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Thread: Northstar Advantage - Screen doesn't display anything

  1. #1

    Default Northstar Advantage - Screen doesn't display anything

    Hello,

    My name is Mark, and I just bought a beautiful Northstar Advantage from ebay. It was advertised as not working, and of course it doesn't. The only thing it does do is beep when I switch it on. I can hear the fan running, and the CRT screen makes the high pitched scream. Sometimes when I press enter one of the floppy drives lights up. I tried reseating the processor, but I'm not sure what I should do next. I've seen a ton of videos about restoring old PC's, and I would consider myself somewhat knowledgable about vintage computers. The only problem is that I'm not at all experienced with actually fixing them. Is there a checklist of things I should be troubleshooting? My goal here is to just get the "LOAD SYSTEM" prompt on the screen. The floppy drives are going to be a whole other nightmare. Any info would be greatly appreciated! I can provide pictures if needed.

  2. #2
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    Are the brightness and contrast (if any) controls set mid-position? Can you tell if the CRT cathode heater is glowing? The "high pitched scream" sound - do you think this is not the normal CRT whine? That may mean a problem with one or both of the deflection circuits. If you turn the brightness up, do you see a line on the screen? Be sure to remember to turn it back down again.

    The access attempt of the floppy drive when you press return/enter probably means the CPU is running the BIOS code well enought to attempt to boot from the floppy.

  3. #3

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    I have tried adjusting the brightness knob with almost everything I change, there isn't a way to tell which direction is brighter. Either way, it still has nothing. The noise sounds like a normal CRT noise, a lot like a normal TV from the era. However, compared to a Macintosh screen, it's extremely loud. Where would the glowing occur? I haven't seen anything glow, but I haven't been looking for it.

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    Quote Originally Posted by Robuck View Post
    Where would the glowing occur? I haven't seen anything glow, but I haven't been looking for it.
    The glow will be near the end of the neck of the CRT close to where the wire harness attaches to the rear of the CRT neck. If you can't see in through air ventilation slots, you will need to remove the cover. When you power on the unit, the cathode heater should start to glow.

    Use care when the cover is off, high voltage will be present, and a CRT will have several thousand volts across it internally to accelerate the electron beam.

  5. #5

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    Took it apart, there is a piece at the very back of the tube that seems to be glowing. I also tried removing all the cards from their slots, still nothing.

  6. #6
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    The issue is probably in the video drive circuitry, or possibly the video controller. Do you know if there are schematics available for your unit? I've not used a Northstar computer since 1980, so probably can't help much unless there is a schematic I can look at. Do you have an oscilloscope and a high voltage probe for a DVM or oscilloscope? Those could be useful to diagnose it.

  7. #7

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    Check the soldered joints in the video circuit for cracks, especially near the flyback transformer (that's the one with the big fat wire coming out of the top that plugs into the side of the tube). But note, there are dangerous voltages in there even when switched off. The CRT itself is like an enormous capacitor, so do not attempt to remove the fat wire from the side of it. If you are not sure, ask a professional.

    If you find cracked joints, reflow them with an iron and a bit of solder. I had this on my Superbrain's video board: https://stardot.org.uk/forums/downlo...9948&mode=view

  8. #8

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    We need to know what test equipment you might have. What ever the problem is, without some test equipment, all we can do is give you some vague guesses.
    Dwight

  9. #9

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    The solder joints look fine to me right now, but I am unable to take a very close look. I don’t have any wires to discharge the tube with and I’m afraid I’ll electrocute myself while taking it apart if I don’t. Here is a pictures of where the flyback transformer meets a board:

    486F2624-B66E-4FA6-B0B4-D28C1F737A17.jpeg

    And here is one of the rest of the tube circuitry:

    B092D558-CA31-48F2-BE2A-700334E59508.jpeg

    As for the schematics, all I was able to find was the original technical manual PDF. Sections 3.6 and 4.1.6 jumped out at me as useful. Thanks so much for reaching out!
    Last edited by Robuck; May 18th, 2020 at 12:35 PM.

  10. #10

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    I have a volt meter, but no oscilloscope. Vague guesses can be helpful too!

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