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Why would there be voltage beng fed back to the aerial??

Discussion in 'Hobby Electronics' started by APR, Jun 24, 2007.

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  1. APR

    APR Guest

    Hi all,

    A computer fitted with a TV card has been progressively failing by not
    wanting to go through the boot sequence, It was going past the password
    screen, then began locking up at the password screen. Some drivers appear to
    have become corrupted.

    The owner started to remove cabling from the rear of the computer and got a
    light zap when removing the aerial cable from the TV card. Now the computer
    will not boot up at all.

    I got involved and, upon measuring the potential from the aerieal lead plug
    where it would plug into the computer TV card to a three pin earth in a wall
    socket, measured around 90 volt using the AC measurement range on a meter.

    The aerial has no amplifier, however, it does attach to an LG video and then
    on to a TV.

    Measuring potential on the antenna connection out of the TV gives basically
    nothing, measuring the potential on the LG video player aerial connection
    gives the high voltage reading.

    Can anyone shed some light on this issue. Both the TV and Video player are
    double insulated.

    I would appreciate comment from PA on this.

    Thanks

    As an aside, after getting the zap when removing the aerial lead from the TV
    card the owner now finds the computer will not respond to the power switch.
    The power supply is working and delivers the correct voltages, it appears
    the motherboard has failed.
     
  2. James

    James Guest

    If you had a circuit diagram for the VCR you could probably see for
    yourself. Look for a couple of ceramic capacitors between the primary and
    secondary of the SMPS. Getting a little tingle from the aerial socket is
    very common.

    James
     
  3. Getting a small tingle like that is very common on such gear due to
    very small leakages. You are obviously measuring the voltage with a
    DMM which has a high impedance.

    I would not rule out the computer failure just being a coincidence.

    Dave.
     
  4. Franc Zabkar

    Franc Zabkar Guest

    Remove all the unnecessary cards and devices and then try to boot with
    a bare minimum system. Add components until the machine fails to boot.

    - Franc Zabkar
     
  5. APR

    APR Guest

    Hi Frank, I have been through this process. With only the processor
    installed the motherboard won't respond to shorting the power switch
    connectors on the board. The power supply does fire up and give corrent
    voltages when the green Power Supply On connection is run to ground.
     
  6. Phil Allison

    Phil Allison Guest

    "APR"

    ** Didn't you read this when I gave the link a while ago ?

    http://sound.westhost.com/articles/external-psu.htm

    particularly this section...

    http://sound.westhost.com/articles/external-psu.htm#kil


    BTW

    External ( outdoor /rooftop) TV antennas are required to be earthed.

    How come this one is NOT ???



    ........ Phil
     
  7. James

    James Guest

    I thought about that myself, and am guessing that the VCR was not connected
    to an external aerial whilst being connected to / disconnected from the PC.
    In haste its quite possible that the RF lead to the external antenna (could
    have been using rabbit ears too?) was disconnected before the PC tuner was.

    I actually have a user manual somewhere here for TV or STB that states that
    a small tingle may be felt when disconnecting / connecting cables and that
    this is normal. I'm guessing the manufacturer added this to the manual to
    prevent false warranty returns on the basis of the unit being unsafe. All
    manuals for equipment I've actually bothered reading tend to make a note
    regarding connection or disconnection whilst the equipment is switched off.

    James
     
  8. Doug Jewell

    Doug Jewell Guest

    FFS!!! I wasn't aware that Govco was working toward this insidious little
    piece of work. Friggin ridiculous what they are doing in the name of
    "greenhouse effect" and "climate change". Next thing you know they'll have
    us all huddled together in a cave eating raw beef, since we won't be allowed
    to start a fire to cook it.
    Thanks Phil, will work toward drafting a few letters to get to MP's and to
    the media.
     
  9. Phil Allison

    Phil Allison Guest

    "Doug Jewell"
    "Phil Allison"

    ** Go for it, need any help - just ask here.

    Insidious is exactly what it is.




    ........ Phil
     
  10. APR

    APR Guest

    Hi Phil,
    thanks for the reply and for the links, it explains a lot. The house is
    rented, a tile roof with the aerial nounted off a timber barge board and the
    lead going down the side and under the house to enter through the floor. I
    can imagine this being done by the owner of the building, with him knowing
    nothing of earthing requirements or any other installation requirements.
    (makes you wonder if he may have done any 240v wiring in the house)
    The renter is a bloke I know well, I will steer him in the direction of this
    newsgroup to have a read and let him determine where he goes from here. I
    think a good start may be earthing that aerial, though.
     
  11. Phil Allison

    Phil Allison Guest

    "APR"
    "Phil Allison"

    ** An unearthed, roof top antenna can easily acquire a static charge of 10
    kV, or more, on a dry, windy day.

    Ouuuccchhh !! !

    SMD size semis are likely to use even stronger language ....




    ....... Phil
     
  12. Friday

    Friday Guest

    Also,

    Antennas can pick up quite a big static charge from a dry wind. And a
    dipole antenna in rain picks up a charge from the rain drops. During a
    heavy shower I had a disconnected cable for a dipole for the 80 metre
    band which had a continuous arc from the center pin to the outer part of
    the connector.
     
  13. Franc Zabkar

    Franc Zabkar Guest

    I propose that to offset the planet killing impact of these dastardly
    devices, each of us spend an additional 15 minutes of each day in the
    prone position. In this way we will lower our metabolisms, reduce our
    individual CO2 contributions, and entitle ourselves to carbon credits.

    - Franc Zabkar
     
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