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Faint, high-pitched noise

Discussion in 'Electronic Design' started by Robert Latest, Mar 1, 2007.

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  1. Hello,

    I just bought a used Wi-Fi router / ADSL modem combo. Everything works fine,
    except that after the thing has warmed up for about 20 minutes, it emits a
    faint, audible whine of a few kHz that's pretty annoying once you start
    listening to it (as one does, and doesn't stop any more. I find myself
    "searching" for that noise once I enter the room where the thing is).

    I'm guessing that the noise comes from some switched POL regulator inside
    the device. The wall-wart is one of the old, clunky, linear ones so that's
    out. Question is what can be done about it, other than moving the bugger to
    the attic. Something inside the case must be oscillating mechanically, and
    the only thing I can think of that would do that is some inductor of the
    switcher. My idea is to locate something that looks like a magnetic
    component and soak it in liquid epoxy.

    Good idea or bad idea?

  2. ian field

    ian field Guest

    Just a thought before you pot it in epoxy and make any subsequent repair
    impossible, see if any electrolytics are getting warm, it could be an ESR
    problem - although this normally improves as the unit warms up, still its
    worth a try.
  3. Genome

    Genome Guest

    It could be the other way around...... I think you'll find that electrolytic
    capacitor ESR goes down as they warm up. If it is a stability problem, which
    is a good reason for things whining then that is a possibility.

    Depending on how 'good' the manufacturer think they designed things it can
    work the other way around. As electrolytics age their ESR goes up and that
    might affect stability at the other end of things. Which sounds like a
    possibility since it is second hand and presumably old. (!)

    I assume you did not buy this from a freind or, if you did, they did not
    hear it themselves.....

    Since you are Robert Latest you probably have a scope so it would be worth a
    prod to see what the supply voltage looks like....

  4. ian field

    ian field Guest

    You obviously have as much difficulty understanding English as you do with
    washing yourself!
  5. Well, I didn't really want to pot it, like into a whole block of plastic. I
    have this lacquer-like stuff that I can brush or spray, so it would just
    soak into the windings of the coil.

    Thanks for the electrolytics/ESR tip.

  6. Genome

    Genome Guest

    You appear to have used reasonable English to provide a scenario whereby the
    problem, as described, should in fact go away as the unit warms up. I'm just
    pointing out that, technically speaking, your argument might be incorrect.

    Then again, it's not uncommon for people with a good grasp of English to
    have difficulty with technical issues. Logic appears to be a problem as
    well. Still, one out of three isn't bad.

  7. On a sunny day (1 Mar 2007 14:53:41 GMT) it happened Robert Latest

    My idea is to locate something that looks like a magnetic
    I have use plastic spay in the past on noisy TV magnetics.
    Worked sometimes, does little harm.
  8. ian field

    ian field Guest

    Capacitors with ESR problems typically improve slightly as they warm up,
    although in my years of experience servicing TVs, monitors & SMPSUs I've
    seen caps react in a way contrary to usual expectation in this regard, my
    suggestion to the OP that the fault presenting once the unit had warmed up
    shouldn't automatically preclude paying attention to the caps, like I said -
    "still its worth a try".
  9. ian field

    ian field Guest

    The Maplin PCB lacquer is even better - the can I bought never sets, it
    stays gooey forever!
  10. Genome

    Genome Guest

    Well, at least my shoes don't smell!

  11. ian field

    ian field Guest

    How could they? - You don't wear any!
  12. Genome

    Genome Guest


    Signed, sealed, delivered and slayed.

  13. I bought it from ebay at a very good price, and if I can't get rid of
    the noise the thing will have to be move to a back corner. I'm just too lazy
    to lay the wiring there.

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