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Dumb TV question

Discussion in 'Electronic Repair' started by oldfogie, Jan 28, 2008.

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  1. oldfogie

    oldfogie Guest

    What is the purpose of the small neon bulb by the flyback on older TVs? (I haven't noticed it in any modern tvs.)
    Someone asked me that the other day and I couldn't answer! Seems like I used to know (I'm an old-timer) but can't remember.
  2. Charles

    Charles Guest

    What is the purpose of the small neon bulb by the flyback on older TVs? (I
    haven't noticed it in any modern tvs.)

    A voltage clipper? Neon lamps have very high resistance and then drop to a
    low resistance when the gas ionizes. About 60 volts, I think.
  3. Sjouke Burry

    Sjouke Burry Guest

    50-100 volt spike clamp??
  4. msg

    msg Guest

    Could be a voltage reference (~60 - 90 V), voltage drop or translation,
    a hysteresis element in a multivibrator, clamp or even indicator of
    some circuit's status.


  5. Bob Shuman

    Bob Shuman Guest

    Are you sure it is really a neon bulb? I'm thinking it is possibly a high
    voltage spark gap which serves as overvoltage protection... Just my
    thoughts. Without seeing it it would be hard to say for sure.


    What is the purpose of the small neon bulb by the flyback on older TVs? (I
    haven't noticed it in any modern tvs.)
    Someone asked me that the other day and I couldn't answer! Seems like I used
    to know (I'm an old-timer) but can't remember.
  6. Marty

    Marty Guest

    I'll bet it's a solid state GE from the early 1970s. They included the
    bulb as a high-voltage indicator. This was to deter arcing with a
    screwdriver as a test.
  7. oldfogie

    oldfogie Guest

    Yes, I'm sure, was a small glass neon bulb soldered to chassis.
    I looked through a good many old Sams folders, back into the '70s & '80s but never found a neon bulb in one. Problem is, just because it was near the fly doesn't have to mean it was in the fly circuit. So it must have been in some other circuit. I think it was for spike protection, for whatever circuit it was in.
    I'm ready to just forget it, not important.
    Thanks to all for replies!
  8. Guest

    A real neon bulb? I wish I owned a million of them, I love neon
    lights/neon signs.I own an old broken neon NO VACANCY sign I bought at
    the Goodwill store years ago.It was broken when I bought it.I like it
    anyway.I think it dates back to the 1940s or 1950s.Nothing beats neon
    lights and neon signs.
  9. I think it was used as a high-voltage clipper... sort of a high-voltage
    zener diode. During normal operation it's never supposed to light up. I
    often saw those on neck boards of old monochrome video monitors. Only once
    did I ever see one light up and that was due to picture tube failure (the
    precise details of which I can't remember now). The firing voltage of neon
    bulbs is pretty high... at least 50 volts, some go as high as 90 volts.

    BTW: If the bulb is placed across DC, only one of the interior electrodes
    will light up. If the bulb is placed across AC, then both internal
    electrodes will light up.
  10. Arfa Daily

    Arfa Daily Guest

    If it was a true neon bulb, then it wouldn't actually need to be connected
    into any circuitry in order to light up as an indicator of flyback activity.
    Most gas filled tubes will light in mid air if they are in close enough
    proximity to a flyback tranny. Back when I was an apprentice, like 35 years
    ago, the guy that I worked under in a TV workshop, used to keep a short thin
    flourescent tube on his bench. He used this to test for horizontal output
    stage activity, simply by waving it around the FB tranny. As I recall, he
    used to reckon that he could tell a lot about how that stage was working,
    when he had a lack of picture fault, just by the 'way' in which his little
    tube lit up.

    Most of the similar 'bulbs' that I've seen on CRT base connector boards,
    have been gas-filled spark gaps. I seem to recall that they used to put
    argon in them, and when they went off as a result of an inter-electrode
    short in the tube, they lit up white, rather than the orange glow of a neon.

  11. PeterD

    PeterD Guest

    Years ago, we'd just tape or glue a neon bulb on either a dowel or a
    pencil. Then someone realized that pencil lead conducts and the dowel
    method became much more popular!
  12. Bill Jeffrey

    Bill Jeffrey Guest

    Assuming the HV supply is still good, you should have it rebuilt. Neon
    signmakers are plentiful, generally do good worrk, and can duplicate any
    tube you bring them.

    Just a thought.

  13. James Sweet

    James Sweet Guest

    Even if it weren't, used neon transformers are cheap.

    I haven't heard anything but incoherent rantings from that guy though, I
    dunno what he's doing on this group. Nothing he posts makes any sense, or
    has anything to do with electronics repair.
  14. Guest

    I know all about this.
    I run a farm. Around the livestock I have electric fences that
    normally put out about 5 or 6KV. (less when wet or being touched by
    weeds). Although I have a tester that connects to the wire and tells
    the approx voltage based on the number of neon bulbs that light, it
    requires connecting it to the fence and a ground. Often I just want
    to know if the fence is working. I have a short florescent bulb that
    I took out of an old kitchen range. All i need to do is hold it a few
    inches from the fence and it will light if the fence is working. I
    even made a holder to keep it in place so I can just look out the
    house window and see if it's glowing. That worked until a pony decided
    it was something to play with.

    Either way, it works well as a quick test.

    To the OP of this message, I do recall seeing the neon bulbs in the
    older tvs

    Which reminds me. I have an old neon bulb nightlight. This thing is
    confused and retarded. If the room is dark, it does not light. But
    when I flip on the ceiling lights in the room, or point a flashlight
    at it, the neon lights up....
    I think it's just plain old, but I leave it there just because it's a
    conversation piece.
  15. Believe it or not, this is more or less normal for an old neon bulb.

    Ambient light knocks electrons off the neon atoms in the bulb, _lowering_
    the threshold voltage. This bulb has aged to the point where the line
    voltage isn't high enough to light it _until_ room light or a flashlight
    "kicks butt" on them-there outer-orbital 'lectrons.
  16. Guest

    My old NO VACANCY neon sign is an old metal box, the old brown paint on
    the box is very faded.The neon glass tubes, some parts of the glass
    tubes are completly broken away, missing.The box still has the old high
    voltage power thingy in it.I don't think there is anybody in my local
    city area who builds/repairs old neon signs.I only hang onto it because
    it is very old and I like to collect all kinds of very old things.The
    Postman Always Rings Twice old movie.That poor cat got zapped on that
    neon sign.
  17. JeffM

    JeffM Guest

    William Sommerwerck wrote
    In the '60s, electronic organs used neons in their percussion circuits
    (e.g. a Xylophone tab).
    Incandescents were positioned inside the unit near the neons
    so that NE-2s were "biased".
    When someone said his precussion stopped working,
    you made sure your stock of mini incandescents was up to snuff.
  18. JeffM

    JeffM Guest

    Old tricks are so cool.
    Well, Argon would be the cheapest
    --but the Argon stuff I've seen glows purple.
    Maybe those are a different gas--or a gas mixture.
  19. Tim

    Tim Guest

    You could always use those Cold Cathode light ropes they sell for
    computer case modding. They could bring the neon look back to life, with
    no HV problems to deal with.

    - Tim -
  20. James Sweet

    James Sweet Guest

    Or just send the glass out to have it fixed properly. I dunno how it's
    possible to live to far from a neon shop, there's lots of mom & pop joints
    still spread around the country. The craft nearly died out for a while, but
    in the last couple decades there's been somewhat of a revival. Once have the
    sign box with the transformer, it's not terribly expensive to have new units
    made for a simple sign like that.

    There's LED "neon" out these days, but nothing comes close to the real
    thing, it just doesn't look right.
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