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Arcade game monitor problem

Discussion in 'Electronic Repair' started by Ken G., Oct 22, 2008.

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  1. Ken G.

    Ken G. Guest

    I have an old asteroids full size vintage video game .
    I has a black & white monitor .

    When it first comes on its fine . Within 10 minutes the whole image gets
    bigger , things disappear off the edges . The center of the screen gets
    a bit darker .
    I suspected the hi voltage diode off the flyback and adjusting the hi
    voltage pot but replacing that did not help .

    I dont have any model #s the monitor has one medum circuit board and
    one small board with the flyback in a metal box .

  2. Ken Layton

    Ken Layton Guest is the correct newsgroup.
  3. Franc Zabkar

    Franc Zabkar Guest

    The "blooming" does sound to me like a low EHT problem. I'd check the
    B+ going to the FBT, and the base drive to the LOP transistor. Make
    sure the duty cycle doesn't change. I've seen this happen in an old
    IBM monitor as a ceramic capacitor in the horizontal oscillator warmed
    up. However, in my case the EHT gradually increased until the HV
    protection shut down the monitor.

    - Franc Zabkar
  4. Ken G.

    Ken G. Guest

    Thanks Franc .. I dont know which conection is B+ on this . It seems to
    be a Wells Gardner monitor according to a website provided i an arcade
    newsgroup .
  5. Franc Zabkar

    Franc Zabkar Guest

    I would think that one end of the primary winding would be connected
    to the collector of the switching transistor, the other to the B+.

    If the FBT has any part numbers, try locating a substitute here:

    The HR substitute will have a wiring diagram with voltages.

    I notice that the Wells Gardner Quadrascan colour monitor at the Atari
    link provided by Dean uses a HV regulator circuit that senses a 180V
    pulse from a tap on the primary winding and uses this feedback to vary
    the B+ to the FBT, and thereby control the EHT.

    - Franc Zabkar
  6. Yukio YANO

    Yukio YANO Guest

    Sounds to me, more like the Filament Voltage is dropping, cold solder
    joint in filament circuit ?

    This describes what happens to the image when I reduce the filament
    Voltage on my Transmission Electron Microscope !

    Yukio YANO
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