Converting a TI model 924 VDT into an Oscilloscope

Discussion in 'The Projects Forum' started by magnet18, Apr 6, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, I recently got it running, so now I'm going to try to turn the old TI model 924 Video Display Terminal I got into an oscilloscope so I can probe things over 20V without fear of ruining my small digital scope.

    From what I've read, the easiest thing to do is to take the vertical deflection signal (lower frequency) and attach it to the horizontal deflection coils, and then do what you wish with the vertical deflection coils.

    This also requires that the entire vertical portion of the screen be covered, and currently all I get is the message "Keyboard Failure Detected"

    So would it be easier to input a fake keyboard signal (I detected 2 pins that aren't either connected directly to +15V or Ground), or just build new horizontal sweep circuitry.

    I don't expect to be analyzing waveforms, just seeing how steady a voltage level is or what kind of waveform there is or if there even is one. But I would still like to be able to go from approximately 1Hz-50kHz (probably a little optimistic).

    Here's links to the pics
    http://forum.allaboutcircuits.com/attachment.php?attachmentid=29292&d=1302059545

    http://forum.allaboutcircuits.com/attachment.php?attachmentid=29293&d=1302059545

    http://forum.allaboutcircuits.com/attachment.php?attachmentid=29323&d=1302117364

    Thanks in advance, any input is appreciated!

    [EDIT]
    I hope this isn't considered too dangerous and needs to be locked, but if it is I understand.
     
  2. debe

    Well-Known Member

    Sep 21, 2010
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    The horizontal sweep freq will need to stay the same as it also drives the EHT for the pic tube. If you can find the Vert deflection amp & disable it you will get a Horizontal line across the screen. You then need to drive the vert amp to show your signal. It will be limited to the fixed horz freq.
     
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  3. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    What would happen if the EHT frequency was changed?
    and what would need to happen to be able to change the horizontal frequency?
     
  4. debe

    Well-Known Member

    Sep 21, 2010
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    The horz freq cant be altered very mutch as the efficiency of the transformer will be badly affected, Ie the picture will shrink for a start & the EHT voltage will diminish.
     
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  5. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    So would it be possible to keep the EHT frequency the same and power the horizontal coils separately?
    Similar to driving the vertical coils?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    magnet18 likes this.
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Why not build a pre-amplifier/scaler/attenuator for your current o'scope?

    John
     
  8. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Is it possible to build a deflection plate?
    (I probably sound like a total n00b now)

    2 reasons in my mind
    1. I'll probably end up wiring it wrong and frying my scope anyway (not an excuse, I know :p)

    2. I really, really, REALLY, want to do something with this CRT... because it's old... and green (my motives are pathetic, but strong :p)
     
  9. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    This is what it looks like when I disconnect the vertical coils...
    It only shows where the x values for the text were before.
    would disconnecting the control grid help this? Or changing the charge on it?
     
  10. debe

    Well-Known Member

    Sep 21, 2010
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    The deflection plate is no as it is built inside the glass envelope. The best ican see you doing is live with the horz scan limitations & use the vert amp for your signal. That alone should be enough of a challenge at this stage. & probably doable. The whole concept has serious limitations. Looking at the pic it doesnt look encouraging, I expected a horiz line across the whole screen.
     
  11. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Well, I will have to work on it next week (see my outdoor hobbies post in off topic)
    looking at the control circuitry, the horizontal and vertical deflection wires are clearly labeled and I think I can disconnect the horizontal and patch my own signal in without disrupting the flyback any, but I could easily be wrong.

    The entire thing is designed in a way that it is cake to work on it (connectors and phillips screws abound), and the whole thing is powered through an isolation transformer and the few HV caps are in a little metal cage with paths to bleed charge (I checked) so the mod's (and I) should be able to rest easy.

    I also checked and there are no warning labels anywhere regarding servicing by untrained persons, but that might just be because it's older than the laws regarding that ;)

    In any case, please continue to post any information you may have/think of and don't do anything to get the thread locked while I'm gone ;)

    PEACE!
    :D
     
  12. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, IIRC, there should be a control grid responsible for stopping the electrons when they aren't supposed to hit the screen, right?
    Would disabling this provide the horizontal line I need?

    Also, debe, I know you have some schematics of this stuff, could you please show one that shows the horizontal coils and how they control the frequency of the flyback? (a relatively simple one if you have any)
    I believe you, it just confuses me :(
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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  14. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    It wasn't ignored, I read it, but to apply the first one I need a continuous line, which I don't have, and the second is excellent information, but I can't build one like that.
     
  15. debe

    Well-Known Member

    Sep 21, 2010
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    Only just arived back from my trip. Suspect the reason no continuous horz line is the processor board is blanking out the video amp, will have to think allitle more about this & how to overcome that.
     
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  16. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Ok, so what would happen if I took where the horizontal coil output is now and hooked it to an inductor of the appropriate value?
    Would this work?
    (and I'm currently in the process of reverse engineering the board with the flyback and all the CRT driver circuitry on it, bout 80% done)
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    No, it wouldn't work.
     
  18. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    why not?
    and heres what I think the schematic is.
    Theres another chip on the board and a couple dozen components but it seems to be contained to the vertical coils.
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    It will have to have a LOT of turns in order to deflect the beam sufficiently.
    If it has a lot of turns, it will have a lot of inductance, so it will be a real problem trying to change the current through the inductor. Same reason that the existing yoke (deflection coils) won't work very well.

    An O'scope CRT is long and skinny compared to a television or monitor CRT, because it has the deflection plates instead of a deflection yoke.

    The deflection plates can respond vastly more rapidly than the deflection yoke can.

    I'm really not going to spend any more time on this.
     
  20. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    I don't expect it to work well, I just want it to work a bit.

    By all means, there are plenty of others that need your time much more than me.
     
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