CRT Phosphor Video

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Inspired by commenter Katemonster, I’ve put together a short clip with a couple of CRTs from my collection, demonstrating various types of phosphors. There are charts out there that talk about persistence using vague terms like “medium” (compared to what?), so it’s nice to see a real video showing what such a CRT actually looks like.

For the video I’ve used my “orbiter” demo that uses Newton’s law of gravity and Newton’s 2nd law of motion (F=MA) to generate simulated planets that orbit around a sun. It’s a nice way to demonstrate persistence (the way the phosphor fades as the electron beam moves away).

P1 Phosphor
This is the basic green phosphor. At 525nm primary color wavelength, it looks slightly more blue than common super-bright green LEDs. The chart linked above lists the persistence time as 20ms which seems reasonable. The formulation for this phosphor varies between manufacturers so some tubes might be slower than others. It’s very common in early oscilloscopes and oscillographs, and apparently some radar systems as well.

P2 Phosphor
The P2 phosphor color has even more blue in it than the P1–it’s very close to “stoplight green”. The persistence is much longer as you can see in the video (30 seconds or more, depending on the ambient light levels). The charts and reference documents I have list the primary applications as oscillography and radar.

P7 Phosphor
P7 is a very interesting phosphor. It is a cascade phosphor, meaning that it has two layers of material. The electron beam strikes the first (outer) layer which emits a bright blue light with some light near ultraviolet. This high energy light excites the second layer (inner, in contact with the glass) which is a much slower material that emits a yellowish-green light with a very long persistence (around a minute). In the video I move the “orbit” trace off to the side so you can see that original afterimage persists.

It was used mostly for radar and sometimes in oscilloscopes to capture one-time events before storage tubes were invented.

So why use a cascade phosphor? One source states that it was originally designed to be used in intensity-modulated displays (varying brightness levels), but it turns out it also helped prevent radar jamming. Since the jamming signal was not synced to the radar pulses, a long persistence phosphor could average out the jamming signal and allow the operator to see the true signal as viewed on an A-scope (time-based pulse waveform monitor). [Cathode Ray Tube Displays, MIT Radiation Laboratory Series, pg. 626]

P12 Phosphor
This one is my favorite. It’s an orange medium-persistence (a few seconds) phosphor that was apparently used for radar indicators. I don’t know of any that were used in oscilloscopes.

P31 Phosphor
The P31 phosphor was invented as an improved P1 phosphor. It’s much brighter (P1 is 32% as bright) and has short persistence (<1ms). The color has a bit more blue in it–in fact, very close to the P2 phosphor’s color. I would say most analog oscilloscopes from the 70s to today use CRTs with the P31 phosphor.

In many cases these CRTs would be installed behind a colored piece of plastic acting as a color filter. For example, P7 CRTs were often installed with an orange plastic filter in front to make the blue/white phosphor look more similar to the secondary yellow phosphor. P31 CRTs usually have a blue or green plastic filter.

For further reading:

CRT Driver Boards, Now With Altium Sources

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Take a look at my crt-driver GitHub repository. I tidied things up a bit and more importantly, released the Altium project files, schematics, boards, and even the output job file. It’s all licensed under the Creative Commons Attribution-ShareAlike 3.0 license. Read the Creative Commons page for the full terms, but basically you can share or adapt any of it as long as you give me credit (a link to this blog would be appreciated) and make sure that you keep the same license so that others can do the same.

If you don’t have Altium (expensive, closed source), you can at least open and edit the schematics with CircuitMaker (free, closed source, limited). Sadly, CircuitMaker will not let you edit the Altium PCB layout.

CRT Magnetic Deflection Driver Design

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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.

 

Vacuum Tube Op Amp Experiments

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At the electronics flea market I recently found something particularly interesting…

This is a vintage Philbrick K2-W vacuum tube operational amplifier! Turns out they have quite the following. Sadly the original 12AX7 tubes were gone–somewhere, someone probably has rare GAP/R marked tubes in their guitar amplifier. I put in some generic replacements.

I decided to build a little jig to try it out.

This is based on the inverting amplifier schematic given in the K2-W datasheet I linked above. I added a simple linear power supply to generate the +/-300V rails. If you build your own supply, be sure to add bleeder resistors so you don’t get a nasty surprise after you turn it off and try to work on it.

After connecting a 10K series and 100K feedback resistor to the op-amp, I ran a 1KHz 5Vp-p square wave from my function generator into the circuit and saw this:

Neat! The top trace is the input and the bottom trace is the output. The bottom trace has a magnitude of 50Vp-p, as expected.

It’s really interesting to see how the short paragraph of specifications at the bottom of the first page of the K2-W datasheet developed into the formal electrical characteristics tables you can see in more modern op-amp datasheets, like the 741.

Build Guide for Mini Arcade Machines

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Edit: Updated instructions to discuss the game ROM

Have an STM32F407 Discovery board? Have a CRT scope with XY inputs? Try out Asteroids and Battlezone for yourself.

Read the rest…

Battlezone Mini Arcade Source, STM32F407 Discovery Board Sound Mods

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First, the source code for the mini Battlezone arcade machine is now online (GitHub), along with the code for the mini Asteroids arcade.

If you build either of those and try it out on an STM32F407 Discovery board, you will notice that no sound comes out. The reason is that the onboard audio codec has a LRCK line (left/right framing clock for the I2S audio stream) wired up to GPIO PA4, which happens to be one of the DAC outputs that we are using for the vector graphics generation. You’ll need to disconnect LRCK (U7 pin 40) from PA4 and wire it to PA15. I did it by cutting a trace (indicated with a black arrow) on the back of the board to isolate the PA4/LRCK net from the STM32:

And by adding a wire (indicated in red) on the top of the board from R48 to pin 77 (red arrow) of the STM32 (PA15):

Oh, and one more thing. To keep the audio mixer marching along, we had to connect PC6 to PC7. This ties the I2S MCLK to timer 3 channel 1, which triggers a periodic interrupt that mixes new samples into the audio output buffer. The easiest way to do this is to connect a jumper across pins 47 and 48 on the GPIO header P2. They are clearly marked on the board and the pins are right next to each other so this is easy.

Battlezone – Mini Vector Arcade Machines

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If you follow me on Twitter, you’ve probably seen the picture of a CRT deflection yoke undergoing surgery. Well, it’s now installed in a mini Battlezone arcade cabinet that I built for Maker Faire this year.

It uses a 5-inch B&W CRT taken from a broken security camera monitor. I had to rewind the yoke to make the vertical windings fast enough to handle vector graphics. The horizontal windings were already pretty close, but they did need to be rewired to operate in series to reduce the drive current.

The CRT is a bit different than most of the old-style CRTs I work with. For the curious, it uses an EIA E7-91 style base which has an additional grid for acceleration purposes. The lower circuit board in the photo is a potentiometer resistor divider that develops a ~300V bias voltage to drive this additional grid. The upper board is a Cockcroft-Walton multiplier circuit to step up the 1KV power supply voltage to the 4KV necessary for the post-deflection acceleration (PDA) anode. I’m running this CRT a bit under voltage, and it works fine because I’m not trying to fill the whole screen with a solid raster.

The metal enclosure houses the power supplies–it is made out of soft steel with the idea of containing the stray magnetic fields generated by the power supply switching inductors. It works a bit, but I’ve had better luck in the past with mu-metal shields around the CRTs themselves. There are three power supplies: one steps 120V down to +12V, +5V, and -12V. Another steps +12V to +1KV, and the final one steps +12V up to +60V. These last two power supplies are detailed here. They are open-source hardware so you can build your own.

Outside the shield you can see the video amplifier board at the lower left, which is the same circuit used on my electrostatic deflection board but without the deflection amplifiers. At the bottom right is my new magnetic deflection amplifier board which drives the deflection yoke. It is based around an LM4765 audio amplifier IC. I’m not totally happy with the design but eventually I will release the board design. The green board in the upper middle is an STM32F4 Discovery board. The board to the right of it is an audio amplifier that drives the speaker.

The joysticks I machined out of small aluminum blocks. It took some experimenting to figure out how to arrange the springs to get the joystick to return to the center position. The joystick handle on the right is hollow to allow the wires for the fire button to pass through.

I also build a second unit with the same driving circuit but a different CRT–a 5-inch round 5AXP4. This CRT would originally have been used by a TV technician to look for problems with the chassis or the full size picture tube in a customer’s set. The glass is extra thick because the tube was designed to be handled more. This is the same tube that I started experimenting with last year.

Tiny CRTs in Action

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Richard built up some of my CRT driver boards and put them to use, testing a variety of CRTs with interesting characteristics, such as the rare 1DP11 or the vintage RCA 913.

He’s also got a nice video as well:

If you plan to build up the CRT boards, I highly recommend you visit Richard’s page. He’s drawn up an excellent connection diagram that shows how to interconnect the HV power board, the video power board, and the deflection/video amplifier board.

Deuterium Arc Lamp

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On Saturday I found a deuterium arc lamp at a local surplus store. It was used, and most likely pulled from an ultraviolet spectroscopy machine. I could not find data on the specific lamp model, but I found a similar lamp. On the chance any of you might know what it is, the lamp is marked

D 805 K

56066349

West Germany

H9

Before running any tests with the lamp, I wiped it down with isopropyl alchohol to remove any fingerprint oils. When heated, they can cause the glass envelope to bubble and even melt, destroying it.

To run this lamp, which is a gas-discharge type, you first have to heat up the cathode. There is a very thick double-spiral tungsten filament inside that uses 2V at 4.5A (or 9 watts!). Once it’s warmed up for a minute or two, you apply the high voltage to the anode. I connected it to a current-limited electrophoresis power supply set to 50mA. The lamp started at 350V and settled to an operating voltage of about 84V. Incidentally, the heat generated by this helps keep the cathode hot, and the filament current can be reduced to improve its lifetime.

Here’s a quick video showing what it looks like.

Deuterium is an isotope of hydrogen: hydrogen has one electron and one proton, and deuterium takes that and adds a neutron. It is not a radioactive isotope, unlike tritium, which has two additional neutrons. According to Wikipedia, Deuterium is used in these lamps because it emits more UV with a wavelength less than 400nm.

If you’ve got one of these lamps and you plan to light it up, you’ll need eye protection. I ran it at a very low beam current (most likely it was designed for 300mA!) and the light was not so intense, but you might want more than just a pair of sunglasses if you’re going to full power…

Miniature Nixie Power Supply – How it Works

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For more details and the video about the miniature Nixie power supply, see my original post.

First off, this circuit is not a Royer oscillator. As summarized by Jim Williams in his famous app note AN65, Royer developed a power converter using a transformer that saturates every cycle.

A coil saturates when the magnetic field (the B field) has reached the maximum that the magnetic core material can support: if the current (which is what creates the magnetizing H field) increases more, the magnetic field increases very little. The inductance, which is proportional to (B ÷ H), rapidly drops off, causing the current in the coil to increase at a much faster rate. Royer’s design detects this current spike and uses it to switch the transistors (2N74s, in his original paper) into their opposite state.


My circuit is a more common LC resonant converter. There are two transistors, Q1A and Q1B. Resistor R1 provides the bias current for the transistors and gets things started. Current (represented by the large red arrow) flows through the center tap of the coil T1 out to Q1A through its collector. The current in the upper half of coil creates a magnetic field, and the magnetic field induces a voltage in the feedback coil. This voltage reduces the base drive for Q1A and increases the base drive for Q1B (represented by the small blue arrow). When that happens, Q1A shuts off and Q1B turns on. The current in Q1B’s collector (represented by the large blue arrow) creates a magnetic field of the opposite polarity in the coil, and therefore causes the feedback winding voltage to reverse polarity (see the small red arrow), turning off Q1B and turning on Q1A. The cycle repeats as long as there is power.


Above are some approximate waveforms. You can see that the transistors go back and forth, driving the coil first one way and then another. Capacitor C1 and inductor L1 help determine the resonant frequency of the circuit. If you measure the voltage across the entire coil, you’ll see a sine wave.

The output winding of the coil has a lot more turns than the input winding, and it increases the voltage (at the expense of the current) dramatically. This high voltage AC goes through the half wave rectifier formed by D1 and gets filtered to DC by C2. R3 limits the current into the Nixie tube.

You might be wondering why T1 is actually an inductor–an 8.2 millihenry one. It just makes the project easier to build. You only have to wind 12 turns on an off-the-shelf part instead of buying a hard-to-find transformer core and adding all the windings yourself.

If you feel so inclined, try adjusting the component values. Start with C1 and then maybe R1 or even L1. Try changing the number of turns on the coil.

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