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Mystery component in Sam's PhotoFact

Discussion in 'Electronic Repair' started by j, Feb 22, 2007.

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  1. j

    j Guest

    There's a schmatic symbol in Sam's PhotoFacts # 2908 and 2753-1
    (covering JVC TV sets av-2749s, av-2759s, and av-2779s) that I don't
    recall ever having seen before. It's NOT listed in the parts section
    so I have no original or replacement numbers for reference. It's in
    the grid (G1) circuit of the CRT.

    Here's my attempt to describe it for those without access to the
    PhotoFacts that I mentioned. It looks like a circle with a vertical
    line running through the middle. On the upper left half of the circle
    there's what appears to be a "black pie" section extending from a
    point in the middle of the circle with an angle from about 11 to 12
    o'clock. It almost touches the circumference. On the lower right half
    of the circle there's a simular "black pie" section, with an angle
    from about 5 to 6 o'clock. There's no line where the two "pie
    sections" meet so it doesn't appear to be some sort of dual diode

    If anyone knows what it is, please let me know. I have researched
    electronic schematic symbols on the web and in books but have not
    found the symbol or what it represents.
  2. David

    David Guest

    Spark gap. These are often part of the tube socket itself.

  3. j

    j Guest

    Thanks for replying. Have you worked on JVC TV's or have seen the
    symbol before, or are you making an educated guess ?

    The reason I ask is that there's no other such symbol on the other CRT
    terminals in the schematic. I know in the past, Sam's has had two
    arrows facing each other as the symbol for a spark gap. If you can
    supply a reference book or web page, that would be great.
  4. Sofie

    Sofie Guest

    You should trust David's reponse to your question..... likely a spark gap.
    This is not just a JVC thing. Many brands and models use them andin
    particular on the grids of the CRTs where HV potenetial can do lots of
    component and circuitry damage upstream.
    And, by the way, what is wrong with an educated guess anyway???
    - - - - -
  5. j

    j Guest

    Thanks for responding. I apologize if my description was unclear but
    sometimes it's hard to describe a graphics figure. Unfortunely, it's
    very likely NOT a spark gap. I had not noticed before because it was
    greyed out in the schematic, but all the CRT terminals have labeled
    spark gaps (still two arrows facing each other - the arrows don't

    Perhaps a circuit description would help. It starts with a 200 Volt
    DC source connected to a 1 M ohm Resistor. A 10 uF cap is connected
    in parallel with the R. The other side of the R is connected to the
    anode of a 1N4007 diode, whose cathode is connected to ground.
    Connected in parallel with the diode is the mystery component. Also
    connected to the anode of the diode is a 1k ohm R, whose other side is
    finally connected to the spark gap and grid of the CRT.
  6. Radiosrfun

    Radiosrfun Guest

    Why don't you post a picture of it in a binaries group - let the folks here
    know WHERE - and then they can look at it and tell you? Then you won't be
    confused as to how to describe it and they can see what the hell it is
    you're trying to describe. Just take and scan the thing OR photograph it
    with a arrow pointing to it.
  7. clifto

    clifto Guest

    I've seen what you're talking about. I want to say it's a transient
    protection device but I honestly don't remember it that well.
  8. David

    David Guest

    This is David. It was an educated guess.
    Your circuit description makes it clear that the device is not directly
    between the CRT grid and ground. This seems to be a strange circuit if in
    fact the 200v supply has no signal component on it.
    Have you visually looked at this mystery component? If so, what does it look

  9. j

    j Guest

    I agree, it does seem to be a strange circuit. I'll provide a
    physical description next, but see my comments for clifto.
  10. j

    j Guest

    Do you remember what type or model number of the equipment had the
    device ?
    Maybe I can jog your memory. This is from Herman's fourth edition of
    Electronics for Electricians:

    "(MOV) Metal Oxide Varistors are a type of thyristor that exhibit a
    change of resistance with a change of voltage...the MOV is a bilateral
    device... "

    The problem is that the schmatic symbol for the MOV in the book and on
    the web are the same but way different than what's on my Photofact

    Still, I'm leaning towards your suggestion. My wild guess from the
    circuit description is that the purpose of the device is to protect
    the diode and the CRT. The diode, in turn, protects the CRT by
    limiting the voltage to 0.6 volts.

    But I want to be sure. Perhaps by providing a physical description I
    can get more info. I'll do that next.
  11. Bill Jeffrey

    Bill Jeffrey Guest

    Unlikely to be a transient suppressor, given its position in the circuit
    you described. (I don't have a better idea, BTW!)

    Actually, the whole circuit you described doesn't seem to make much
    sense. Starting with 200 VDC (not VAC, right?), the power supply will
    push 200 microamps at most through the 1 M resistor. If the whole 200 uA
    passes through the diode, the 1N4007 (a rectifier diode) will be barely
    in conduction, so the voltage developed across it will be perhaps 0.5
    volts. If some of those microamps are shunted through the mystery
    device, then the voltage will be even less.

    And this small positive voltage is applied to the grid of the tube
    through 1K, probably acting solely as an isolation resistor. I can't see
    that there is enough voltage, or available fault current, at that
    location, to warrant a transient suppressor. Hmm.

    By the way, that grid isn't connected internally to some other element
    in the tube, is it? I don't think you specified the tube type.

    And the 200 volts is DC, not AC, right? In that case, I can't make any
    sense of the 10 uF cap, either.

    Sorry not to be of more help.

    Bill Jeffrey
  12. Radiosrfun

    Radiosrfun Guest

    Again - why not post a scanned picture or digital picture of the part of the
    schematic in question - with an arrow pointing to the part in question - in
    the Binaries group so ALL can see it and know for SURE - what the hell it
    is. I've seen guys offer suggestions and the OP seeming to refute each one
    of them. At this rate, no one will ever know - including the one who needs
    to know - the OP.
  13. Jim Land

    Jim Land Guest

    Yeah, what he said. Also, a closeup photo of the part in question.
  14. j

    j Guest

    Actually, "clifto" wrote that, but he was probably right (more on that

    Now, about the 200 VDC source, it also goes (thru a coil and resistor)
    to the collectors of the 3 color output transistors. The video output
    at the collectors goes (again thru a coil and r) to the respective
    grids in the CRT. Apparently, transients are supposed to turn on the
    diode, not the 200VDC. It still remains a strange circuit with the 10
    uf cap in there.

    After I added "sci.electronics.basic" to the group line in my posts
    and posted the physical description of the mystery device, one of the
    guys in sci.electronics.basic appears to have identified the device.
    Knowing how to identifying a device from the charactors on the part is
    sure helpful, but his messages have not appeared here.

    Although it works like a Metal Oxide Varistor (MOV), Panasonic says
    it's a Zinc oxide Nonlinear Resistor (ZNR).

    Sam's PF did designate the component as "R364", but the symbol they
    used led me to suspect that was an error. I haven't seen the symbol
    on any recent or past schematics, only the 2 (1990-1991) in my
    original post. I've already spent too much time and effort on this
    apparently rare symbol so I won't post a pic in a binary group (I
    don't know how anyways). If someone still wants to have an idea of
    what the symbol looks like, a can give a different description but
    that's all.
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