I've read about ground strap arcing causing implosion, Zenith safety cap arcing (HV button to yoke) causing implosion, and I've heard of the islands arcing to each other. I don't know of any cases of island arcing causing implosion, but it is possible....Arcing generated intense local heat, uneven heat of glass causes fracturing, fracturing of glass under vacuum causes implosion.
I rather not risk it.

Agree it's best to error on the safe side. However my gut feeling is you'd be hard pressed to heat up an area of a CRT through conduction of it's dag coating, considering the typical resistance of the material and it's low thermal mass. Might be a fun youtube experiment. :)

It's not conductive heating of the dag at all. An arc essentially forms a super heated conductive plasma in the air to carry current. If you've ever experimented with high voltage arcs (like a Jacob's ladder or a plasma speaker or an automotive spark ignition system) you know that an arc generates enough heat to easily light paper, a cigarette, gasoline, and even burn/erode metal.

If you put your finger in the middle of a 20KV arc and you are not connected to either supply pole you will feel more pain from the plasma burn than the shock (as I know first hand).

I got the recoated CRT installed without any troubles :)
https://farm2.staticflickr.com/1444/...a741ca33_z.jpg

I believe this chassis was originally designed for round CRTs because of a couple clues.
One - This tube bumps into the CRT and there's no room to install a shield.
https://farm2.staticflickr.com/1594/...0c6d4e93_z.jpg

Two - The discriminator coil is mounted under the chassis. That gave me a headache while doing the alignment as the instructions for adjusting the top and bottom slugs are the opposite of what I needed to do.
Sound is excellent now :)
https://farm2.staticflickr.com/1501/...7e4551be_z.jpg

I then fought with the ion trap magnet and centering rings to eliminate corner shadows and minimize distortion. Still not entirely happy, but it's not very noticeable while viewing programming.
https://farm2.staticflickr.com/1510/...f925632e_z.jpg

So that's a wrap for the chassis restoration. Next up I'll arrange for a trip to reunite it with it's cabinet.
https://farm2.staticflickr.com/1544/...01bb1688_z.jpg

Very nice Bob. You do superior work!

Another restoration that looks awesome! What's next on the restoration bench? Possibly a porthole or a remote set?

Nope. A Stromberg Carlson 21 inch tabletop I'm restoring for a client. I'll be starting a new thread soon.

Excellent restoration, Bob. I'm surprised they didn't recess that tube socket 1/4" or so below the chassis.

Yeah that would have been a simple solution. Perhaps I could drill out the rivets and recess it :scratch2:

Tom, I'm sure you realize that with the Zenith color safety cap failures, it was because the high voltage would soar insainly high, at which point the HV could arc "through" the CRT neck glass to the yoke or convergence poles. But that's not the same thing that happens with sparks between adjacent dag islands or a grounding strap on the outside surface of a b&w CRT.

Right. The arcky sparkies you'd get there would be more akin to rubbin' a cat's fur vigorously. But it will generate hash that'll sure play hobb with sync.

I have a set with bad safety caps and it appears to arc over the surface of the glass, and not through it.

The other example I gave of a poor arcing ground strap connection was a surface arc too and that surface arc DID cause an implosion.

I noticed the aquadag coating on the CRT bell was flaking off (there is a large bare spot where the coating should be, even though most of the rest of the dag is still on the tube). I'd replace the tube, as it could be very weak--especially if the TV was made in the 1950s and was used a lot by its former owner(s). There could be high-voltage issues as well if the dag coating (or any part thereof) is missing, including arcing or even no raster. Because the dag is one part of a large HV capacitor in all CRT TVs, it is very important that the coating is intact over the entire surface of the tube bell.

Edit: I just looked at some other posts in this thread and found that Bob had recoated the CRT bell, thereby restoring the tube to normal, or at least as close to normal as one can come with a 1950s-era CRT. I was not aware (until just now) that flaking aquadag could be replaced, but I learned something tonight when I saw the references to Slip-Plate and the other compound mentioned.

BTW, the picture on the newly-recoated CRT looks good from here. Bob said there were still some issues with the image the tube produces, but from what I can see, the picture is pretty darn good. I would think defects in the raster, etc. would show up more prominently on a crosshatch pattern than on a standard program picture anyway.

Entries #25&31 show a different CRT, than the original one. It was one Bob had that tested good, but the a-dag coating was flaking off.
I had read somewhere that Warwick aquired Pacific Mercury sometime in the 50's. They were building the Thomas organ there as well. :scratch2: