PSU fried drive, but whose fault was it? What now?

Discussion in 'Computer Support' started by Damon Malkovich, Apr 7, 2004.

  1. I bought a barebone system a few weeks back that came with an ULTRA
    brand 500w PSU and Sony DVD-ROM. Assembled it and tested her out for a
    few days -- all was good. The PSU however never lost it's new
    electronic smell (this may be relevant later).

    Last week I decided to do some mods, beginning with sleeving the PSU
    cables. I was very careful and systematic, ensuring that everything
    was reassembled properly. I was in no rush and after a couple days
    finished the job.

    Today I planted the PSU back in my system and gave her a boot. It
    locked at the "Detecting IDE Drives" message. It took removing both
    the HD and DVD-ROM for it to get beyond this stage. I tested the HD
    and DVD-ROM on my old system; HD was fine, but the DVD-ROM caused even
    this system to now lock at boot.

    Initially I assume the DVD-ROM just died, so I test an old Sony 4x
    CD-ROM on the old system (checks out okay) and then plug it into my
    new modded system. When I turn the system on, the CD-ROM immediately
    goes Rice Krispies (snap crackle pop) and smoke appears. UGH.

    Which brings me to my dilemma. Where is the problem? It's easy to
    suggest I screwed something up while modding the PSU (which I won't
    discount), but I feel confident in the job I did. The PSU itself
    certainly isn't fried, at the fans still spin, lights turn on, and I
    can enter CMOS. If the PSU fried the DVD/CD-ROM, then why is the HD
    fine? Is it too unlikely that the DVD-ROM merely went bad and the 4x
    CD-ROM couldn't handle a 500w PSU, meaning the PSU is actually fine?

    I'm nervous to plug any drives back into the PSU, even though it still
    appears to be fine. I'm really stumped on this one -- any advice??
     
    Damon Malkovich, Apr 7, 2004
    #1
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  2. Damon Malkovich

    philo Guest


    Sounds like you must have mixed up some wires...

    if you did that it would not necessarily destroy the powersupply
    but has evidently destroyed your entire machine...
     
    philo, Apr 7, 2004
    #2
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  3. Damon Malkovich

    Palindrome Guest


    Stick a voltmeter on all the flying leads and check they are ok. I have
    had new power supplies with the 12v and 5v crimp sockets in a drive
    power flying lead reversed and I have had one with one of the -5 and +5
    volt swapped - faulty assembly with the wires soldered in the wrong
    place on the power supply printed circuit board. Stick 12v onto the
    CDROM 5 v rail and it will rice crispy a treat. It may be just one
    flying lead is wrong and the HD one is fine. You can buy a test meter
    for less than a fiver (that is 5GBP to the rest of the world).

    OK, switch mode power supplies do need a minimum load without which
    their regulators will go unstable. That is why they have a bloody great
    resistor in the power supply box - just in case someone tries to power
    it up off load. So avoid connecting a light load (say just one drive) to
    a power unit, with the mobo disconnected. The bigger the rating of the
    power supply, the bigger the minimum load needed. If you run it on no
    load or very light load for a long time you run the risk of the power
    supply failing - depending on the design. It's like a car with an on/off
    accelerator rather than a linear one - ok on the motorway but impossible
    in town. (OK, so I am a coach driver..You want to make an issue of it?)
    Hope that helps a little.
     
    Palindrome, Apr 7, 2004
    #3
  4. Damon Malkovich

    PuddleNuts Guest

    Sounds like you switched some wires on the Molex connector that you
    plugged your optical drives into.
     
    PuddleNuts, Apr 7, 2004
    #4
  5. Thanks for the insight everyone! I did indeed remove the molex
    connectors while sleeving and would have bet my soul that I'd
    reattached them properly... but low and behold, while attempting to
    verify the voltage with a new multimeter, I had the +12V and the +5V
    swapped.

    Anyway, all's been corrected now and I can use the machine like normal
    (well, minus a DVD and CD-ROM). Would you say there's need for
    concern for any other components now (PSU, mobo?) because of the whole
    frying experience?

    Thanks!
     
    Damon Malkovich, Apr 7, 2004
    #5
  6. Damon Malkovich

    Palindrome Guest

    If it still works, you got away with it. 5v on the 12 v rail wouldn't
    have caused much of a problem, but 12 v on the 5v rail:

    Devices such as DVDs and CDROMS use tantalum capacitors to filter the
    rails. These capacitors normally go dead short when over-voltaged.
    "Luckily" the 12v rail that you were shorting down has a very limited
    current capability (compared to the 5 v rail) and so, rather than the
    capacitor exploding, it took the overload and stopped it spreading. Thus
    protecting the rest of the computer. Now, if you had swopped the ground
    and 5v lines, kablooooie! The 5v rail does have enough current
    capability to blow the capacitor clean off the board. That way, the
    fault could have propogated through everything, with your own little
    mini-gatling gun going off in the case. Aren't you a lucky bunny?
     
    Palindrome, Apr 7, 2004
    #6
  7. Woah, cool! I may have to dig up an older machine and just give that try! <g>

    Damon
     
    Damon Malkovich, Apr 8, 2004
    #7
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