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Re: [midatlanticretro] Kaypro II retrace/blanking problem

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  • Mike Hatch
    Hi, Video can be either +ve or -ve depending on the system, I have no knowledge of the Kaypro (is there a circuit somewhere ?), but the following info from my
    Message 1 of 6 , Nov 19, 2012
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      Hi,

      Video can be either +ve or -ve depending on the system, I have no
      knowledge of the Kaypro (is there a circuit somewhere ?), but the
      following info from my basic video knowledge.

      In a basic Black/white video signal (usually 1V p-p) 0% is the sync
      pulse level, 33% is black, 100% is white. The Black level is usually
      "clamped" in some way and the sync pulses both line and field are
      imposed on on black as a pulse to 0%, the actual video is then left to
      fill the remainder between 33% to 100%.

      It would appear that your 100% "white" level is ok, but that the 33%
      "black" level is changing so the flyback is seen on screen. Some systems
      generate a "flyback blanking" signal from the video to turn the CRT beam
      off during this period, if the Kaypro does it may have failed. Others
      rely on the video to block the beam during flyback purely by the video
      being below the 33% level.

      As The Kaypro seems to get worse with time I suspect a component/s in
      the video chain is drifting so that the 33% level drifts towards the
      100% level, it could be either in the video generation or the display.

      The basic structure of the video signal is good as the text is displayed
      in the correct place and time, so the sync signals are correctly placed,
      just that the "black" background level appears to drift up from black.

      Basic video structure here - http://www.ni.com/white-paper/3020/en -
      there are many examples on the web.

      A circuit of the Kaypro video system would help, anybody ?.

      Regards,
      Mike


      On 17/11/2012 06:52, Douglas wrote:
      > I have a Kaypro II that shortly after power on the display looks like this:
      >
      > Bad Video
      > <http://xa.yimg.com/kq/groups/14379032/sn/1525488828/name/1117020054.jpg>
      >
      > (stored in the DC Album of this group)
      >
      > For a few seconds, the background is normal; nice and dark.
      > The text stays the same.
      > The the abnormal retracing fades up.
      > The video intensity knob in the back changes the text brightness but
      > does not affect the
      > brightness of the abnormality.
      >
      > Googling on Kaypro video subjects turned up no definitive trouble
      > shooting/trouble spots.
      > (some just say swap out the video board...)
      > Need some pointers, thanks in advance for any shared wisdom!
      >
      >
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