Electrostatic CRT Driver Design

Projects 45 Comments

Update 23-December-2014: Richard at LabGuy’s World has wiring diagrams for the RCA 913 and the 3JP7, and other neat stuff.

Update 23-June-2014: Please see the CRT Board BOM Updates page for important changes.

Update 3-June-2014: Added OSH Park links so you can order PCBs easily and quickly!

Well, it’s here! I’ve put together an open source release package for my CRT driver design. See all the glorious details at the GitHub project. It contains the fab data (Gerbers, NC drills, etc) as well as the CSV-formatted bills of materials for three boards. All the parts are available at Mouser Electronics (and the Mouser numbers are listed in the BOM). It’s the same set of boards that I used for my Asteroids and Flappy Bird arcade machines at Maker Faire.

The design can drive most 2″, 3″, and some 5″ electrostatically-deflected cathode ray tubes, such as the popular 3BP1 or the precision 3RP1A. It can be modified to drive some of the oddball 2″ and 3″ CRTs that have a shared heater and cathode connection.

The three boards are:

The 1KV power supply takes +12VDC and uses a Royer oscillator with an off-the-shelf CCFL transformer to create filtered 800-1200VDC. The voltage is tuned by adjusting R1. Use a plastic-handled pot tweaker tool please! The supply can easily source several milliamps, which at 1KV could stop your heart. This is a serious power supply, so please be safe if you decide to build it. Keep one hand in your pocket while using it. Turn the supply off before poking at it, waiting at least a second or two after switching off the DC supply to allow the sense resistor divider to bleed off the output voltage. Don’t work alone.
The design is inspired by CCFL circuits designed by Jim Williams–see the very long and wonderful AN65, especially Jim’s hand-drawn notes at the end. While Jim used a switching regulator to drive the Royer circuit, I used a simpler linear FET approach which works well but generates more heat. You will need to screw a metal heat sink to the plated through-hole located above Q1. Use a mica washer and a nylon screw if you want electrical isolation since this node is NOT ground.

If you want to put this in an enclosure, keep a clearance gap of 1cm or more between HV connections and low voltage nodes or a metal chassis. The power supply produces stray magnetic fields that will cause the electron beam in a CRT to deflect in ways you don’t want, so you should keep it far away from the CRT and use a CRT shield made out of high magnetic permeability material, like mu-metal or soft steel.

You can also add a voltage multiplier circuit, tapping in to diode D1, to generate post deflection acceleration (PDA) voltages. A 2-stage multiplier (as shown on the schematic) can generate 3KV which is fine for 3″ and 5″ tubes that require it.

At the heart of the design is the electrostatic CRT deflection and video amplifier circuit, which takes 0-3.3V analog signals for X and Y deflection as well as the video input, and outputs high voltage drive signals to the CRT.
There are two deflection amplifiers, one for X and one for Y. The core of the differential amplifier is a matched dual NPN transistor with a bias circuit for setting the gain and offset (width/height and position potentiometers). A cascode stage made out of two high-voltage NPN transistors increases the bandwidth slightly and level shifts the signal up to about 1KV. There are two 220K resistors that form the load for the circuit. Don’t substitute these with plain carbon composition resistors–these parts have to be rated for high voltage use. Bandwidth of the amplifiers is about 10-15KHz, so you could drive ramp signals in to X and Y and generate an NTSC sweep if you wanted.

The video amplifier is a pretty simple class A design with a cascode stage to increase the speed. Bandwidth is about 6MHz, so the circuit can easily handle NTSC video. A simple-minded approach for driving a CRT electron gun is to ground the cathode and drive the grid negative, but the circuit design ends up being tricky. An easier way is to drive the grid to ground (or in my case, to an adjustable brightness voltage from a potentiometer) and connect the cathode to the amplifier output. The end result is the same, although there is a slight impact on the focus electrode. The video amplifier uses a +60V bias voltage which needs to be rather stiff since the drive current is fairly high (required to get 6MHz bandwidth).

A focus potentiometer produces a bias voltage for the focus grid in the CRT. It’s adjustable over a fairly wide range covering most 2″, 3″ and some 5″ CRTs, but if you’ve found some oddball CRT, you could always change some of the other resistor values.

The astigmatism potentiometer changes the final accelerator voltage relative to the average deflection plate voltages. Adjust this potentiometer if the spot on the screen looks oval instead of round.

There is a connector for a 6.3VAC filament supply. You don’t have to use an AC voltage, but be aware that pin 1 is tied to ground. You might want to measure the actual output voltage of the filament transformer (under load of course). I’ve found that most of them tend to run high (7V is not uncommon). Adding a series resistor can help prolong the life of the CRT.

Finally, a video amplifier bias supply generates +60V for the video amplifier. As an “easter egg”, you could modify this circuit to operate as a Nixie tube power supply by changing the value of R1. You’ll also need to increase the voltage rating on C5.
If you’ve already looked at the GitHub project, you might have noticed that there are no design files. I use Altium Designer for my schematics and PCB layouts. It’s a fantastic tool, but Altium’s decided that hobbyists are not a target market so this is a very expensive product that not many hobbyists own, so I don’t think there’s much point to posting design files. Maybe if Altium releases a noncommercial free license, but  I doubt that will happen. Altium recently raised their prices by $2K so the direction they are going in is pretty clear.

I’ll be posting updates with additional information, such as a CRT compatibility table, pin connection tables, and so forth.


CRTs with Magnetic Deflection

Projects, Uncategorized 2 Comments

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.