GoKawiilLabGuy's World: Electrostatic Cathode Ray Tube Project 1 Project Start - 20141030
PROJECT GOAL: Display television pictures on a small oscilloscope or radar display tube with my Tiny TV Mark 2 video rasterizer board. We will accomplish this using a three board CRT driver circuit designed by my friend Eric Schlaepfer. I had assembled his CRT driver boards several months ago. They have been awaiting the arrival of the fully debugged Tiny TV board. Adding a scope clock board kit is in the plans as well! So, let's go!
Tiny TV Mark 2 and Eric's 3 boards driving a 3JP7 radar display tube - 20141212 The main distinction of this project is that the tube uses electrostatic deflection. The electron beam is passed between two parallel metal plates that are charged to a particular voltage. If both plates are at zero volts, the beam passes between them uneffected. When one plate is driven more positive than the other, the beam will be attracted to the more postively charged plate and repelled by the negative plate. The deflection plates, and other elements within the tube, operate at and require very high driving voltages. Thousands of volts in many cases. The function of Eric's boards is to convert low voltage drive from Tiny TV to the extremely high voltages required by these cathode ray tubes.
Circuit Schematic - 20141212 The Tiny TV Mark 2, PCB1, provides buffered, gamma corrected, video to Eric's CRT driver boards. It also creates two scanning ramp voltages, in sync with the video, for horizontal and vertical scanning. These are also sent to PCB2 via J3. Horizontal, vertical and video drive are known as X, Y and Z signals when discussing graphic or oscilloscope displays. Eric's CRT driver board set: 60 Volt Bias, Deflection & Video, and 1,200V Power Supply - Update 20141221 First photo: 60 Volt Bias Board, PCB3. Converts +12 volts to +60 volts for the CRT cathode driving video amplifier. Second photo: Deflection and Video Driver Board, PCB2. This board runs on four power supplies. They are; 6.3 V for the tube heater, +12VDC for low voltage circuits, +60VDC for the video amplifier and up to +1,200VDC for CRT second anode and deflection plate driver amplifiers. Then the board provides all of the DC levels and other appropriate driving voltage to the CRT. A generous array of potenetiometers allows for very flexible set up so that it can support the greatest number of CRTs. Last photo: 1,200 Volt High Voltage Power Supply. Steps 12 volts up to +800 to +1200 volts at several milliamps. Voltage is set with potentiometer. Work around this board with great care.
3,000 volt external power supply for the post deflection anode - 20141212 Here we see my plus and minus 3,000 volt bench power supply. I use this for when a CRT is equipped with a PDA or Post Deflection Anode. This is a conductive coating on the inside of the CRT between the deflection plates and the screen phosphor. This way, low velocity electrons pass between the deflection plates and are deflected more than fast electrons. After passing the plates, the electrons are now influenced by the PDA electrode and accellerated to the phosphor. This gives high deflection gain along with higher brightness than a tube lacking this feature.
3JP7 screen shots - 20141212 So, here we go. Fired up the boards. No smoke. Tweaked the pots until I got the best image possible. Began debugging the problem visible in the photos when my Tektronics TDS744A oscilloscope died. Dead as a door nail! No scope, no more troubleshooting this weekend. Looking at the pictures above, there is an obvious problem with the horizontal scan on the left side. Before the scope died, I was able to prove to myself that the Tiny TV board was putting out three good signals at about 3 volts amplitude. I probably missed an update somewhere on Eric's page. Will investigate that next time I sit down to the project. Obviously, this is now on hold. Let's discuss P7 radar phosphor. P7 is a two stage phosphor. A long persistance yellow phosphor with a very short persistance blue/ultraviolet phosphor deposited on top. The electron beam does not have a strong effect on the yellow phosphor. The yellow phosphor is, instead, excited brightly by ultraviolet light. The yellow phosphor's second desirable property is that it glows for a usable period time after stimulus is removed. Up to 30 seconds in a darkened room. This long persistance was used effectively as an image storage memory in the earliest days of analog radar and before computer memory was plentiful and cheap. If one wishes to view the long persistance effect of P7, a yellow filter can be placed over the tube to block the blue light. Conversely, if one wishes to view, and perhaps photograph, the blue phosphor, a blue filter is used.
Video Vectorscope Display - the color signal analyzer - 20141213
Image Source Credit: Wikipedia The scale above represents the relationship between the color difference signals in composite video. The vertical axis is R-Y and the horizontal axis is B-Y. I chose this example as it shows the primary colors and their inverse evil twins. Additive and subtractive primary colors. Note the color boxes on the vectorscope display above. The opposite color of blue is yellow. The other primary colors and their negatives are red/cyan and green/magenta. In fact, many broken NTSC color TVs fail to either a green or magenta picture when the color decoders malfunction. In the case of a black and white video signal, the vectorscope displays a dot at the center - no chroma info, only lumanance. In this view of the video signal, the luminance is the vector going straight into and out of the screen at the center of the scale. To read more about that at Wikipedia, click the link below the image. I present this to you to visualize the color relationships. To summarize, the P7 screen images in the photos look grey. They are. This is because both the blue and the yellow light are summing together to produce white light. Or at least as far as the human eye is concerned. It happens that blue and yellow are primary opposites. Blue is an additve primary while yellow is a subtractive primary color. When two primary opposite colors are mixed, the hues cancel out and only the luminance, or brightness information, remains. Hence, the appearance of grey scale. This is only valid for still images on P7. Moving images have the most annoying yellow comet tails and blinding bright blue edges. Not good for TV viewing. Perfect for radar.
1EP1 screen shots - 20141213 Hooked up the 1EP1 CRT to Eric's boards. Turned the high voltage down to 800 volts and it worked just fine. Not good for a TV as the beam spot is too large even when focused as well as possible. Still pretty impressive.
Glow, baby, glow! (1EP1 heater) - 20141213
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