Mercury TV-220A restoration

[Kevin Kuehn]

Originally Posted by Electronic M

Also rarely does a significant portion of the dag come off without "islands" forming, and the islands tend to arc to each other/other dag and grounds which may cause implosion

Is this really a well documented phenomenon, the resulting in implosion part? Don't recall ever reading about that before.

[Electronic M]

Originally Posted by Kevin Kuehn

Is this really a well documented phenomenon, the resulting in implosion part? Don't recall ever reading about that before.

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.

[Kevin Kuehn]

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.

[Electronic M]

Originally Posted by Kevin Kuehn

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).

[Kevin Kuehn]

Originally Posted by Electronic M

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).

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.

[old_coot88]

Originally Posted by Kevin Kuehn

...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.

[Electronic M]

Originally Posted by Kevin Kuehn

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.

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.