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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