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enter image description here

I have noticed that a lot of CRTs have a coloured coating inside of them that would appear to have practical purpose other than maybe protecting the aquadag. Paints and plastics tend to be avoided in vacuum scenarios, so I can't imagine that the green coating is paint. Would anyone happen to know what the green layer is and why it's there?

To be clear, I know from breaking one of these open that it's a flat layer of green. In this image, the CRT PDA (The black lines) is helical, but I've also seen this green in banded PDAs.

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  • \$\begingroup\$ please provide all of the available information ....are those concentric rings or is it a helix? \$\endgroup\$ – jsotola Jan 12 at 2:56
  • \$\begingroup\$ That is a helical PDA. \$\endgroup\$ – Seif Mehanna Jan 12 at 7:26
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It looks like rings with graphite between an algae green silicate binder to dissipate static build up on the tube to prevent glass partial discharge. If discharge occurs between ring of dielectric, the capacitance is in series and only a partial discharge would occur if it failed from contamination in the vacuum. A conductor then bleeds the mA current off somewhere.

Aquadag™ consists of a dispersion of colloidal graphite in distilled water.

The aquadag coating has two functions: it maintains a uniform electric field inside the tube near the screen, so the electron beam remains collimated and is not distorted by external fields, and it collects the electrons after they have hit the screen, serving as the return path for the cathode current

In some CRTs the coating performs a third function, as a filter capacitor for the high-voltage anode supply. -||-||-||-||- < 1nF max.

Aquadag™ brand is now owned by Henkel.

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Below is a typical small electrostatic-deflection CRT - looks like my old 3JP1 (diagram from this book.

enter image description here

Typically the accelerating electrode runs at around double the voltage of the intensifier electrode (eg 4kV and 2kV).

My speculation is that the concentric dark rings are relatively conductive and the green coating is much more resistive so that a voltage divider is formed along the axis that the electrons are accelerated. The concentric conductive rings ensure that the voltage distribution is uniform about the central axis so as not to improperly deflect the beam.

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Most likely a lead containing compound to reduce radiation exposure to users. If your referring to the green, I believe that is a wire coil.

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  • \$\begingroup\$ I don't that would be a lead compound, since the radiation protection is usually the glass itself being doped with lead and barium oxide. I doubt that it's a wire coil either since it is a flat layer of something green, while the black spiral is a thin conductive coating used for post-deflector acceleration. I've edited the question to say that it's flat. \$\endgroup\$ – Seif Mehanna Jan 11 at 23:42
  • \$\begingroup\$ Yes, with your update my answer is not of value and most likely wrong. \$\endgroup\$ – MadHatter Jan 12 at 0:07

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