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Discussion in 'Electronic Repair' started by andy12345, May 26, 2005.

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  1. andy12345

    andy12345 Guest

    It has come to my attention recently - after using computer
    communications voice software that I can reduce the hum of the
    microphone recording by lightly touching my pc case.
    I was always under the impression that the earthing worked in my room.
    However, I have bypassed the psu earth temporarily simply by
    connecting a wire from a clamp in the pc case(unpainted surface) to
    the earth contact hole of the mains socket.
    This does not reduce the hum at all. However, by touching the case
    and touching the floor with the other hand I can reduce the hum on
    the signal. Therefore, is the earthing failing at some point in the
    house or is there something that I am ignorant of?

    I can supply any information if you require more detail.
  2. Main Account

    Main Account Guest

    I may not have an answer to your question but I have some healthy advice for
    you. I worked in electricity transmission and distribution for the last 15
    years, got within the limits of approach at voltage level going from 120V to
    750kV on two continents. My main specialty is work safety so what I'm
    telling you is mostly from this perspective.
    Remember that the ONLY protection you have, as a living creature is the
    ground (what you call "earthing"). If that fails you're virtually dead -
    meaning that you're not breathing, seeing, talking, eating, f..., etc - the
    fact that didn't happen yet doesn't mean it cannot happen, just that it
    didn't happen yet. If you have any doubt, even the smallest one, that your
    house grounding is not 100% safe have it checked ASAP before what I just
    said happens.
    As a rule of thumb, for normal people like you and me (and probably 99% of
    the guys writing here) everything above 24V is lethal and should be treated
    accordingly. There's only one thing that makes the difference between life
    and death: your skin resistance. As long as that holds you're ok, as soon as
    it fails you're gone. What's underneath your skin is mostly salted water
    which guarantees a massive failure due to the electrical current passing
    through. Examples of how your skin resistance can decrease are: moisture
    (sweat, hands recently washed, hand lotion, etc), cuts, burns, scratches,
    etc. In other words, there is one thing that works for you, one million
    things against you.
    Household grounding failure is a main cause for many accidents, some lethal,
    some not, some with long term consequences. Remember that copper oxidizes,
    the oxide increases the contact resistance which diminishes the efficiency
    of the ground connection. Also contact pressure has a tendency of decreasing
    in time with the same effect. Also bending the solid copper wires can reduce
    the copper section increasing the resistance thou reducing the overall
    grounding efficiency. In this scenario, one hand on the metal case of your
    PC and the other hand on the floor establishes a current path dangerously
    close to your heart, you had no problem when you first tested it because
    your hands are dry, next time you'll try it after eating an orange, you'll
    have some juice still on your finger and the next person entering your room
    will find you immobilized there and if this happens after more than 4
    minutes your brain already suffered irreversible damage... Even if they
    manage to re-start your heart they'll never manage to fully re-load your
    operating system...
    Again: if you have the smallest suspicion that your house grounding may be
    faulty or not necessarily faulty, just if you thing it lost some of it's
    efficiency, have it checked. If you know exactly what you're doing you can
    do it yourself.... which I believe you would've done already... if you knew
    what you're doing... remember that money are of no value if you're not
    around to spend them, hire a certified electrician to have your grounding
    checked and eventually brought back within proper operating parameters
    before something unwanted happens.
    This may also fix your problem... but again, it may not. The switching power
    supplies used in computers are converting the household AC voltage into DC,
    converts it back to AC but this time with a high frequency that allows use
    of a little high efficiency step down transformer to reduce the voltage.
    Once reduced it's converted to DC that's used by all components. To
    eliminate - as much as possible - interferences between the household AC
    frequency and whatever goes out of the power supply they use all sorts of
    filters (capacitors, inductors, etc). As you know, all filters have a
    certain range where their efficiency is high, outside of that range the
    efficiency decreases so it may allow some system frequency components or
    harmonics to enter your PC. If the ground is ok it may be that you, acting
    as a capacitor, change the filter range a little, sufficiently to reduce the
    hum to more acceptable levels. In which case you may want to try a different
    power supply, one that may better filter the primary components.
  3. Guest

    Could be light dimmers or other equipment causing the problem.....
    try shutting off lights, television, etc ...... also move the
    microphone several feet away from it's present location, and definately
    away from your computer and monitor. You can purchase an inexpensive
    plug-in wiring checker at most hardware or electrical supply stores
    that will verify if your outlet is wired and grounded properly.
  4. However, by touching the case and touching the floor with the >other hand

    No, no, no...... You must touch the live conductor with one hand and a
    water pipe with the other...............
  5. Mark

    Mark Guest

    if you are holding the mic in your hand, it is probably because you are
    grounding yourself to the computer.

    Otherwise your whole body is picking up 60 Hz hum and coupling it into
    the mic

  6. andy12345

    andy12345 Guest

    Thank you very much for all of your replies. I believe maplin
    electronics sells a checker for such things. The points you make
    are just like walking down the street without any body armour. You
    never know if body armour is going to save your life one day until it
    happens. Unfortunately, at that point, the knife blade will penetrate
    and kill you because you did not have the body armour on......
    It is easy to electrocute yourself when you are not thinking
    correctly or fail to understand whats going on. I do understand such
    things of course, but, i never thought of the implications of doing
    what i did, as i havent thought to actually be aware of a potential
    earthing failure.
    You know what its like.... No one expects to suddenly be potentially
    electrocuted. So, thanks again and I will get onto the matter at
    hand. By the way, the microphone is a combo headphones/microphone set
    which I do not hold at any point, except for on my head. Maybe the
    wire screening is crap.
  7. andy12345

    andy12345 Guest

    Hello again,
    After reading my initial post, I was shocked (ahem) to notice that I
    said 'I have bypassed the psu earth.........' Let me elaborate on
    that. What I meant was that the pc was connected to the mains in the
    totally normal way. All I did was attach an additional cable to the
    earth contact of the extension lead socket to the internal pc case
    chassis. I know that I could still be dead of course.
  8. andy12345

    andy12345 Guest

    Hello Again,
    I just remembered that i have one of those screwdriver tester devices
    which has a led that lights up when touched or hovered near wires and
    plug sockets. I touched the case with it and nothing lit up at all.
    Does that give new ideas?
  9. andy12345

    andy12345 Guest

    OKay, its me again.
    This time I thought hmmmmm. Use a multimeter. I set it from 0-600vac
    and 0-600v dc and nothing showed all on the case, inside or outside.
    I can stop the microphone hum just by touched painted, non conductive
    pc case surfaces therefore I diagnose that there is not an earth
    problem. I trust that I am correct however, I dont want to be wrong
    in anyway so if you can just give your verdicts when you have a
    chance, I believe that the matter can be closed and put down to
    something else other than earthing problem.
  10. has a led that lights up when touched or hovered near wires and
    Yes, the LED is fucked!
  11. Mark

    Mark Guest

    did you read my post?
  12. stu_e

    stu_e Guest

    using a straight wire to ground is shorting the two points out. however they
    are already shorted by chassis grounds internally. so, the only difference
    between the 2 points is distance and that allows noise to get on the line.

    your body acts like a filter ( a capacitor). if you want to try installing a
    more permanent filter, try installing a 0.1 micro farand,200vac cap between
    the pc case and earth ground....about 50 cents at radio shack.
  13. stu_e

    stu_e Guest

    i believe you are seeing two separate issues:

    1. the 58vac hot to ground and neutral to grd are because when you unplug
    the input power cord you lose your reference ground. during an outage, with
    input and load power cord present, your safety ground reference would still
    be present....the voltage would be close to 0 neutral to ground.

    2. the voltage change:

    when input utility is present the apc just passes that through to the load
    with some isolation. when the input is lost, the dc to ac conversion is not
    perfect and the load does not really care. you would see this change with an
  14. andy12345

    andy12345 Guest

    Hello everyone,

    Just another theremin fan:- Thanks for the water pipe idea. I will
    try that when having a bad hair day. The screwdriver power tester
    led works fine just by hovering it near cables as I checked

    main account/sm:- Thanks for your highly detailed post. I will chew
    that over for some time.

    Mark:- I am in england and therefore on a 50 hz system and could
    potentially be coupling that into the microphone. I am not holding
    it, but, it is on my headphones set isolated by some headphone
    padding. Can it couple under such circumstances?

    Stu_e. I do not see the 58vac that you refer to anywhere in this
    thread. Am I missing something? Okay, so the 0.1uF capacitor
    stores charge, but why is that relevant? Is it to just remove some
    interference at a quicker pace than it builds up or something like
    that (I am a noob in some aspects) o.0 ?

    I should definitely buy a quality headset or quality standalone
    microphone before I post anything else I suppose. Do ferrite filters
    work on microphones leads?
  15. They only do a rough check. You need an earth loop impedance tester to do
    the job properly, and they aren't cheap.
  16. I'd say by the Maplin (bit like RS) reference Andy is in the UK.

    In the UK, neutral and earth may either be connected at the substation, or
    at the home in many modern installations.

    On an older installation where the neutral and earth aren't connected in
    the home, it's common to see several volts between them.
  17. Is the comms unit self powered? Ie by a wall wart? If so it's likely to be
    double insulated so no *safety* earth required. So what you're dealing
    with is an audio ground. Is the casing of the mic connected to the screen
    of the output cable? The mic *might* be a balanced type with the screen
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