Magnetic Fields on a CRT

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RODALCO

Well-Known Member

Was playing around with some old CRT's over the weekend, and trying to convert one as a simple oscilloscope.
Well I got a bit side tracked as I had one TV smoking when I disconnected the horizontal coils. So ended up repairing the TV, one bulged capacitor and two resistors fried, which were in parallel with the hor. Coils.
Coincidentally I held a magnet near the screen and the standard blue default colour produced some amazing magnetic lines.
Here is a short video I made.
Enjoy.
 

I'm rather bemused about someone 'discovering' the effect a magnet has on the shadow mask on an old colour CRT
 
I accidentally did this as a kid (held a magnet too close to the TV). There were purple and green splotches on the screen for the next couple of years
 
DerStrom8 said:
I accidentally did this as a kid (held a magnet too close to the TV). There were purple and green splotches on the screen for the next couple of years
Click to expand...
I did the same on a Philips TV and it took ages for the purple spot to disappear. Wonder if older TV's used degaussing coils.
That was in the 1980's
 
RODALCO said:
I did the same on a Philips TV and it took ages for the purple spot to disappear. Wonder if older TV's used degaussing coils.
That was in the 1980's
Click to expand...

Mine certainly had a degaussing coil (I remember tearing it apart when it finally died) but it really didn't do much good
 
I will do colds starts on the 2 TV's used in this demo and see how they come up. They are sitting in our garage.
 
Nigel Goodwin said:
I'm rather bemused about someone 'discovering' the effect a magnet has on the shadow mask on an old colour CRT
Click to expand...

I had the same experience as DerStrom8 when I was a kid, and made a purple spot in our Philips TV, and got told off by my parents. Now I have lots of donated or roadside TV's which I often overvolt , strip for parts or blow up for fun.
May as well have some fun with them and the effects are very cool.
Some TV's keep working up to close to 500 Volts AC before they let go.
 
DerStrom8 said:
Mine certainly had a degaussing coil (I remember tearing it apart when it finally died) but it really didn't do much good
Click to expand...

It should do, but they only work from absolutely stone cold - I've spent a LOT of time over the decades deguassing CRT sets

As far as I'm aware all colour sets had deguassing coils/circuits, although in later years there was some talk of stopping fitting them.

One of my memorable stories was a lady who kept complaining about 'funny colours' on her TV, it was a GEC valve set, and every time we went out we turned the set on, and it was perfect. Anyhow, one day we were driving past (and had a quiet moment), so thought we'd just pop in and see how she was doing. This was when it all got explained - catching her un-awares stopped her clearing everything away from the TV (to give us room), and right next to the TV was a huge great loudspeaker. Every time she called us out she moved it out of the way, and set deguassed itself when it came on
 
I love those historical stories from the field. Brilliant!!
That are experiences never to be forgotten.
 
I have two deguassing coil winding machines in the top of the barn. Have not been turned on for 10 years. I though about making metal detector coils.

Recently I re-purposed another machine. I doubt anyone will want coils 3 meters across. I have been in too many businesses that no longer exist.
 

Ron,

I recall, as a cadet, being on board of a vessel with her own degaussing system, used to reduce the magnetic "signature" thinking she could have to navigate an area planted with magnetic sensors fitted mines.

In an old shipyard they even had a degaussing system to treat vessels under repair.

Had you any experience on that area? Not even sure what vessels nowadays do on that regard.

In the past it was a must for minesweepers.
 
atferrari said:
Ron,

I recall, as a cadet, being on board of a vessel with her own degaussing system, used to reduce the magnetic "signature" thinking she could have to navigate an area planted with magnetic sensors fitted mines.
Click to expand...

In WWII ships were commonly fitted with deguassing systems, to avoid magnetic mines.
 
If I remember right, the process of welding will leave magnetic effects on the ship.
It will take a great deal of energy to fix a ship in a timely manor.
 

One of my huge transmission line speakers did a similar thing to our beautiful Sony 27" color TV, although it had built-in degausing. In the end I got a hand held degausing tool that fixed the TV for a couple of days before the rainbows started creeping back again. The degaussing procedure was a great entertainment for the kids, with the swirling colors on the screen and the loud hum from the degausing tool.

The TV interference was an additional reason why my speakers got ejected from our front room. The chipboard finish, size and the four exposed speaker chassis and port filled with black wool didn't help much either.

spec
 
Last edited:
Hi,

I remember doing that with a magnet on my color CRT monitor several years ago. To get rid of the effect i simply turned the magnet around so the opposite pole faced the spot, then moved it around a little.

What always interested me was that magnets either attract other magnets or iron based objects or repel other magnets, so they move directly away from the first magnet or directly toword the first magnet, BUT if the object is a moving electron, the Lorentz force pushes it either up or down (assuming horizontal orientation of the magnet). It does not attract the moving electron nor repel it it the same direction as the magnet is facing like other objects, but instead pushes it at a RIGHT angle to the orientation of the magnet (and thus the field). If the electron was like any other object it would turn toward the magnet or at least move away (right or left not up or down). I did a video on this using the scope beam as the electron flow. The dot starts out at the center of the screen, then moves down when the magnet is brought near the side of the scope body.
So there is something very different about the electron than other objects that interact with magnets (and thus their fields).
 
MrAl said:
So there is something very different about the electron than other objects that interact with magnets
Click to expand...
Electric charge?
The electron has charge, and is moving, I am told that constitutes an electric current.
Now think about the right hand and left hand rules for motors and generators, where the current the field and the force are in mutually orthogonal directions.

JimB
 
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