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28139Re: Kaypro II retrace/blanking problem

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  • joshbensadon
    Nov 17 6:26 AM
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      --- In midatlanticretro@yahoogroups.com, "Douglas" <touchetek@...> wrote:
      > For a few seconds, the background is normal; nice and dark.
      > The text stays the same.
      > The the abnormal retracing fades up.

      > (some just say swap out the video board...)

      Wow, that's great advice, I would have said swap out the computer ;)

      Seriously, here's what I would do to start breaking down the problem.
      Two roads.
      1. the problem occurs much after the capacitors charge, the picture tube warms up, so I would guess some part is getting excessively hot. Get some cold spray and gently spray some area of the video board. Spray with the straw 2 or 3" from board. Cover the whole board a few times, do this within 30 seconds of the problem appearing. If there are no changes, power down, repeat but be more agressive with cold spray, say 1". If you do encounter a change, wait for the problem and find the component(s) that are causing it. When you find the component, you'll decide then what to do about it? replace? supplement a heat sink? trace the heat problem to some other source ie voltage too high?

      2. Scope the pins to the CRT. In general, there could be 3 grid voltages I think they were Bias, Focus and Video? Then there's the cathode and the heater. I think most of these voltages are low, perhaps the bias voltage might be high? but nothing a scope can't handle. Stay away from the high voltage line that is a single wire to the side of the tube, here you can find 15,000 volts (up to 35K on large colour TV's). While scoping, look for which line (if any) shifts it's voltage. If none of the lines shift voltage, then the problem could be with the high voltage or the picture tube? But I've never heard of a picture tube causing this problem and a change in high voltage typically results in a loss of focus.

      PS. I have a Kaypro in my repair queue too, but it will be a while before I get to it.

      Good luck!
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