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Thread: IBM 5150 EGA card vertical offset/roll issue

  1. #1

    Default IBM 5150 EGA card vertical offset/roll issue

    Hey everyone, I was hoping someone might have some insight into an issue I've been having with an EGA card.

    The vertical position of the screen will be offset, cutting off and wrapping around the image upon boot randomly. Sometimes, if you leave the card on for long enough, it will correct itself. It's always been a little funky, but would usually correct itself within a few seconds. Not so much now.

    Very weird, and very frustrating. I've not been able to find any similar issues online. I've already tried the following: recapping the tantalum caps, changing slots, removing other cards. Any help would be mucho appreciated.

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  2. #2

    Default

    Sometimes, if you leave the card on for long enough, it will correct itself.
    Smells like cold solder joint for me. Obviously you ruled out monitor itself?

  3. #3

    Default

    The setup you have shows a 5153 monitor displaying CGA format image. So your EGA card is running in GCA system mode.

    The black bar you see is the vertical blanking interval, which is where the vertical sync pulse normally resides. That sync pulse is supposed to precede the start of vertical scan which is off the screen, above the top of the displayed image.When, of course, the monitor's vertical oscillator is correctly locked to it. So the vertical scan oscillator, in the monitor, has locked to a position in the picture timing area, where there is no (or should be no) vertical sync pulse.

    There could be a few possibilities:

    1) If the monitor is ok, and the card defective, the vertical sync pulse could have disappeared from the blanking interval, and the vertical scan oscillator is locking to some random part of the picture information. A failed gate could do this.

    2) the card is producing a vertical sync pulse in the wrong place with respect to the picture information.

    Both of the above are easily sorted out by putting a scope on the vertical sync pulse signal to the monitor and the other scope channel on one of the picture signal lines, Say G, and seeing if the vertical sync pulse is in the correct position.

    OR:

    Apart from the exotic possible explanations above, which blame the video card, the fact that it is changing suggests there may be nothing wrong with the card at all, and merely the V.Hold control on the monitor is out of adjustment and the monitor vertical scan circuit/oscillator is running a tad fast and just randomly locking to some picture information and not the Vsync. So if you have not tried it yet, find and adjust the V. Hold control and see what happens.

    When the vertical hold control is set correctly , prior to lock, the vertical scan oscillator is just running a little slower than in its locked state, so just before lock, that black vertical blanking bar should be seen to be rolling upwards toward the top of the screen. If it rolls downwards prior to lock, the vertical scan rate (hold control) is set too fast and its not a stable condition.

    Finally, there is something else happening. A number of objects have been placed on the top of the monitor, obstructing its cooling vents. These monitors rely on convection cooling and if that happens, it will cook it up.

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