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…

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