Compensating CRT Deflection Coils

8:31 pm Projects

Things look really weird when you use an uncompensated deflection coil with a vector graphics display. This is because the coil looks like an inductive load to the driver amplifier, and the parasitic capacitance makes it ring. Ringing and overshoot create the strange-looking display, basically extending every line past its destination.

One way to compensate for that is to add a series RC snubber in parallel with each deflection coil. You can perform some calculations to figure out the values of the resistor and capacitor but they won’t get you very close to the answer. There are just too many parasitics to model. It’s much faster just to build a RC substitution box and tweak the values until you get the result you want.

16 Responses
  1. Richard :

    Date: July 31, 2014 @ 9:44 am

    Eric. Great job, fella! Does this apply to directly driving raster scan ramps, at non-standard rates, into the deflection coils? Please reply to me via email.

  2. Richard :

    Date: August 13, 2014 @ 10:17 am

    Howzabout an update giving the construction details of your final deflection yoke?

  3. eric :

    Date: August 13, 2014 @ 6:11 pm

    I’m still refining it. Same with the board design. Check back–eventually I’ll put it here!

  4. Richard :

    Date: September 17, 2014 @ 3:32 pm

    Did you know that the high voltage connector on your CRT is known as a “J1-21”? This is the generic name for that connector. The cuter connector that resembles a clothing snap, like used on a 3JP1, is called the “J1-22”. I recently tried to purchase ten of them from a defense contractor, the “onlyest” source I could find. I was given a quote of $38 for twenty five of them. Not being too observant, it turned out to be $38 EACH! Or $950 for 25 connectors. No. Not going to happen. An alligator clip works too.

  5. eric :

    Date: September 23, 2014 @ 3:31 pm

    Yes, I looked into obtaining some connectors, and they’re hard to find. I have a couple of the metal clips, and I was able to 3D print (with a soft rubbery material) the insulating shell.

  6. Richard :

    Date: September 19, 2014 @ 12:46 pm

    At what second anode voltage are you running your 5AXP4? I just got a sweet self contained 15KV power supply that is perfect for this tube. There is only one left. Write to me off line and I will tell you where. $148 shipped out the door.

  7. eric :

    Date: September 23, 2014 @ 3:33 pm

    The 5AXP4 worked fine at 5KV. I air-wired a capacitor multiplier to my HV board (the schematic has a diagram showing where to connect it).

  8. Richard :

    Date: September 25, 2014 @ 11:21 am

    I got the 15KV power supply. It is huge and heavy. At least 25 pounds! I guess its a neon sign 60Hz transformer inside. A lot of iron in this baby! It’s also a lot larger than the CRT itself. Like this supply was made for a 14″ or larger tube.

  9. Labguy :

    Date: October 5, 2014 @ 8:15 pm

    Fired up my 5AXP4 today. Check it out:

  10. eric :

    Date: October 8, 2014 @ 10:03 pm

    Looks really good! I like the support blocks.

  11. Rick :

    Date: November 12, 2015 @ 5:27 am

    In the CRT Deflection coil compensation video, what tube are you using?

  12. eric :

    Date: November 21, 2015 @ 3:42 pm

    It’s a 5AXP4 TV test CRT.

  13. Rick :

    Date: November 12, 2015 @ 3:18 pm

    Also, any details on the homebrew deflection coil would be appreciated.

  14. eric :

    Date: November 21, 2015 @ 3:43 pm

    Check for details on the winding techniques.

  15. Phillip Jordan :

    Date: October 17, 2016 @ 12:49 am

    Saw your little explanation of RC snubber network on vector game well done.

    thank for giving the final clue to compensating a Vetrex Vector display.

    I have a Related question

    I have been looking for a Deflection coil of equal electrical specs for vector based work using Commodore 1702 color display for experiments, but I simply cant find Vector Yokes of any type.

    I would like to know more about your research into building homebrew deflection Coils, as it seem this will be the only way i will get what i need.

    The Commodore 1702 is blessed with a horizontal flyback design that allows the HV circuity to operate normally with the Horizontal & vertical yoke coils disconnected from the Deflection board with no ill effects, and is a great monitor to be modded to a color xy Vector display.

    I have several 1702 stored away for this reason ;-).

    My Initial experiments have been very promising using a peavy m-2600 audio amplifier at about 2 watts to drive the 1702 Defection coil to test some basic ideas.

    the Main problem is the differing electronic charecteristic of each Yoke coil H & V winding that make the “video yoke” unsuitible for vector X/Y work.

    I need a some help to design yoke that will work on the neck a 1702’s CRT tube, and rolling my deflection yoke seems the only way I will get one.

    Any help would be appreciated.

    thanks Eric


  16. eric :

    Date: October 17, 2016 @ 12:54 am

    Phil, I have some details in this post. I used the MIT Radiation Laboratory Series, Volume 22: Cathode Ray Displays as a resource, but I haven’t really done anything further than what I documented in the blog post. Best of luck!

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