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Thread: IBM 5151 Monitor, bright spot when turning off

  1. #1
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    Default IBM 5151 Monitor, bright spot when turning off

    Last week i picked up an IBM 5151 together with a clone XT computer. I'm new to pre-386 computers and their accessories and didnt used a 5151 monitor till now.

    Monitor is functioning without problem, image is clear and crisp, but when i turned off the computer, monitor turns off with a very bright spot around the middle part of the screen. Is it normal for these monitors or something needs repair? (aged capacitors in monitor, or something bad on clone MDA card maybe?) In color monitors there is a circuit which turns the electron guns off just before turning the deflection off during power loss to prevent phosphor burn caused by bright spot at the middle of the screen, but i have no idea such a measure is built on these monitors too. Before starting to troubleshooting, i wanted to ask.

    I recorded a video, http://vid1376.photobucket.com/album...ps3gfn7lmu.mp4

  2. #2

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    Older TVs do this, too.
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  3. #3
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    My 5151 has always exhibited this same behaviour since I obtained it, and I've had it for 15+ years.

  4. #4
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    If you have the 5151 power cord plugged into the rear of the 5150 power supply.......turning off the 5150 power switch turns
    off the 5150 and 5151 at the same time. Have you tried just unplugging the 5151 power cord to see if the symptoms are the same ?
    Last edited by mikey99; January 17th, 2016 at 03:12 PM.

  5. #5

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    Tried that with mine and it is an improvement. When I power off cpu and display at the same time, the display collapses to a bright line down the center. When I unplug the display first, it collapses to a one inch band
    down the center then extinguishes so not as harsh. I don't see any phosphor burn so it does no damage anyway.

    Larry G

  6. #6

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    Here's a pretty good description of what's happening:

    http://www.madsci.org/posts/archives...1922.Eg.r.html

  7. #7

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    Interesting, I had training on CRT's and the Sony Trinitron but had not heard of the use of an aluminum film to protect the phosphor.

    Larry G

  8. #8
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    Then it looks like this behaviour is by design and most probably won't harm the phosphor coating. My 5151 is 1984 made and should be powered off like that hundreds of times up to today. Anyway, i can make a short cable with a switch on it between computer and monitor to be able to turn off 5151 before shutting down the computer.

  9. #9
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    Quote Originally Posted by retrogear View Post
    Tried that with mine and it is an improvement. When I power off cpu and display at the same time, the display collapses to a bright line down the center. When I unplug the display first, it collapses to a one inch band
    down the center then extinguishes so not as harsh. I don't see any phosphor burn so it does no damage anyway.

    Larry G
    You're right, mine's the same. I'm very curious about the reason behind this. The beam raster is created in the circuitry of the monitor, not in the PC. Maybe it is easier for the raster to collapse without an input signal for some reason? Nevertheless, I'm going to pull the power on the 5151 first from now on. It seems kinder than the alternative.

  10. #10

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    The MDA display is unique because the horizontal and vertical drive to generate the raster from the pc IS the drive signal, not just for synchronization so when the PC is powered off, the H&V drive to deflect the beam is instantaneously gone but the
    high voltage has not had time to bleed off so the beam continues to the center of the tube.

    Larry G

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