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IBM PS/1 Monitor (CRT)

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    #1

    IBM PS/1 Monitor (CRT)

    Hello, I have an IBM PS/1 SVGA CRT monitor, it was working ok untill it started to make a disturbing noise and the image was as if it was turning off constantly,
    I opened it and i found that the flyback transformer was discharging to ground, I've tried isolating the leak, no success, I've replaced the transformer with exact replacement, it turned on fine but now i have extremely washed out picture see IBM_21.jpg, after that I replaced the RGB amplifier and the image got a little better with colors but almost no contrast see IBM_22.jpg

    I've checked diodes around flyback transformer, I tried to adjust flyback transformer, any ideas?
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    #2
    Re: IBM PS/1 Monitor (CRT)

    Looks like when i lower the resistance of that adjustable resistor the maximum contrast improves but still not enough could it be one of the caps? or both?
    Attached Files

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      #3
      Re: IBM PS/1 Monitor (CRT)

      Do you have any schematic for the monitor? It looks like a Taiwan build. Look for any text or markings on the board.
      I would check some power supply voltages are still in spec. because contrast should not go down, unless maybe the wrong flyback transformer and voltages are lower - but you say it's an exact replacement. Maybe a small rectifier diode got damaged from the arc.
      The neck board also can have broken solder joints there, it is common.

      There should be a service switch (maybe on the neck board) and potentiometer(s) for setting the CRT screen voltages and cathode drive voltages. These affect the contrast.

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        #4
        Re: IBM PS/1 Monitor (CRT)

        I don't have schematic I've attached a photo with some labels
        If the pwoer supply had a problem shoudn't be a problem with brightness also? the brightness seems to work fine.
        As you can see in the photos the flyback transformer is the same model.
        I've checked some diodes especially around the flyback transformer and the neck board.
        I'm going to reflow the neck board solder just in case but i doubt thats the problem.

        I've played with the marked potentiometer and the contast got better but i've set it to max (or min) and it didn't got good enough.
        I've also posted a picture of the IC i've replaced the old one is LM1203N and the new one is LM1203BN there is a small diference I hope that's not the problem.
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          #5
          Re: IBM PS/1 Monitor (CRT)

          If the monitor picture was good prior to changing the flyback, (due to it arcing) it could be that the flyback is just a bad replacement. I have seen this many times with aftermarket flybacks.
          Does the screen go dark if the brightness is turned down all the way or is there still a visible screen?
          Does the contrast control have any effect on the picture? does the voltage on pin 12 of the lm1203 change when you vary the contrast control?
          The LM1203BN is wider bandwidth etc, but should not effect the operation.

          What are the numbers on the back cover? It looks like a 8511 but I don't see any schematics for it.
          Last edited by R_J; 10-30-2022, 06:45 PM.

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            #6
            Re: IBM PS/1 Monitor (CRT)

            After arching, I tried covering the hole in the flyback and it worked for a few hours and the pictures was as the first picture.
            The screen doesn't go dark if i turn the brightness down and the contrast control does have an effect on the picture but after I turn it like a quarter down the image disappears, the voltage on the pin 12 is between 7V and 4V when i turn the brightness.
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              #7
              Re: IBM PS/1 Monitor (CRT)

              look for a control called "screen"
              it's probably either near the lopt or on the neck board - it's like a master-brightness control

              Comment

                #8
                Re: IBM PS/1 Monitor (CRT)

                Originally posted by stj View Post
                look for a control called "screen"
                it's probably either near the lopt or on the neck board - it's like a master-brightness control
                You mean the one on the flyback?
                that seems to work fine, my problem is the contrast for example in DOS the text should be bright white but it's faded and yellowish if I turn up the brightness the text will get brighter but also the background


                I also added a picture with the board code
                Attached Files

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                  #9
                  Re: IBM PS/1 Monitor (CRT)

                  could it be that the RGB IC is bad? I have three more I could replace it.

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                    #10
                    Re: IBM PS/1 Monitor (CRT)

                    This is the best I can get, the 60% grey seems to get a bit better over time, when i turn on the monitor it's inexistent
                    1. 100% contrast / 50% brightness
                    2. 100% contrast / 100% brightness
                    3. 75% contrast / 100% brightness
                    4. 50% contrast / 100% brightness
                    Attached Files

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                      #11
                      Re: IBM PS/1 Monitor (CRT)

                      can the tube be damaged during arching?

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