APRIL 2010 [time-nuts] [OT] degaussing thread highlights
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Dave Baxter dave at uk-ar.co.uk
Thu Apr 15 08:02:32 UTC 2010
[...] One site, a MRI research facility, the "ambient" magnetic field is strong enough to screw the display off the screen if you are not careful how you position the analyzer. You can also feel your toolbox being pulled sideways when walking around the outside of the building.
[...]
(The main Tech guy at that site could somehow screw an alloy plate into the field near the MRI scanner, let it go, and it'd sit there in mid air! If you tried to get it out, not knowing how, the harder you tried, the hotter it got. The Miensner effect I believe.)
[...]
I've also seen an old guy at a customers site in the past demag a small colour tube using a small strong permanent U shaped magnet on a stick, spun between his fingers while moving it arround the affected area. Absolute magic to watch him doing that.
[...]
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Craig S McCartney CMcCartney at on-sitetraining.com
Thu Apr 15 15:21:03 UTC 2010
A bit off topic, but likely interesting to time-nuts:
In the early days of HDTV (late 80s - early 90s) we were at a European trade show and had to borrow, at the last minute, a large (~40") CRT-based HD monitor from a Dutch company that was also exhibiting there. We liked it so much that we purchased it for use at other trade shows. The very next show was in Sydney, Australia. The monitor would not converge and had funny rainbow patterns no matter how many times we degaussed it. Thinking it had been damaged in its half-way around the world shipment we called in the local representatives of the Dutch manufacturer and they were as stumped as we were. Finally, after they talked to engineers at the factory, the explanation was: "Of course, that is a Northern Hemisphere monitor; the earth's magnetic field is much different in the Souther Hemisphere. It can't work there.". After the show we shipped it back to our headquarters in California and, sure enough, the picture was perfect.
So, we had a HDTV monitor that doubled as a earth hemisphere detector, using magnetic flux differential. Needless to say, we only used it in northern hemisphere shows after that.
Craig McCartney
WA8DRZ
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Magnus Danielson magnus at rubidium.dyndns.org
Sat Apr 17 15:27:06 UTC 2010
Did you ever tried to run it upside down?
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Craig S McCartney CMcCartney at on-sitetraining.com
Sat Apr 17 16:51:44 UTC 2010
Magnus,
No, we did not think of that at the time. A good idea, but if it worked the customers at the trade show would have had to stand on their heads to see the picture properly.
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Magnus Danielson magnus at rubidium.dyndns.org
Sat Apr 17 17:41:16 UTC 2010
Well, besides that it would be fun to see, it would at least have been a good test to clear out if it was broken or just not adjusted. I would assume that near equator would be another trimming.
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Robert Atkinson robert8rpi at yahoo.co.uk
Sat Apr 17 21:23:29 UTC 2010
Hi Magnus,You beat me to it. Many years ago a friend brought a high end CRT computer monitor back from Australia. The colours were off (Purity). I told him that it was because Australian monitors only work upside down. He didn't believe me, so one bet of a 6 pack later I turned it upside down and enjoyed my beer ;-) It wasn't perfect upside down, but it was a lot better.
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Kit Scally kScally at BYTECAN.com.au
Thu Apr 15 23:58:34 UTC 2010
Craig,
If memory serves me correctly, Philips "better quality" PAL colour TV's in the late 70's to early 80's "inverted" the CRT for us in the Southern
Hemisphere. The TV's were clearly marked as such on the packing box.
For reasons unknown, Japanese-made colour TV's never seemed to have this problem. Even though I was working professionally with these beasts at the time in a TV station, I never figured out why Jap TV's seemed immune to this problem whilst those made by our Dutch friends were not.
[...]
Kit
VK2LL
Sydney
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jimlux jimlux at earthlink.net
Fri Apr 16 04:35:36 UTC 2010
And, of course Trinitron tubes, since they have only one gun, no shadowmask, and synchronize by looking for the backscatter from the face, are immune to such convergence effects.
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Chuck Harris cfharris at erols.com
Fri Apr 16 11:54:23 UTC 2010
Are they really? For some reason, every Trinitron I have ever seen has clusters of little stick on magnets placed here and there on the back of the glass envelope.
The trinitron has a shadowmask. It is a grill of highly tensioned wires that are positioned just behind the screen. The original trinitron tube was a little 5 inch diagonal CRT. It had to be small because the wires tended to vibrate if the set was bumped, and that made for some very odd displays. The later larger tubes had horizontal titanium wires welded to the backs of the shadow mask wires every 5 or 10 inches, to prevent the psychedelic color fest that happened when the CRT got bumped.
The trinitron has three very carefully aligned cathodes in the gun. They are positioned side-by-side, creating the slight different projection angles necessary to cause the long vertical slots formed by the shadow mask to eclipse the appropriate color bands on the screen. I'm not sure what you are describing; it sure sounds cool; but it isn't a trinitron. Can you find some references? I'd like to read up on it.
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jimlux jimlux at earthlink.net
Fri Apr 16 13:48:54 UTC 2010
You're right...
I must have been thinking about another scheme.. Now I'll have to go find it.
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Arnold Tibus Arnold.Tibus at gmx.de
Sat Apr 17 12:04:41 UTC 2010
[...]
The small magnets on the back of the [Trinitron and Diamondtron tubes (Mitsubishi) ] tube are necessary to linearize the dynamic field of the deflecting coil and to compensate other small steady magnetic distortions around the tube. There are some more magnets on the neck of the tube for convergence and beam forming.
A long and distracting work to to when you had to replace the tube or coils and then to adjust for white and clean colors and sharp picture...%-))
Older systems needed an earth field compensation in situ. [...]
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Henk henk at deriesp.demon.nl
Sun Apr 25 15:51:21 UTC 2010
Up to the Philips 20AX tubes they used adjustable multipole units around the neck of the tube. These multipoles can be readjusted if needed. From the 30AX design on, the used multipoles that were internal, thus inside the neck. The required correction was measured during manufacture and the internal multipole magnetized. Turning the tube upside down will help down under if the tube was manufactured in the northern hemisphere. Then tune the deflection yoke back or swap line and frame connections.
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Steve Rooke sar10538 at gmail.com
Mon Apr 26 04:34:35 UTC 2010
When I moved from England to New Zealand I brought my Sony Trinatron TV with me and had the tuner replaced so it would work over here (we still have VHF TV and a different sound sub-carrier). The set worked fine after the conversion and I noticed no problems at all with the picture but that's not to say that the REALLY good TV guy had not sorted things out.
I tried power cycling the monitor in it's new position, and it went back to looking great! Haven't found out about this autocalibration yet, but it's neat. I Always thought well of the ViewSonic because it came together with a pair of IRIS Indigos and a graphics tablet..