Re: New GS owner questions

["Michael J. Mahon" <mjmahon@aol.com>]

Michael Black wrote:
> On Fri, 16 Mar 2012, Michael J. Mahon wrote:
>
> > D Finnigan <dog_cow@macgui.com> wrote:
> >
> > > Wholly Mindless wrote:
> > >
> > > > The good news is that the THUNK is what they all do - apparently 
> > > > this the
> > > > automatic degaussing feature that many crts had.
> > >
> > > I had assumed that it was just the sound of high tension entering the 
> > > CRT.
> > >
> > > I may be mistaken, but I don't believe that the AppleColor RGB has a
> > > degausser.
> > >
> > > I once was party to quite the shenanigan involving my GS monitor: I 
> > > had been
> > > fooling about with a rare-earth metal magnet (liberated from a 1992-era
> > > Quantum hard disk drive), pulling the electron gun until rather a nasty
> > > blotch took up residence on the screen. Well of course it had no 
> > > intentions
> > > of leaving!
> > >
> > > The solution I devised was to lift the GS monitor and hold its screen 
> > > flat
> > > against the screen of my Mac mini's 17" CRT. I powered on the larger
> > > monitor, whose automatic degausser was only too happy to assist with 
> > > the GS
> > > monitor as well.
> > >
> > > An amusing solution, I'm sure.
> >
> >
> > I think almost all color monitors (and TVs) came with degaussers--a
> > practical necessity on a planet with a magnetic field. ;-)  But the were
> > never intended to deal with strongly magnetized shadow masks, which is 
> > what
> > you get if you get close with a rare earth magnet.
> That's my impression too.  Very early color tv sets didn't, you 
> certainly would see lots of reference to external degaussing coils back 
> then.
>
> But then they disappeared, and it does seem like after a certain point 
> (and I think predating the rise of home computer monitors) they did all 
> have degaussing coils built in. Of course, perhaps they burn out. They 
> have something to make sure they come on when the set is just turned on, 
> and then off after enough time to degauss, and I think the whatever 
> means that if you turn off the monitor/tv and then turn it immediately 
> back on, there's no degaussing.  Given that, the whatever could break 
> down at some point, no automatic degaussing.

Built-in degaussers are simply connected to the AC supply through a
positive temperature coefficient (PTC) resistor.  When the device is
cold, turning it on permits a surge of AC to pass through the coil,
which, over the course of a second or so, decreases to a very small
current, as the PTC resistor heats up.  It usually continues to be
powered while the monitor is on, with a very small current flowing
while the resistor remains hot.

That is why the degausser only "thunks" when the monitor/TV is turned
on from a relatively cold state.

Since the most important time for it to work is after the monitor/TV
has been moved from one place to another (with consequent change in
orientation), it usually is cold when the degausser is needed.

To demagnitize a material, it must be subjected to an alternating
magnetizing force greater than its coercivity, which is then smoothly
decreased in magnitude to near zero.  This has the effect of "walking"
the material down its hysteresis loop until the remanent magnetization
is almost zero, that is, until it is essentially demagnitized.

That's why the manual degaussing coils needed to be brought close to
the thing to be demagnitized, then moved away from it within a few
seconds (to decrease the AC field) until the field at the object was
near zero, then switched off.

The same coil can be used to magnetize by bringing it close to the
object and then switching it off.  (If the current is interrupted near
a peak, then the object is left magnetized most strongly, so it can
take a few tries to switch off near a peak.)

-michael

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