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'[EE]: Re: Question for an Electrical engineer'
2000\10\30@133738 by Russell McMahon

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>Why does my monitor go "Boink....Crackle..crackle" when I push the
>little button with the green light next to it? I know a bit about crt's
>and high voltage, but what physically happens that causes the noise?


Bbob,

Mega-ish energy and strong magnetic fields.
This is (presumably) the "degaussing" button.
Mains is applied to a coil around the CRT rim which generates a very healthy
50 Hz (possibly 60 Hz if you don't live in God's own country) magnetic
field. A positive temperature coefficient resistor is in series with the
coil and as it heats up with the large current flow it increases in
resistance thereby decreasing the current and thereby also the field. The
field therefore starts at a very high value and decays away nicely to almost
nothing in a small time. The initial current is largish.

Why do this?, you may well ask.

If the screen is subject to stray magnetic fields it may acquire random
localised magnetisations which will deflect the electron beam in an equally
random manner giving an interesting picture. The strong field overrides
these small error magnetisations and force everything that is easily
magnetised to march in step with the changing field.As the field decays all
the camp followers follow less and less until the field gently falls below
the level at which they will continue to be moved. Ultimately they all
settle gracefully into place in a randomish pattern thereby removing the
stray magnetisation.

Modern TVs and monitors often do this automatically at turn on.
In the good-old-days you had to use a "degaussing wand" which was a portable
AC magnet which you waved at offending spots while slowly pulling it away to
reduce the field. TVs could not be moved without risking needing degaussing.
Woe betide the person who eg turned off a vacuum cleaner adjacent to the
screen :-).

A permanent magnet brought near a modern screen will demonstrate the effect.
This will not usually do any damage.
BUT
Murphy says YMMV.



Russell

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2000\10\30@135400 by hard Prosser

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Just something that others may not be aware of:-
Some TV's do not in fact do a degauss at turn-on. They only do it when
initially plugged into a wall socket. We had the ocurrance of gradually
diminishing colour purity despite many turn off / turn on cycles. (Front
Panel Switch - not just remote standby cycles either). Eventually I asked a
repair guy and he advised unplugging the set completely for 20 mins or so
(to allow the PTC to cool).
Problem solved after 2 cycles.
I guess the mains switches aren't rated for the degaussing current.

I would guess the crackle is the increase in EHT capacitively coupling to
dust etc in the back of the set and causing mini-arcs until it stabilises.
Never really thought too much about that one!


Richard P





>Why does my monitor go "Boink....Crackle..crackle" when I push the
>little button with the green light next to it? I know a bit about crt's
>and high voltage, but what physically happens that causes the noise?


Bbob,

Mega-ish energy and strong magnetic fields.
This is (presumably) the "degaussing" button.
Mains is applied to a coil around the CRT rim which generates a very
healthy
50 Hz (possibly 60 Hz if you don't live in God's own country) magnetic
field. A positive temperature coefficient resistor is in series with the
coil and as it heats up with the large current flow it increases in
resistance thereby decreasing the current and thereby also the field. The
field therefore starts at a very high value and decays away nicely to
almost
nothing in a small time. The initial current is largish.

Why do this?, you may well ask.

If the screen is subject to stray magnetic fields it may acquire random
localised magnetisations which will deflect the electron beam in an equally
random manner giving an interesting picture. The strong field overrides
these small error magnetisations and force everything that is easily
magnetised to march in step with the changing field.As the field decays all
the camp followers follow less and less until the field gently falls below
the level at which they will continue to be moved. Ultimately they all
settle gracefully into place in a randomish pattern thereby removing the
stray magnetisation.

Modern TVs and monitors often do this automatically at turn on.
In the good-old-days you had to use a "degaussing wand" which was a
portable
AC magnet which you waved at offending spots while slowly pulling it away
to
reduce the field. TVs could not be moved without risking needing
degaussing.
Woe betide the person who eg turned off a vacuum cleaner adjacent to the
screen :-).

A permanent magnet brought near a modern screen will demonstrate the
effect.
This will not usually do any damage.
BUT
Murphy says YMMV.



Russell

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2000\10\31@190317 by Jinx

face picon face
> A permanent magnet brought near a modern screen will demonstrate the
> effect. This will not usually do any damage.
> BUT
> Murphy says YMMV.
>
> Russell

I recently stripped down monitors of various make and age. I'm keeping
the very fine mesh masks for some reason that will become apparent to
me one day. They are magnetic and so I guess could be permanently
distorted by a strong magnetic field. My speakers are next to the monitor
and seem to do no harm there, but I wouldn't dream of putting them in
front where they could. My mother's TV has a pronounced and very long-
lived pink area in the top corner which I've tried to remove. And bless
the flatmate who once plonked a bare big speaker on top of my TV once.
It was unwatchable after that. Admittedly it was a 15" 200W speaker. More
than adequate a magnet to crush a flatmate with.

Getting back to the short exchange about parallel importing, one of the
comments made by Philips NZ was that the TVs they sold were "set up"
for Southern Hemisphere magnetic fields and that the cheaper discount
store Philips brand weren't, and as such could be liable to bad pictures
and/or expensive "tuning". Is that hooey ?

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2000\10\31@213629 by Graeme Zimmer

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face
Jinx,

> And bless
> the flatmate who once plonked a bare big speaker on top of my TV once.
> It was unwatchable after that.

A pass or two of a TV serviceman's degausing wand should have fixed that....

> one of the
> comments made by Philips NZ was that the TVs they sold were "set up"
> for Southern Hemisphere magnetic fields and that the cheaper discount
> store Philips brand weren't, and as such could be liable to bad pictures
> and/or expensive "tuning". Is that hooey ?

Nope. Many monitors imported to the Southern hemisphere have a decided
keystone tilt unless they are re-aligned.....

....................... Zim

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'[EE]: Re: Question for an Electrical engineer'
2000\11\01@004640 by Dale Botkin
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On Wed, 1 Nov 2000, Graeme Zimmer wrote:

> Jinx,
>
> > And bless
> > the flatmate who once plonked a bare big speaker on top of my TV once.
> > It was unwatchable after that.
>
> A pass or two of a TV serviceman's degausing wand should have fixed that....

I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a
handle and momentary switch, basically.  Start close to the tube, wave it
around while slowly backing off.  Works every time.

Dale
---
The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
               -- Isaac Asimov

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2000\11\01@005510 by Sean H. Breheny

face picon face
I know a guy who can (roughly) degauss monitors with a _permanent_ magnet.
We waves it around in circles of varying radius and varying distance from
the monitor. He also varies the aspect angle (so he may wave it in a circle
around  the side of the monitor). He is able to take the magnet, severely
magnetize the monitor, and then get it almost back to normal.

Sean

At 11:45 PM 10/31/00 -0600, you wrote:

{Quote hidden}

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2000\11\01@021124 by Nigel Goodwin

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In message <000e01c04397$73915180$65c8a7cb@joe>, Jinx
<.....joecolquittKILLspamspam.....CLEAR.NET.NZ> writes
>> A permanent magnet brought near a modern screen will demonstrate the
>> effect. This will not usually do any damage.
>> BUT
>> Murphy says YMMV.
>>
>> Russell
>
>I recently stripped down monitors of various make and age. I'm keeping
>the very fine mesh masks for some reason that will become apparent to
>me one day. They are magnetic and so I guess could be permanently
>distorted by a strong magnetic field. My speakers are next to the monitor
>and seem to do no harm there, but I wouldn't dream of putting them in
>front where they could. My mother's TV has a pronounced and very long-
>lived pink area in the top corner which I've tried to remove. And bless
>the flatmate who once plonked a bare big speaker on top of my TV once.
>It was unwatchable after that. Admittedly it was a 15" 200W speaker. More
>than adequate a magnet to crush a flatmate with.

I don't think you can permanently damage a CRT with a magnet, everytime
a set is switched on from cold degaussing coils around the CRT de-
magnetise it. However, if the shadow mask has been subjected to an
extremely high field the inbuilt coils might not have sufficient power
to correct it. In this case a TV engineer would be able to effect a cure
using an external degaussing coil, it's a common procedure. Speakers for
use with a TV usually have shielded magnets, which is why special
speakers are used for the centre channel in Pro-Logic systems.

>Getting back to the short exchange about parallel importing, one of the
>comments made by Philips NZ was that the TVs they sold were "set up"
>for Southern Hemisphere magnetic fields and that the cheaper discount
>store Philips brand weren't, and as such could be liable to bad pictures
>and/or expensive "tuning". Is that hooey ?

Yes, it's a load of crap!.
--

Nigel.

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       | Lower Pilsley   | Web Page : http://www.lpilsley.co.uk       |
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2000\11\01@021813 by Nigel Goodwin

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In message <@spam@4.3.2.7.2.20001101005209.00ac9b60KILLspamspampostoffice2.mail.cornell.e> du>, Sean H. Breheny <KILLspamshb7KILLspamspamCORNELL.EDU> writes
>I know a guy who can (roughly) degauss monitors with a _permanent_ magnet.
>We waves it around in circles of varying radius and varying distance from
>the monitor. He also varies the aspect angle (so he may wave it in a circle
>around  the side of the monitor). He is able to take the magnet, severely
>magnetize the monitor, and then get it almost back to normal.

'Almost' is the key word, to de-magnetise a CRT (or anything else
really) you need a slowly decreasing AC magnetic field. This is achieved
inside the set by feeding the degaussing coils from mains via a
posistor, which is a combination of negative and positive temperature
coefficient thermistors. In the handheld degaussing coils it's done my
slowly moving the coil (fed from AC mains) away from the tube.
--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : RemoveMEnigelgTakeThisOuTspamlpilsley.co.uk           |
       | Lower Pilsley   | Web Page : http://www.lpilsley.co.uk       |
       | Chesterfield    | Official site for Shin Ki and New Spirit   |
       | England         |                 Ju Jitsu                   |
       \--------------------------------------------------------------/

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2000\11\01@023300 by staff

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Dale Botkin wrote:
>
> On Wed, 1 Nov 2000, Graeme Zimmer wrote:
>
> > Jinx,
> >
> > > And bless
> > > the flatmate who once plonked a bare big speaker on top of my TV once.
> > > It was unwatchable after that.
> >
> > A pass or two of a TV serviceman's degausing wand should have fixed that....
>
> I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a
> handle and momentary switch, basically.  Start close to the tube, wave it
> around while slowly backing off.  Works every time.

Yeah, I have one too. I also have an expensive "professional" type
degauss wand, the cheap radio shack one is MUCH more powerful. Have
to be a bit careful you dont get too close to the picture tube or
you can wreck the shadow mask. Very nasty - like buy a new picture
tube!

They are superb for bulk erasing floppy disks too, I have even had
floppies that had bad sectors for months that could not be fixed,
actually come good perfectly after a severe magnetic erasing! :o)
-Roman

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2000\11\01@023712 by staff

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Sean H. Breheny wrote:
>
> I know a guy who can (roughly) degauss monitors with a _permanent_ magnet.
> We waves it around in circles of varying radius and varying distance from
> the monitor. He also varies the aspect angle (so he may wave it in a circle
> around  the side of the monitor). He is able to take the magnet, severely
> magnetize the monitor, and then get it almost back to normal.
>
> Sean


Yeah, that's a party trick used to amuse TV apprentices.
The "small magnet on a stick" things do it better, the ones
you use to get a screw out of the tight spot it fell into.
-Roman

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2000\11\01@044753 by Alan B. Pearce

face picon face
> I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a
> handle and momentary switch, basically.  Start close to the tube, wave it
> around while slowly backing off.  Works every time.

Another good trick is an old power transformer with only the E cores in it (No
not that toroid out of the stereo dummy!). Drive the mains winding from a low
voltage output of another transformer or it will get real hot real quick...

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2000\11\01@052351 by Andy Howard

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From: "Dale Botkin" <EraseMEdalespamBOTKIN.ORG>
To: <RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU>
Sent: Wednesday, November 01, 2000 5:45 AM
Subject: Re: [EE]: Re: Question for an Electrical engineer


> On Wed, 1 Nov 2000, Graeme Zimmer wrote:
>
> > Jinx,
> >
> > > And bless
> > > the flatmate who once plonked a bare big speaker on top of my TV once.
> > > It was unwatchable after that.
> >
> > A pass or two of a TV serviceman's degausing wand should have fixed
that....
>
> I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a
> handle and momentary switch, basically.  Start close to the tube, wave it
> around while slowly backing off.  Works every time.


This works very well, but don't get *too* close to the screen. A friend
trashed his TV by playing around "making pretty colours" on his screen with
a permanent magnet. No amount of degaussing restored the correct display so
presumably he managed to distort the shadowmask with his antics.

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2000\11\01@104721 by Lawrence Lile

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When I was a TV tech years ago we just used an electric soldering gun (the
type that use a big trtansformer and electric resistance tips)  for
degaussing.
-- Lawrence

{Original Message removed}

2000\11\01@114833 by Dale Botkin

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face
On Wed, 1 Nov 2000, Sean H. Breheny wrote:

> I know a guy who can (roughly) degauss monitors with a _permanent_ magnet.
> We waves it around in circles of varying radius and varying distance from
> the monitor. He also varies the aspect angle (so he may wave it in a circle
> around  the side of the monitor). He is able to take the magnet, severely
> magnetize the monitor, and then get it almost back to normal.

I've done that too, with good results, but the degausser is a lot faster
and easier.

{Quote hidden}

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The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
               -- Isaac Asimov

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2000\11\01@162533 by Russell McMahon

picon face
And here's the 'your mileage may vary" confirmation :-) !!!!!!!!



-----Original Message-----
From: Andy Howard <musicaSTOPspamspamspam_OUTUKONLINE.CO.UK>
To: spamBeGonePICLISTSTOPspamspamEraseMEMITVMA.MIT.EDU <KILLspamPICLISTspamBeGonespamMITVMA.MIT.EDU>
Date: Wednesday, 1 November 2000 23:24
Subject: Re: [EE]: Re: Question for an Electrical engineer


{Quote hidden}

once.
{Quote hidden}

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2000\11\03@063417 by Peter L. Peres

picon face
>I know a guy who can (roughly) degauss monitors with a _permanent_
>magnet. We waves it around in circles of varying radius and varying
>distance from the monitor. He also varies the aspect angle (so he may
>wave it in a circle around the side of the monitor). He is able to take
>the magnet, severely magnetize the monitor, and then get it almost back
>to normal.
>
>Sean

A trick: place a silicon diode in series with the power lead of a monitor
or TV with switching power supply. The device will magnetize its screen in
an unthinkable way at the first turn-on. This was discovered when someone
decided that SMPSU powered monitors and computers could be run safely on
240V DC ;-) He forgot about the degaussing problem (which needs ac). I
have seen such a monitor and I can assure you that it is VERY HARD to
remove the magnetization and restore the screen to normal.

Peter

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