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Advice on yoke (scan coil) / CRT replacement

Discussion in 'Electronic Repair' started by Adrian Smith, Feb 29, 2004.

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  1. Adrian Smith

    Adrian Smith Guest

    I have an Iiyama Visionmaster Pro 410 with a severely scratched CRT face
    which would not polish out. I replaced the entire CRT assembly including
    scan coils with one from a scrap Dell P791 which uses the same type of tube
    (Mitsubishi Natural Flat 17") but different coils. A quick check on the coil
    resistances proved them to be the same readings as the original but I cannot
    check their inductance. I connected the rotation coil and the coils for
    convergence and found the image when powered up is sound geometry wise
    indicating the scan coils are probably compatible.

    However, the line output transistor seems to be running a few degrees hotter
    than normal, but the unit worked fine for two days. The greyscale needs
    adjusting slightly but this is not possible as I don't have Iiyama's
    software to set it up.

    It is too much hassle to swap the scan coils over between the two tubes and
    I can live with the greyscale problem but would the monitor be OK long term
    with these substitute coils? It was suggested to uprate the line output
    transistor to a 25A transistor instead of the fitted 20A but I don't think
    it will make much difference.

    Any suggestions before I bin the old CRT?
    Also does anyone have the service software for this monitor?

    Any help would be greatly appreciated.

  2. Adrian Smith

    Adrian Smith Guest

    Degrees centigrade; the transistor is not too hot to touch but notably
    hotter than another working unit with the original CRT.
  3. Sofie

    Sofie Guest

    Adrian Smith:
    A 25 A transistor instead of a 20 A transistor WILL NOT solve the heat
    problem........ the same amount of power will still be dissipated in the
    form of heat..... the SAME amount of heat. Also, the gray scale problem
    is NOT caused by scan coils but rather the CRT itself and the support
    circuitry in the monitor...... usually there are adjustments..... over a
    limited range... that will compensate for the RGB gun imbalance inherent in
  4. Adrian Smith

    Adrian Smith Guest

    Thanks for the info but the monitor has no adjustments for cut off and gain
    of the guns as this is done via a service mode or via service software.
    There is a connector on the mainboard labelled 'SC' which has nothing
    connected. Two of the pins are the SDA and SCL lines of the I2C bus so I'm
    guessing SC means Service Connector and some sort of service jig attaches to
    this for adjustment.

  5. Sofie

    Sofie Guest

    Adrian Smith:
    Yes, the monitor circuitry DOES have adjustments for gray scale tracking
    but, as you discovered, these adjustments are carried out in software just
    like many models consumer televisions in the past 10 years or so. .... no
    matter whether or not the gray scale is aligned via physical controls or
    through software.... they are adjustments never the less.
    At the current low prices of new CRT based computer monitors these days you
    might be best advised to stop wasting your time and money and just replace
    it with a new one..... or a used monitor may be available usually at good
    prices at computer repair places, etc.
  6. RE: How hot the HOT?
    If it's not too hot to hold your finger on it (insulated of course!), it's
    probably OK. Did you install it with heatsink compound and/or a heat
    conducting pad?

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  7. Adrian Smith

    Adrian Smith Guest

    Its mounted as originally with a thermal transfer insulating pad. Although
    very warm, its not too hot to touch and you can leave your finger on it
    without burning so I guess I am worrying over nothing.
    Thanks for your help.

    As for the greyscale problem, a circuitry hack will probably work if I can
    just find a datasheet for the video IC...
    Its all good experience...

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