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enter image description here

This question is regarding a matte red/orange coating around the anode socket on a cathode ray tube which is made to be connected to via a cable with a suction cup, the coating seems to typically be a circle just a little bit larger than the suction cup, although in some images I have seen this being a much larger extended area. In my case, I found this on an oscilloscope CRT, although I have also seen this in images online appearing on CRTs from old TV sets. I have found very few mentions of what this actually is online and the information is inconsistent! The consensus seems to be between this having no purpose (which I doubt), and this being insulation which prevents arcing (which makes me wonder why it is necessary in between glass and the suction cup which are already insulators). Would anyone knowledgeable be able to tell me what this substance is and what it does?

The reason I need to know is that I accidentally removed a good part of this coating on an oscilloscope CRT while discharging the anode and cleaning the area under the cup, and need to figure out whether I need to do something to replace it. I have heard about silicone paste being used in this area to prevent arcing, however if the original coating was there for the same purpose, I wanted to find out whether whatever this substance is works better.

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  • \$\begingroup\$ Just out of curiosity, can you post a picture of it? \$\endgroup\$ – Ricardo Dec 21 '14 at 19:42
  • \$\begingroup\$ My apologies - I'll have to link to someone else's photo online as I don't have a camera (and if I did it wouldn't be too useful to describe what it USED to look like) - however here is a photo of the same thing on a TV CRT, hope the webpage's owner won't mind me linking here: electronicrepairguide.com/images/anodecap.jpg \$\endgroup\$ – ew218 Dec 21 '14 at 20:00
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    \$\begingroup\$ I seem to recall that the outside of the tube as well as the inside was typically coated with aquadag (the outside grounded with the mounts and/or a spring contact), so the insulation (which may well be silicone- the color is right) would prevent arcing across the surface to the outside aquadag. A silicone vacuum grease may work for this purpose. \$\endgroup\$ – Spehro Pefhany Dec 21 '14 at 20:39
  • \$\begingroup\$ Thanks for your response - I can see how grease would help here by preventing air gaps (I also didn't realise silicone grease was made in red), however I don't think this was what was on my CRT - it is hard, dry and set on the CRT like paint (although quite easily scratched and came off with isopropanol); even though the scope is rather old (Kikusui COS6100A) I am having difficulty imagining grease setting like this (though I could be wrong) - and all of it on the CRT, none on the suction cup. If it is not grease, it would not improve the seal, so I don't understand how else it could help! \$\endgroup\$ – ew218 Dec 21 '14 at 21:16
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    \$\begingroup\$ My guess is that it's a high resistance coating designed to minimize field gradients under the cap and foil any attempts of the EHT to zap tracks to air. \$\endgroup\$ – EM Fields Dec 22 '14 at 10:26
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Probably it's Red Alkyd hv paint,which is rust-red in color. It might be there to halt leakage currents across hygroscopic glass surface during extremely humid weather. Older trade name was Glyptal, but its the same as any hv insulating paint, Alkyd Enamel.

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  • \$\begingroup\$ Great! I've done a bit of research following this answer and this certainly seems like what I was looking for. It appears that one specific product name for this substance for which I found an actual datasheet is Glyptal 1201. Descriptions and images I found are all consistent with the substance I encountered. This seems to be used for multiple purposes and I found just a couple of mentions of this being used for the purpose described in CRTs, so I'm not too surprised I never came across it during my initial search. Many thanks! \$\endgroup\$ – ew218 Jan 26 '15 at 17:11
  • \$\begingroup\$ It's commonly sprayed on motor windings to'heal' any shorted turns. I have a tiny bottle for dripping onto bad TC vac gauges to find leaks in poor welds. \$\endgroup\$ – wbeaty Jan 28 '15 at 8:11
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I know from personal experience that bare glass in the area of the anode connection can take oily fingerprints which will conduct an arc from the anode connection to the external grounded coating (aquadag). Makes for a very (electrically) noisy TV.

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  • \$\begingroup\$ Interesting - however any idea as to what specifically this substance could be? Following all responses It seemed that the coating is indeed related to arc prevention following the responses I got here - so I removed the rest of my damaged coating and replaced it with Super Corona Dope ( mgchemicals.com/products/protective-coatings/insulating/… ), so that at least there is something else in place there for the same purpose - however I'm still interested in finding out what this red/orange substance originally was as its use seems to be quite widespread. \$\endgroup\$ – ew218 Jan 11 '15 at 13:10

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