6 ms·

How Cathode Ray Tubes Work [video]

60 points by hggh 2y ago

Sakos 2y ago
Great video. Seeing what it looks like inside with the phosphor and the mask was so enlightening. I also recently watched the one by Technology Connections, which was quite good: https://www.youtube.com/watch?v=l4UgZBs7ZGo https://www.youtube.com/watch?v=l4UgZBs7ZGo I feel like I have a much better basic understanding of CRTs now. They felt like utter magic before. Now it just feels a bit like magic. The only thing that still confuses me is how the electron gun itself actually works. I don't really get how electrons are being shot at all and why we can shoot them in a beam.
- emchammer 2y ago
  Think of the electrons not as being shot, but as being boiled off a cathode where there are a lot of them, and then suddenly they find themselves attracted to an anode which has nothing blocking them from reaching it.
  - willis936 2y ago
    Don't think of the bullet as being shot, but as being released and finding itself attracted to a low pressure space with nothing blocking it from reaching it. ;)
- Cerium 2y ago
  The basic idea is you have a source of electrons and a way to get at least some of them going where you want. A simple way to do this is to have a hot charged filament (source of electrons) and a plate with a hole to attract them. Check out a google image search for "electron gun schematic" to get a better idea.
  - MisterTea 2y ago
    You can also build a glow discharge gun and use an aperture which is way simpler than a hot cathode.
- Bluecobra 2y ago
  One thing that I still find magical is vector graphics on a CRT, even when playing games like Asteroids or Lunar Lander. AFAIK there is no way for any other display to replicate the intensity of vector graphics, except on a real CRT. It would be a sad day when they eventually die off.
  - drivers99 2y ago
    Maybe not the same thing but you could do something similar with a lasers and I'll let this random video I just looked up show how it works: https://www.youtube.com/watch?v=MsaYcX5aIac https://www.youtube.com/watch?v=MsaYcX5aIac Looks like there are some commercial products like that as well.
darajava 2y ago
Great video and explanation, but he still doesn’t really explain how the magnets in the monochrome CRTs actually draw the image. How do they know where to point? How does it move so fast?
- Sakos 2y ago
  For this, I recommend the Technology Connections video https://www.youtube.com/watch?v=l4UgZBs7ZGo https://www.youtube.com/watch?v=l4UgZBs7ZGo. He actually demonstrates how the beam is redirected by the electro-magnets wrapped around the base of the tube. It's very cool.
- kube-system 2y ago
  > How do they know where to point? With a VGA signal, a pulse on the vsync line tells the monitor to start at the top left. A pulse on the hsync line tells the monitor to move down a row. In between those pulses, the analog values of the RGB values determine the color as the beam scans left to right. The CRT is dumb -- it's up to the device sending the signal to time the pixel values correctly so they hit the screen as the beam scans over the correct spot on the screen. If you were making a device that needed to display analog video, you'd have to make sure the signal you were outputting would make the electron gun fire at the right time. (e.g.: http://www.tinyvga.com/vga-timing http://www.tinyvga.com/vga-timing) > How does it move so fast? They're electromagnets, so they move in sync with the timed signal sent to them.
  - MrRadar 2y ago
    In theory the electromagnets controlling the vertical and horizontal sweep of the electron beam should react instantly to the incoming sync signals but in practice it's much more complicated (because the magnetic fields store energy which you need to deal with somehow). Bob Anderson has an entry in his series on the general process of restoring vintage televisions which goes in depth on the topic: https://www.youtube.com/watch?v=HdEfo8jS3FI&list=PL4aHiwoXvLGA6lr13xGxkxFV3Y20pv6Nb&index=15 https://www.youtube.com/watch?v=HdEfo8jS3FI&list=PL4aHiwoXvL...
  - dclowd9901 2y ago
lawlessone 2y ago
Be interesting if we kept developing this tech. Where would CRT's be now?
- ethagnawl 2y ago
  What's interesting and possibly surprising is that the core tech didn't evolve much past the 50-60s. Sony's Trinitron were the pinnacle, prior to modern inputs and HD. The original patent expired in the mid 90s and everyone else raced to implement their version of it. So, if evolution had continued, _I think_ we'd have marginally lighter and more efficient displays with HDMI et al and the surrounding electronics and the "smart" features seen on modern screens.
  - dylan604 2y ago
    Not sure about them being lighter. The vast majority of the weight of a CRT was the thick glass on the front. CRTs that claimed to be "flat" used very thick glass that was curved like a lens so that the front the viewer saw was flat. The larger the screen, the thicker the glass would need to be to keep the edges/corners from bowing. That glass was very heavy. The 32" 16x9 broadcast Sony monitor we had was extremely heavy and required two people at a minimum to carry/place it.
    - dclowd9901 2y ago
      I had a 32” 4:3 trinitron I used to use for vintage gaming and damned if that thing didn’t weigh 130 lbs.
      - ethagnawl 2y ago
        I've got a 36" in my office and it's a monstrosity. I need to vacate my office in the near future and moving it out is the part I'm least looking forward to. Granted, I was a much younger man when I moved it in but I kind of can't believe I did it by myself with only the help of a skateboard and some movers' straps. It's a wonder I didn't kill myself or the set. There's no way I'm doing that again.
ycombinatorics 2y ago
Nothing makes me feel older than seeing a youtube video trend about CRTs. This was part of my high school curriculum.
z500 2y ago
When I was a kid we had a Trinitron. I used a magnet on it once and pretended not to know anything about it as my dad called the cable company. Apparently it was able to degauss itself because the color went back to normal eventually.
- dclowd9901 2y ago
  Those trinitrons looked amazing. I had a 22” I had bought myself when I was a senior in high school. But god were they heavy.
  - sixothree 2y ago
    That’s a beast. Largest CRT I ever had on a desk was 21 and that was nuts.
  - alok-g 2y ago
    Anyone knows why they were so much heavier? Just size difference or some other interesting reason? :-)
MisterTea 2y ago
There's an old site with a nice run down of building simple DIY CRT's using a glow discharge electron gun and laboratory flasks: http://www.sparkbangbuzz.com/crt/crt6.htm http://www.sparkbangbuzz.com/crt/crt6.htm The glow discharge stuff is very easy to build as you dont need a filament/heater and power supply, it's a cold cathode gun. I wound up coming across another simple and fun project that demonstrates a very simple glow discharge gun in a glass bottle: https://hackaday.com/2011/08/30/diy-electron-accelerator/ https://hackaday.com/2011/08/30/diy-electron-accelerator/ I built one of those bottle accelerators but with some mods using a smaller Stuart's soda bottle. Cathode is a 0.5 in diameter Al spacer bushing mounted to one end of a 0.25 inch aluminum tube punched through a rubber stopper and the other to a tube adapter screwed to a KF25 vac flange. I then rigged up a stand to hold a vacuum manifold connected to my Alcatel 2008 vac pump and could pull a nice vacuum down to 0.4 Pa. The bottle gun's anode is a loop of 12 AWG copper wire inserted into a drilled hole and sealed with Faraday wax (safe and easy to make but messy and does not easily clean up.) Finally, wired it to my 200W Bertan +10kV 20mA supply. The +10kV lets me setup the gun as a common cathode putting the electron gun parts at ground potential. Only the anode is at high potential. I then decided to see if I could focus the beam and MacGyverd a lens out of a big 2 inch conduit bushing I had lying around in the electrics bin. Wound a bunch of 20 AWG magnet wire around it and found that building a big enough field to focus and sat it around the bottle neck. That big hunk of steel took a crap ton more power than I thought - I could focus the beam to midway of the bottle at around 300W (30V DC @ 10A)into the focus lens (yes it got very hot)! At full beam power I can melt holes in tin foil sheet using a focused beam.
Clamchop 2y ago
Trinitrons and workalikes were very good, but the shadow mask displays that this video focuses on to my memory didn't have the excellent contrast and black levels cited. I remember most of them looking pretty dull, and downright silvery if there was much light in the room.
deleted 2y ago
[deleted]
crmd 2y ago
In the old days, our cars were powered by gasoline and every house had a ~25KeV particle accelerator in the living room.
ononkota 2y ago
[flagged]

60 points by hggh 2y ago

Great video. Seeing what it looks like inside with the phosphor and the mask was so enlightening. I also recently watched the one by Technology Connections, which was quite good: https://www.youtube.com/watch?v=l4UgZBs7ZGo https://www.youtube.com/watch?v=l4UgZBs7ZGo I feel like I have a much better basic understanding of CRTs now. They felt like utter magic before. Now it just feels a bit like magic. The only thing that still confuses me is how the electron gun itself actually works. I don't really get how electrons are being shot at all and why we can shoot them in a beam.

emchammer 2y ago
Think of the electrons not as being shot, but as being boiled off a cathode where there are a lot of them, and then suddenly they find themselves attracted to an anode which has nothing blocking them from reaching it.
- willis936 2y ago
  Don't think of the bullet as being shot, but as being released and finding itself attracted to a low pressure space with nothing blocking it from reaching it. ;)
Cerium 2y ago
The basic idea is you have a source of electrons and a way to get at least some of them going where you want. A simple way to do this is to have a hot charged filament (source of electrons) and a plate with a hole to attract them. Check out a google image search for "electron gun schematic" to get a better idea.
- MisterTea 2y ago
  You can also build a glow discharge gun and use an aperture which is way simpler than a hot cathode.
Bluecobra 2y ago
One thing that I still find magical is vector graphics on a CRT, even when playing games like Asteroids or Lunar Lander. AFAIK there is no way for any other display to replicate the intensity of vector graphics, except on a real CRT. It would be a sad day when they eventually die off.
- drivers99 2y ago
  Maybe not the same thing but you could do something similar with a lasers and I'll let this random video I just looked up show how it works: https://www.youtube.com/watch?v=MsaYcX5aIac https://www.youtube.com/watch?v=MsaYcX5aIac Looks like there are some commercial products like that as well.

darajava 2y ago

Great video and explanation, but he still doesn’t really explain how the magnets in the monochrome CRTs actually draw the image. How do they know where to point? How does it move so fast?

Sakos 2y ago
For this, I recommend the Technology Connections video https://www.youtube.com/watch?v=l4UgZBs7ZGo https://www.youtube.com/watch?v=l4UgZBs7ZGo. He actually demonstrates how the beam is redirected by the electro-magnets wrapped around the base of the tube. It's very cool.
kube-system 2y ago
> How do they know where to point? With a VGA signal, a pulse on the vsync line tells the monitor to start at the top left. A pulse on the hsync line tells the monitor to move down a row. In between those pulses, the analog values of the RGB values determine the color as the beam scans left to right. The CRT is dumb -- it's up to the device sending the signal to time the pixel values correctly so they hit the screen as the beam scans over the correct spot on the screen. If you were making a device that needed to display analog video, you'd have to make sure the signal you were outputting would make the electron gun fire at the right time. (e.g.: http://www.tinyvga.com/vga-timing http://www.tinyvga.com/vga-timing) > How does it move so fast? They're electromagnets, so they move in sync with the timed signal sent to them.
- MrRadar 2y ago
  In theory the electromagnets controlling the vertical and horizontal sweep of the electron beam should react instantly to the incoming sync signals but in practice it's much more complicated (because the magnetic fields store energy which you need to deal with somehow). Bob Anderson has an entry in his series on the general process of restoring vintage televisions which goes in depth on the topic: https://www.youtube.com/watch?v=HdEfo8jS3FI&list=PL4aHiwoXvLGA6lr13xGxkxFV3Y20pv6Nb&index=15 https://www.youtube.com/watch?v=HdEfo8jS3FI&list=PL4aHiwoXvL...
- dclowd9901 2y ago

lawlessone 2y ago

Be interesting if we kept developing this tech. Where would CRT's be now?

ethagnawl 2y ago
What's interesting and possibly surprising is that the core tech didn't evolve much past the 50-60s. Sony's Trinitron were the pinnacle, prior to modern inputs and HD. The original patent expired in the mid 90s and everyone else raced to implement their version of it. So, if evolution had continued, _I think_ we'd have marginally lighter and more efficient displays with HDMI et al and the surrounding electronics and the "smart" features seen on modern screens.
- dylan604 2y ago
  Not sure about them being lighter. The vast majority of the weight of a CRT was the thick glass on the front. CRTs that claimed to be "flat" used very thick glass that was curved like a lens so that the front the viewer saw was flat. The larger the screen, the thicker the glass would need to be to keep the edges/corners from bowing. That glass was very heavy. The 32" 16x9 broadcast Sony monitor we had was extremely heavy and required two people at a minimum to carry/place it.
  - dclowd9901 2y ago
    I had a 32” 4:3 trinitron I used to use for vintage gaming and damned if that thing didn’t weigh 130 lbs.
    - ethagnawl 2y ago
      I've got a 36" in my office and it's a monstrosity. I need to vacate my office in the near future and moving it out is the part I'm least looking forward to. Granted, I was a much younger man when I moved it in but I kind of can't believe I did it by myself with only the help of a skateboard and some movers' straps. It's a wonder I didn't kill myself or the set. There's no way I'm doing that again.

ycombinatorics 2y ago

Nothing makes me feel older than seeing a youtube video trend about CRTs. This was part of my high school curriculum.

z500 2y ago

When I was a kid we had a Trinitron. I used a magnet on it once and pretended not to know anything about it as my dad called the cable company. Apparently it was able to degauss itself because the color went back to normal eventually.

dclowd9901 2y ago
Those trinitrons looked amazing. I had a 22” I had bought myself when I was a senior in high school. But god were they heavy.
- sixothree 2y ago
  That’s a beast. Largest CRT I ever had on a desk was 21 and that was nuts.
- alok-g 2y ago
  Anyone knows why they were so much heavier? Just size difference or some other interesting reason? :-)

MisterTea 2y ago

There's an old site with a nice run down of building simple DIY CRT's using a glow discharge electron gun and laboratory flasks: http://www.sparkbangbuzz.com/crt/crt6.htm http://www.sparkbangbuzz.com/crt/crt6.htm The glow discharge stuff is very easy to build as you dont need a filament/heater and power supply, it's a cold cathode gun. I wound up coming across another simple and fun project that demonstrates a very simple glow discharge gun in a glass bottle: https://hackaday.com/2011/08/30/diy-electron-accelerator/ https://hackaday.com/2011/08/30/diy-electron-accelerator/ I built one of those bottle accelerators but with some mods using a smaller Stuart's soda bottle. Cathode is a 0.5 in diameter Al spacer bushing mounted to one end of a 0.25 inch aluminum tube punched through a rubber stopper and the other to a tube adapter screwed to a KF25 vac flange. I then rigged up a stand to hold a vacuum manifold connected to my Alcatel 2008 vac pump and could pull a nice vacuum down to 0.4 Pa. The bottle gun's anode is a loop of 12 AWG copper wire inserted into a drilled hole and sealed with Faraday wax (safe and easy to make but messy and does not easily clean up.) Finally, wired it to my 200W Bertan +10kV 20mA supply. The +10kV lets me setup the gun as a common cathode putting the electron gun parts at ground potential. Only the anode is at high potential. I then decided to see if I could focus the beam and MacGyverd a lens out of a big 2 inch conduit bushing I had lying around in the electrics bin. Wound a bunch of 20 AWG magnet wire around it and found that building a big enough field to focus and sat it around the bottle neck. That big hunk of steel took a crap ton more power than I thought - I could focus the beam to midway of the bottle at around 300W (30V DC @ 10A)into the focus lens (yes it got very hot)! At full beam power I can melt holes in tin foil sheet using a focused beam.

Clamchop 2y ago

Trinitrons and workalikes were very good, but the shadow mask displays that this video focuses on to my memory didn't have the excellent contrast and black levels cited. I remember most of them looking pretty dull, and downright silvery if there was much light in the room.

deleted 2y ago

[deleted]

crmd 2y ago

In the old days, our cars were powered by gasoline and every house had a ~25KeV particle accelerator in the living room.

ononkota 2y ago

[flagged]