[time-nuts] [OT] degausing

Craig S McCartney CMcCartney at on-sitetraining.com
Thu Apr 15 11:21:03 EDT 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 Southern 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 


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Dave Baxter
Sent: Thursday, April 15, 2010 1:03 AM
To: time-nuts at febo.com
Subject: [time-nuts] [OT] degausing

The shaker system I used to know of, was used to test early mobile phone
network test systems (with CRT monitors in) that were intended for
"mobile" use, and no magnetic field leaked out of that one.  It was the
size of a small refuse bin, stood about 2' high, (3' dia) verticaly on
the floor, bolted down to a huge block of concrete, and liquid cooled!
The movement was only a couple of mm at most, but the force it could
exert was silly.

As for degausing things after the event...

Hear at the salt mine, we have a HP 3GHz network analyzer with CRT
display we used to cart arround the country when servicing RF power
amps.   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.
Never affected any credit cards though, even when I forgot I had one on
it's own in a coat pocket!

(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.)

We often had to put the whole analyzer through a large home made
delousing coil when back at the office to get the display straight
again.  OK, that was only a monochrome tube (green) but I personaly have
also recovered normal shadow mask colour CRT's in TV's and monitors in
the past, but it takes time patience, and a 9" coil that uses so much
power it gets too hot to hold if you run it for more than 10 seconds!

Remember too, to take the coil WELL away from the CRT before turning it
off!

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.  I guess he's long retired now,
if not pushing daisies.

Regards.

Dave B.




> ---------------Original Message--------------
> 
> Message: 6
> Date: Wed, 14 Apr 2010 12:12:46 -0400
> From: "Bob Camp" <lists at rtty.us>
> Subject: Re: [time-nuts] time-nuts Digest, Vol 69, Issue 31
> To: "'Discussion of precise time and frequency measurement'"
> 	<time-nuts at febo.com>
> Message-ID: <4D4EF6B7D229491EAB69EA5B551DADD0 at vectron.com>
> Content-Type: text/plain;	charset="us-ascii"
> 
> Hi
> 
> I've found that in some cases you need to run multiple passes over 
> multiple days to degauss a badly mag'd monitor. I'm sure that at some 
> point they do indeed become "irreversible". So far I haven't found one

> that a hula hoop sized coil won't eventually fix.
> 
> Bob
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Chuck Harris
> Sent: Wednesday, April 14, 2010 8:15 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] time-nuts Digest, Vol 69, Issue 31
> 
> Dave Baxter wrote:
> 
> > I think you'll find that any stray fields from industrial shaker 
> > systems, are often from the connecting leads, especialy if
> not correctly
> > positioned and layed out.  The internal static and dynamic magnetic 
> > fields are "Very" well contained.
> 
> Not necessarily.  I did some shaker work at one company, and their 
> shaker magnetized the CRT's on any computer that was within several 
> feet of the shaker table.  Sadly, the damage was irreparable.  The 
> screens were left with permanent rainbow splotches here and there even

> after running a degaussing coil over the entire monitor.
> 
> -Chuck Harris
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to 
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> 
> 
> 
> 
> 
> 
> ------------------------------
> 
> Message: 7
> Date: Wed, 14 Apr 2010 12:33:17 -0400
> From: Chuck Harris <cfharris at erols.com>
> Subject: Re: [time-nuts] time-nuts Digest, Vol 69, Issue 31
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Message-ID: <4BC5EE4D.1030806 at erols.com>
> Content-Type: text/plain; charset=UTF-8; format=flowed
> 
> Bob Camp wrote:
> > Hi
> > 
> > I've found that in some cases you need to run multiple
> passes over multiple
> > days to degauss a badly mag'd monitor. I'm sure that at
> some point they do
> > indeed become "irreversible". So far I haven't found one
> that a hula hoop
> > sized coil won't eventually fix. 
> > 
> > Bob
> Sadly, I have.  I used a large degaussing coil that could be passed 
> over the entire monitor, a smaller bulk tape demagnetizer, and the 
> internal degaussing coil.  I could never get rid of the rainbow fringe

> around the outside of the CRT.
> 
> -Chuck
> 
> 
> 
> ------------------------------
> 
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> End of time-nuts Digest, Vol 69, Issue 34
> *****************************************
> 

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