CRT Magnetic Deflection Driver Design

5:32 pm Projects

I’ve uploaded the design files for my CRT deflection yoke driver board. This works for CRTs that use magnetic deflection. For a complete design, you will need the following boards:

  • ScopeMag
  • ScopePower – +1KV power supply
  • ScopeVideoOnly – Video amplifier, focus chain, and nothing else (the ScopeDefl electrostatic design combines this amplifier with the electrostatic deflection amplifiers)
  • ScopeVideo – +60V video amp bias supply

Gerbers are in the repository but you can get them directly from OshPark by clicking the links for each board above.

This board, unlike the others I’ve developed so far, requires both +12V and -12V. My projects typically use an Artesyn NFS40-7608J but it is now obsolete and a bit expensive, so you probably should use something else for power.

There are places on the board (C6/R9, C10/R20) for coil compensation components. You can figure out the values that you need with a little experimentation.

The board is designed for a vector-style yoke, not the far more common raster scan yoke that has a high inductance vertical deflection coil with lots and lots of turns. It can usually drive the horizontal coil no problem, but you’ll want to modify the vertical winding to reduce the number of turns. Check out my blog post on winding deflection yokes for more information on making your own.

The LM4765 audio amplifier, which drives the deflection coils, will dissipate a lot of heat so you will need to bolt it onto a good-sized heat sink (at least 3″ x 3″ aluminum with fins, not some dinky little TO-220 clip-style heat sink).

Circuit operation is pretty straightforward. CRT electron beam deflection is proportional to the magnetic field which is proportional to the current in the coil, so the LM4765 controls the coil current (measured through R14/R25) instead of the voltage. The current is therefore directly proportional to the input voltage (X or Y). An extra gain stage facilitates the width/height and left/right offset adjustments.

 

15 Responses
  1. Vaughn :

    Date: June 30, 2015 @ 4:45 am

    I ordered a ScopeMag PCB. Do I have to buy a CAD pgm. to get a visible BOM and schematic?

  2. eric :

    Date: July 1, 2015 @ 9:47 pm

  3. Fred :

    Date: August 18, 2015 @ 7:23 pm

    What values are C9 and C11 suppose to be? I built this, but the video is anything but recognizable.. Thanks for any help.

  4. eric :

    Date: August 18, 2015 @ 8:38 pm

    Fred, C9 and C11 are for adding an optional pole to the gain stage. They’re not strictly necessary. If you are having trouble with the quality of the video I recommended feeding a square wave into one axis and looking at the output on a scope. The “right” way to do this is to monitor the winding current, either with a current probe or by looking at the voltage across R14/R25. Chances are your deflection yoke needs a compensation circuit (series RC across the winding).

  5. Fred :

    Date: August 18, 2015 @ 10:50 pm

    Thanks for the info. I appreciate it. I’ll have to get a RC substitution box to make it easier.

  6. Fred :

    Date: August 19, 2015 @ 10:28 am

    Eric, I got it working very well. Thanks again for your help. I actually used 2 separate LM1875T audio amplifiers for mine instead of the dual amp. I had those in my parts bin. Any chance you could build a vectrex on one of those development boards? The vectrex stuff is in the public domain so you wouldn’t have any legal issues.. I unfortuanatly have not been able to make your asteroid or battlezone since I do not know Linux to program them…. Maybe someday I can find the time for the learning curve, but at my age I doubt it.. LOL Very Cool projects and thanks again for your great projects.

  7. Charlie :

    Date: May 4, 2016 @ 1:53 pm

    Thanks so much for putting these up, I’m just getting started doing CRT hacking. Could I drive the beam on a small/micro salvaged TV tube with just the ScopeMagA, powered by the ScopeVideoA? I would like to basically visualize audio waveforms on a synthesizer, so I’d drive the X with a ramp voltage between say, 30hz and 4000hz depending on the note, and the Y would be driven by the synth audio signal. Do you think frequencies like this would be possible with standard yokes and your hardware? Thanks again, was very happy to find your site and articles.

  8. eric :

    Date: May 11, 2016 @ 5:19 pm

    Charlie, you might be able to. Keep the original high voltage driver electronics. You will need to rewind the yoke since the vertical coil on the stock yoke will have far too many turns. The horizontal one should be fine.

  9. Bleedin :

    Date: October 26, 2016 @ 6:02 pm

    Hello Eric!
    This looks very interesting. God job!
    Do you think the ScopeMag would work with a old 5AHP7 or 5AHP19 radar crt?

  10. eric :

    Date: October 30, 2016 @ 1:00 am

    I checked the specs, and both the 5AHP7 and 5AHP19 would probably work. The P19 phosphor should look very nice, too.

  11. Bleedin :

    Date: November 1, 2016 @ 8:01 am

    Excellent! I’ll give it a try. Thank you for the reply. I’ll let you know how things work out.

  12. Paul :

    Date: February 12, 2017 @ 10:43 pm

    You may be just the person I’ve been looking for! There doesn’t seem to be much hacking going on with old CRTs, which is a shame.

    I have an old monochrome CRT from a Magnavox Videowriter. This is an amber phosphor tube with a rather unusual aspect ratio – approximately 8:3. However, I have had no luck in finding out any real information on it. The monitor assembly was made by Dotronix, who unfortunately didn’t have any information in their archives.

    Any ideas what I can do with this thing, or where to even start? Thanks!

  13. eric :

    Date: February 25, 2017 @ 12:56 am

    Paul, looks like a neat CRT! It’s probably got magnetic deflection, so you’ll need to get or make a yoke for it. You’ll also need to figure out the base pinout. Chances are that it’s the same as some industry standard pinout. You could always shine a flashlight into the neck and see if you can trace the connections which are hopefully easy to see.

  14. Aivaras :

    Date: May 9, 2017 @ 11:27 am

    Thank you for the project. I am just trying to figure how to get a higher signal for the coils (with original setup i get only about third of crt coverage) As I understand lowering the resistance from preamp to audio amp (49.9KOhm) would increase the amplification factor? Or am i missing something here?

  15. eric :

    Date: May 9, 2017 @ 10:38 pm

    Lower R8 to increase the gain. Be careful that you don’t drive too much current, you might overheat the amplifier chip. You will notice a lot of visual distortion before that happens, however.

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