CRTs with Magnetic Deflection

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Whew, Maker Faire was a lot of work, and a lot of fun!

Now that the Asteroids arcade machines are finished, I’m thinking about some suggestions that people gave me. A lot of people want a larger screen. Even with a precision 3″ CRT (3RP1A, for the curious), playing the game involves lots of squinting and hunching over.

In my collection I have a pile of 5″ CRTs, mostly electrostatic but a few magnetic. The electrostatic CRTs are quite long: 16 3/4″ is a pretty common length but some are even longer. The distance is necessary to maintain a reasonable deflection factor. 70-100 volts applied across a pair of deflection plates leads to 1″ of beam deflection. While I could certainly build a project with a very long case, this gives me a good excuse to experiment with a few of the magnetic CRTs I have.

Starting with cardboard harvested from old toilet paper rolls, I made a tube that can slip over the neck of a CRT. Next I cut 12 notches in both ends so I could hook magnet wire around them. Then I cut the whole arrangement into two halves to make it easier to wind the first set of coils on the inside of the tube.
Hand Wound Deflection Yoke
Then I wound 19 turns of wire on each half, starting with a small set of 3 turns spanning 2 notches, and then winding 7 turns across 3 notches, and finally 9 turns across 5 notches. After taping the two halves back together, I soldered the two sets of windings together in series. The polarity is critical because the magnetic fields need to add together, not cancel out. The second set of windings used the same winding pattern only this time I wound them on the outside of the cardboard tube and rotated 90 degrees.

This coil arrangement is called a semidistributed winding: look at (c) in the figure below.
Deflection Yoke Styles

After wrapping a layer of insulating tape over the windings, I wound a thin strip of soft steel around the whole thing. This provides a high permeability path for the part of the magnetic field outside of the CRT envelope. The idea behind the winding technique and all that is to create a uniform magnetic field in the path of the electron beam. The uniform magnetic field deflects the electron beam according to the Lorentz force law:
F=q(E + v x B)
E is the electric field. In this case, it’s the potential between the cathode and the anodes in the electron gun as well as the final anode. This accelerates the electrons forward towards the face of the CRT. The deflection force is the cross product of the velocity (v) and the magnetic (B) field. You can figure out the direction of force using the left hand rule. Since the electrons are moving towards the screen, a magnetic field in the up-and-down direction pushes the electron beam from side to side. This means that the horizontal deflection coils have to be positioned on the top and bottom of the CRT neck.
5AXP4 With Yoke

And it worked! I slid the yoke onto the neck of a 5AXP4 which is a CRT designed for electrostatic focus and magnetic deflection. It took nearly 1 amp to get a bit under an inch of deflection. To decrease the current I can add more windings. There’s a classic engineering tradeoff there between response speed (bandwidth) and current, since more turns have more inductance and parasitic capacitance. Incidentally, since the magnetic field strength is proportional to the current in the coils, I’ll have to drive them with a linear amplifier design that servos the current instead of the voltage.

The next step is to figure out how to handle magnetic focus. I have a 5FP14 which requires an external permanent magnet or electromagnet to focus the beam instead of the usual electrostatic lens. My friend Kent sold me a military radar display that uses a 5FP7A, and this display has a ring magnet connected with a screw and gear mechanism to adjust the focus.

A great resource for me has been the MIT Radiation Laboratory Series, Volume 22: Cathode Ray Displays, available here as a free PDF download. It’s full of details on how deflection and focus coils were manufactured.

I’ll leave you with this beautiful shot of the zero-first-anode-current electron gun assembly in the 5AXP4. The visible elements are (right to left): cathode, grid, accelerator, focus electrode,  and second anode (electrically tied to the accelerator).
5AXP4 Electron Gun

Asteroids Mini Arcade Machine

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Here’s a tiny Asteroids arcade machine I built from scratch. It uses a vintage 3″ round cathode ray tube driven from an amplifier board and high voltage supply of my own design.

A friend of mine ported his 6502 emulator to an STM32F4 Discovery board so this arcade machine is able to run the original Asteroids program without any modifications. The STM32F407 processor has two DAC outputs which work perfectly for driving the X and Y deflection inputs on the amplifier board.

Turns out the ST Micro part is really good for driving displays like this. Not only do the DAC outputs work great for deflection, but the hardware floating point really speeds up things like 3D vector rotation.

Come find me at the Bay Area Maker Faire! (May 17 and 18–go buy your tickets now!) I will be located in the Fiesta Hall (the dark room with the Tesla coils). I’ll set up a second arcade machine running some additional demos, including a Super Secret Game. You’ll just have to come and find out what it is.

3D Vector Graphics

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It’s back! This is a new vector CRT driver setup. The 3D cube is generated by an Arduino driving an 8-bit DAC.

Three Fives – Discrete 555 Timer

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My friends at Evil Mad Scientist have a new kit for sale. It’s a 555 timer circuit that you can build yourself using discrete transistors. You can wire it into all sorts of 555 timer circuits and then probe individual nodes to see how the chip actually works.The circuit board that you get with the kit has silkscreen labels that mark the functional blocks of the circuit, and silkscreened component designators that match up with the “official” Signetics schematic.

The circuit is full of interesting analog electronic design elements. You’ll be able to play with differential pairs, current mirrors, Darlington stages, diode-connected transistors, and more.

It’s a great kit if you want to learn more about how integrated circuits work, or if you’re a fan of the indefatigable 555 timer and want to have a neat conversation piece, or even if you’re just a beginning electronics hobbyist and you want to practice your electronics assembly and soldering skills.


CRT Driver Kit Update

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It’s been quite some time since I last posted about this. The project has been on the back burner for some time now since I’ve just been so busy with other things. It’s actually pretty far along the process but the cost of the parts is just too high, and the kit has quite a few parts.

I’ve been revisiting the design again to see if I can make it easier to build and less costly.

A question: Would you consider a version without a DAC? Instead of having an 8-bit digital interface (Arduino compatible), it would have analog X and Y inputs and a video/blanking input.

Quick Note

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If the site’s been very slow for you lately, it’s because someone used a PHP injection attack to add some potentially malicious Javascript to the top of the page. It should be fixed now. Thanks to Olli for the tip.

New Table Lamp

Projects 1 Comment

Several months ago at the electronics flea market I picked up a neat bit of brass. I did some internet research and it’s actually part of a 19th century scientific demonstration instrument, most likely a prism. I found a very similar example at Fleaglass. Theirs sold for quite a bit of money, but I got mine for $5, which is probably about the value of the brass in it.

So I turned it into a table lamp.


In the photo I’ve installed one of the many vintage-style reproduction light bulbs that are starting to appear. They don’t really look like a nice carbon filament bulb but I can use this every day and not worry about it burning out.

Now all I need to complete it is to put a shade on it. Ideas?

Panaplex Wall Clock Schematic and Software

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Remember my Panaplex clock project? Here’s a present: the design details!

Panaplex Clock Schematic
PIC18F242 listing – main.asm

Still Alive

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Wow, it’s been a few months since my last post. Sorry for the blog silence. I got very busy with a new job and just haven’t had the time to work on projects at all, let alone blog!

So here’s a quick one–it’s a Heathkit GC-1005 digital clock that uses Panaplex displays (Neon filled). I picked it up at the electronics flea market and it looked like someone had been trying to get it working before me, and they left a bit of a mess. I had to clean up the wiring job and check the electrolytics to make sure they were still good (they were).

The reason it wasn’t working right is that some of the component leads on the bottom of the PC board had poked through a paper insulator and shorted out against the switch contacts on the bottom of the case. All I had to do was trim the component leads and fix the insulator.
Heathkit GC-1005 Clock

Lissajous Figures

Projects 6 Comments

My new CRT driver board is coming along rather nicely. Tonight I tested it out with a 5″ CRT. It uses a P7 radar phosphor so it looks bluish white with a sickly yellow persistence.

5" Cathode Ray Tube - Lissajous Figure

The pattern is a Lissajous figure (LISS-uh-joo). Take two waveform generators and connect one to the X input and the other to the Y input, and you get all sorts of interesting patterns. Since the CRT driver board is not available as a kit (not yet, anyway!) you can duplicate this with an oscilloscope and two function generators.

There’s some interesting math behind Lissajous figures, but I’m more interested in building 3KV power supplies.

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