Degause a B/W CRT?

[TonyP]

Ive always wondered if one should degause a b/w crt? would it help any? would it hurt? or will i have to just live with a little pincushion distortion on a 1950s tv?

[bandersen]

It won't hurt it, but it also won't do anything.

Color CRTs have a metal shadow mask behind the phosphor dots that can become magnetized and cause distortion.

B&W CRTs do not have a mask

[Popester]

There would be no need to do so as they only have 1 phosphor. There is nothing metallic in it to get magnetized.

[Electronic M]

The only things that could effect the raster noticeably are if the face mounts were magnetized about as strong as a speaker magnet, or if yoke mounting hardware were magnetized, and I don't expect a pincushion effect would be the symptom...

[Eric H]

how much Pincushion you talking about? Some minor distortion is normal on old TV's, if it's pulled in so bad you have dark areas on the screen than something is wrong. Bad power supply,bad yoke, etc.

[ppppenguin]

Monochrome TVs are often fitted with geometry correction magnets around the scan yoke. If these aren't fitted or are missing then the result is often pincushion.

The photo is of a 12" professional monochrome monitor and shows several correction magnets. This is more complex than an ordinary TV due to the need for higher performance. A monochrome TV will often have a pair of cylindrical magnets mounted on the yoke.

[dtvmcdonald]

I always thought that you needed to degauss the metal cone CRTs. IS this true?

[ppppenguin]

In the UK there were very few metal cone tubes. A few (monochrome) were made in the early 1950s, I very much doubt if degauss coils were commonplace back then.

In the US you had metal cone colour tubes in the 1950s. I presume these needed degaussing along with the shadow mask. I don't know if monochrome metal cone tubes were used in the US.

[old_tv_nut]

The way to think about metal cone mono CRTs is to note the effects of degaussing metal cone color CRTs. Degaussing the color CRT results in beam landing deviations on the order of fractions of a millimeter (the spacing between color dots). It does not produce any noticeable change in the geometry of the image. So, in a monochrome metal cone tube, it would produce no noticeable effect and is not needed.

[dtvmcdonald]

Originally Posted by old_tv_nut

The way to think about metal cone mono CRTs is to note the effects of degaussing metal cone color CRTs. Degaussing the color CRT results in beam landing deviations on the order of fractions of a millimeter (the spacing between color dots). It does not produce any noticeable change in the geometry of the image. So, in a monochrome metal cone tube, it would produce no noticeable effect and is not needed.

Beam landing POSITION errors in a shadow mask tube, if small, don't cause purity problems. The problem with them is that magnetic fields result in landing ANGLE errors, which send the beam to the wrong place after the mask.

I too would expect that the usual minor magnetic fields in a metal cone
B&W tube would not result in problems with ordinary program material.
For high resolution specialty monitors, that might be very different. That has
already been mentioned.

[old_tv_nut]

Originally Posted by dtvmcdonald

Beam landing POSITION errors in a shadow mask tube, if small, don't cause purity problems. The problem with them is that magnetic fields result in landing ANGLE errors, which send the beam to the wrong place after the mask.
...

Yes, but I just didn't want to get into it, but on to the relevant point that the geometric distortion, if any, is not noticeable.

Originally Posted by dtvmcdonald

...
I too would expect that the usual minor magnetic fields in a metal cone
B&W tube would not result in problems with ordinary program material.
For high resolution specialty monitors, that might be very different. That has
already been mentioned.

[Robert Grant]

(Apology for posting a little OT):

About 40 years ago, I put the magnet of a magnetic-mount CB antenna against the face of the CRT of a monochrome TV set.

The effects were psychedelic. At several inches, a small portion of the picture was distorted and the image in the center was turned clockwise.

Placing the magnet closer to the tube turned it drastically, and created a "Black Hole" in the image.

After withdrawing the magnet, the picture assumed normal shape as if nothing unusual ever happened.

[ppppenguin]

You were lucky. If you put a pwoerful magent close to a colour CRT it will often magnetise the shadowmask to a point where the internal degauss coils can't cope. No problem if you have a handheld degauss coil. I have an old handheld bulk tape eraser which does the job nicely.

[old_tv_nut]

Originally Posted by ppppenguin

You were lucky. If you put a pwoerful magent close to a colour CRT it will often magnetise the shadowmask to a point where the internal degauss coils can't cope. No problem if you have a handheld degauss coil. I have an old handheld bulk tape eraser which does the job nicely.

I think a REALLY powerful magnet might be capable of mechanically distorting the mask so that no degaussing will fix it. I would not try it for fun unless I was willing to trash the CRT.

[old_tv_nut]

Originally Posted by Robert Grant

(Apology for posting a little OT):

About 40 years ago, I put the magnet of a magnetic-mount CB antenna against the face of the CRT of a monochrome TV set.
...
After withdrawing the magnet, the picture assumed normal shape as if nothing unusual ever happened.

Because there is nothing in a glass monochrome CRT face to get permanently magnetized.