k

    The ticking/buzzing was caused by the two large capacitors to the left of the heatsink being bad.

    Quote Originally Posted by TH2002 View Post
    As for the CPU fan I can't be 100% sure, but given that it was literally blanketed in dust before I cleaned it, I'm guessing it's never been replaced.
    Does the fan have resistance when you try to spin it? It should spin freely, if it doesn't, the bearing is shot.

    Those are pretty specialized fans, and you'll need to get an exact replacement. Fortunately, Ebay still seems to have a few:

    https://www.ebay.com/itm/124106745478

    As for the power supply, any generic 300W or greater ATX power supply will work in that computer. I wouldn't get an exact replacement unless you already have one on hand because any replacement is going to be just as old as yours and have the same potential issues.

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    Quote Originally Posted by GiGaBiTe View Post
    Buzzing can either be the fan or coil whine.

    If the fan is buzzing, it usually indicates a bearing failure.

    Coil whine is where the coils on transformers or inductors start to resonate at a frequency humans can hear, this can be caused by a few things like manufacturing defects, improper dampening of the coils or improper design that allows the coils to run at an audible frequency. It can also be caused by capacitor failure, here is an example of a Dell power supply I repaired which had bad line capacitors:

    https://www.youtube.com/watch?v=lgwUadgIM8k

    The ticking/buzzing was caused by the two large capacitors to the left of the heatsink being bad.
    So if I do choose to repair the original power supply, hopefully it won't be too difficult. I'm also starting to think that the CPU fan not working and the PSU buzzing are two separate issues, and it's just a coincidence that the PSU started buzzing the night before the CPU fan stopped working.

    Quote Originally Posted by GiGaBiTe View Post
    Does the fan have resistance when you try to spin it? It should spin freely, if it doesn't, the bearing is shot.

    Those are pretty specialized fans, and you'll need to get an exact replacement. Fortunately, Ebay still seems to have a few:

    https://www.ebay.com/itm/124106745478

    As for the power supply, any generic 300W or greater ATX power supply will work in that computer. I wouldn't get an exact replacement unless you already have one on hand because any replacement is going to be just as old as yours and have the same potential issues.
    The original fan does have resistance when I try to spin it, so indeed the bearings are probably shot. Thankfully, I already ordered a replacement CPU fan of the exact same type (with the correct shroud, connector and case mountings.)

    Regarding the power supply, thanks for the info! That means if I decide to replace the original PSU, finding a replacement will be easy.

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    Quote Originally Posted by TH2002 View Post
    So if I do choose to repair the original power supply, hopefully it won't be too difficult. I'm also starting to think that the CPU fan not working and the PSU buzzing are two separate issues, and it's just a coincidence that the PSU started buzzing the night before the CPU fan stopped working.
    PSU repair is always an option, but I only really recommend it for rare or proprietary units that are hard to find or expensive to replace. You should also have some experience with high voltage electronics because you can easily kill yourself if you don't know what you're doing. Those main line caps store a lot of energy and can do some damage.

    Quote Originally Posted by TH2002 View Post
    The original fan does have resistance when I try to spin it, so indeed the bearings are probably shot.
    You can sometimes resurrect frozen fans by lubricating them with 3-n-1 oil. Peel back the label on the back of the fan and add some oil around the fan shaft and lightly push/pull the entire fan assembly back and forth while rotating it by hand until it frees up. If the bearings weren't run too long without lubrication, you can usually get quite a bit more life out of them. If the fan uses ball bearings instead of a sleeve bearing, it usually isn't possible to recover these without replacing the tiny sealed ball bearings, which are hard to find.

    Though if they're too far gone, the grease usually burns into abrasive carbon soot and acts like sand paper to wallow out the sleeve and the fan is trash after that.

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    Typing and posting this from my Dell OptiPlex 170L!

    I did manage to free up the bearings a bit on the old fan (though admittedly now I don't think they were too bad to begin with). Unfortunately it still doesn't spin when installed in the computer, so I'm guessing the problem is electrical.

    Anyways, I installed the new CPU fan and now everything works fine. Even the PSU buzzing has become intermittent... so I guess I won't worry about it until it becomes constant again, which hopefully won't be for a while.

    Thank you to everyone again!

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