DIY CRT oscilloscope only displays in tiny part of the screen.

Discussion in 'The Projects Forum' started by Tyler Simmons, Apr 14, 2019 at 5:52 PM.

  1. Tyler Simmons

    Thread Starter New Member

    Mar 30, 2019
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    I have wired an old crt tv to display like an oscilloscope. It is working, but the problem is it is only displaying a on a very thin portion on the center of the screen making it hard to read. Is there a solution to this or should I try a different tv?

    Also, is this project in any way dangerous to my eyes or skin due to some kind of radiation?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Give us some more info and maybe we can help you.
    Photos of your DIY circuit, schematics, and a photo of the CRT screen would be useful.
    (I did convert a TV monitor to be used as an X-Y hi-fi stereo display.)
     
  3. Yaakov

    Well-Known Member

    Jan 27, 2019
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    The real danger of a CRT is the high voltage power supply. You have to be very careful around the flyback transformer, the anode cap:

    upload_2019-4-14_18-5-34.png

    and anything it is connected to. The dirtiest parts of the chassis are the most dangerous, the get that way because they attract the dust.
     
    Last edited: Apr 14, 2019 at 6:14 PM
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  4. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    Which way is the thin?
     
  5. Tyler Simmons

    Thread Starter New Member

    Mar 30, 2019
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    The tv model number is srt2319, sylvania. my connections are horizonatal coil----->vertical coil terminal on pcb. vertical coil-------->frequency generator. I adjusted the brightness of the line with the screw on the transformer. 20190414_180301.jpg 20190414_180342.jpg 20190414_180324.jpg second picture you can see the white connector where the horizontal coil is connected. The last picture shows the gm328 I am using for the frequency signal.
     
  6. Tyler Simmons

    Thread Starter New Member

    Mar 30, 2019
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    Thank you for this info!
     
  7. MrChips

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    How much do you know about TV scan and circuitry?

    Horizontal and vertical deflection are accomplished using AC coils for magnetic deflection.
    The horizontal deflection is optimized to work at 15.7kHz.
    The vertical deflection is optimized to work at 60Hz.
     
  8. Tyler Simmons

    Thread Starter New Member

    Mar 30, 2019
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    hrmmmm. I'm not sure what you mean MrChips. I don't know really anything about TV scan and circuitry. Just looking at diy guides and videos about the oscilloscope. But none of them seem to have any info about the wave only showing up in the middle.
     
  9. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    The trajectory of the electron beam is controlled differently in scopes, than they are in tvs.

    One controls with the magnetic and one controls with the electric. The magnetic(tv) moves the beam in a constant manner. No matter the input. It paints a constant full picture or raster of straight lines.

    The electric(scope) only follows the input. It paints only one squiggle line. So...control of vertical and horizontal will be quite different.

    Buy a scope.
     
  10. BobTPH

    Senior Member

    Jun 5, 2013
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    Well, I would say the the drive for the two coils is insufficient.

    Bob
     
  11. Tyler Simmons

    Thread Starter New Member

    Mar 30, 2019
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    Thank you for the suggestion but that is not possible right now.

    Some of the videos just detach one of the coils and add an audio signal and this works. I'm not sure what I can do to increase the drive. Ill just try another tv for now. I've tried about everything I can think of and made the tv do some weird squealing.
     
  12. BR-549

    AAC Fanatic!

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    My advice is to NOT play with anything over 50 volts, until you study and understand electricity.

    It can be most unforgiving to you and your loved ones.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    Let me save you a lot of grief. The problem is not with the TV you now have. Trying another TV will produce the same results.
    The deflection coils on the TV require a lot of power to deflect the electron beam.

    If you are looking for a simple oscilloscope there are some low cost options starting at $0.
     
  14. Tyler Simmons

    Thread Starter New Member

    Mar 30, 2019
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    I was pretty sure this was my 0$ option. Ill look elsewhere. Thanks for helping.
     
  15. LesJones

    Well-Known Member

    Jan 8, 2017
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    I think that disconnecting the horizontal scan coil from the line output transformer will prevent the EHT supply working correctly. I think you would have to have a spare scan coil connected to the line output transformer but not on the tube neck for the EHT supply to work correctly. I agree with the advice not to play round with this project until you have a much better understanding.

    Les.
     
  16. Tyler Simmons

    Thread Starter New Member

    Mar 30, 2019
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    You mean I need a identical coil to trick the board to think it is still connected? Does this matter how close to identical it is? I have some ballast, could these work too if I have one close enough to the same resistance?
     
  17. DickCappels

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    Aug 21, 2008
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    Is the "ground" of your TV set's chassis connected to AC neutral? Many TV sets were made that way. If the TV set circuitry is not isolated from the AC line with a transformer you, your circuits under test, and your scope are all in grave danger. The common solution isi to put an isolation transformer between your TV set and the power line.

    With any luck, an expert on that chassis will be along to comment.
     
  18. Tyler Simmons

    Thread Starter New Member

    Mar 30, 2019
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    It is. Since I do not have an isolation transformer, I would guess it is equally safe to use 2 transformers to step down, then back up?

    I have already taken this tv apart so I will have to try this project later. I will read more about it until then.
     
  19. DickCappels

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    Aug 21, 2008
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    Back to back transformers work sometimes. The main issue is poorer "regilation" on the output. It is probably best to use larger transformers if you have a choice.

    That combination should give you nice isolation.
     
  20. RPLaJeunesse

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    Jul 29, 2018
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