[Sderby] Monitor

[David Bottrill]

On Tue, 2003-09-30 at 20:54, James Gibbon wrote:
> Simon Hales wrote:
> > Hi
> > 
> > Just a quick safety warning I haven't seen anyone mention yet.
> > Turning off a monitor does NOT make it safe to open!  The
> > capacitors in a monitor are so big, and charged to such a high
> > voltage that they are capable of storing a lethal charge for some
> > hours after being switched off and unplugged if you're unlucky.
> > 
> 
> That's true - when I worked at RR all our Sun monitors had safety
> warnings tacked onto them, to the effect that touching the plug
> terminals could kill you, hours after pulling them out of the mains
> socket.  If I remember correctly, eventually we had them all 
> hard-wired into the wall or fitted with screw-down plugs.
> 
> Most unfortunate

Ok guys, here goes

Firstly it is possible to get a shock by touching the pins of a mains
plug shortly after it has been unplugged. This is a rare occurance on
modern equipmemnt and shouln't be possible if you switch it off on the
from first. Or in the case of an ATX power supply switch the power
switch on the Power Supply off if there is one,

Shocks from the power plug are mere flesh wounds compared to what you
can get from the innards.

Lets assume you have disconnected the the monitor from the mains and
from your PC you woun't want to kill you video card if you slipped with
you screwdriver would you.

Leave it for about 30 minutes to make sure the capacitors in the power
supply have discharged

Firstly a warning, the tube contains a vacuum and whilst the front of
the tube is very thick and strong the thin part at the back is made from
very thin glass that is not so strong at all, hit it too hard and the
tube will implode. Ok so you may think all the bits of glass will go
inwards the trouble is they will come out again the other side, so you
really don't want to have all those sharp bits of glass wizzing past
your ears do you so BE CAREFUL.

OK, now look at the tube (the big glass bit) usually at the top edge,
near the front you will see a rubber or plastic sucker  about 2"
diameter, dissapearing into this will be a thickish wire (usually
black). This wire carries 30,000 volts DC to the tube, as the monitor is
turned off it's not the wire thats so much of a problem it's the tube
that it's connected to.

Those of you who did 'O' Level (GCSE) physics will probably remember the
leyden Jar this is a form of capacitor that will hold a very high
voltage charge for long periods, well a Cathode Ray Tube will also act
as a leyden jar or capacitor storing up to 30,000 volts with enough
energy to kill you several times over. 

So before you can safely work on the monitor it is necessary to
discharge the tube. To to this you need 2 long, well insulated
screwdrivers, one of which should have a flat blade. 

Firstly locate the metal band that runs around the front edge of the
tube, this will probably have brackets to fasten the tube to the front
of the case.

What you need to do is carefully slide the flat blade of the first
screwdriver under the sucker, inside the bump at the centre of the
sucker is a live metal contact you need to make the screwdriver touch
this, but please please don't touch the shaft of the screwdriver, make
sure you are holding the insulated handle. Now with the second
screwdriver short the shaft of the first srewdriver to the metal band
around the tube, there will be a crack and a flash as a spark about and
inch long will fly between the screwdriver shaft and the metal band.
Make sure the screwdrivers touch each other and the metal band for
several seconds to ensure the tube is properly discharged. 

If for any reason you wish to remove the high voltage cable you need to
squeeze the bump in the middle of the sucker this will allow the clip
connection into the tube to be pulled out, you may also need to lift the
edge of the sucker to release any vacuum. 

Once this cable has been removed it may be a good idea to discharge the
tube again, this time just put the first screwdriver gently into the
indentation in the top of the tube and use the second screwdriver to
discharge it again to the metal band.

If you monitor has been arcing in the past making cracking of fizzing
noises it would be a good idea to clean around the tube, the inside of
the sucker and the high voltage cable with a DRY cloth, do not use any
liquid.

The high voltage cable will dissapear off into the guts of the monitor
to a device called the trippler, this will be plastic block again clean
around this with a DRY cloth. Look for black charred lines indicating
where arcing has been occuring, this will need careful cleaning.

Due to the high voltages, particles of dust are attracted to these  
components.

Replacing the high voltage connector is simply a matter of squeezing the
the centre of the sucker and pushing it gently into the hole in the top
of the tube, then releasing. Just give the sucker a slight tug to make
sure it has clipped into the tube correctly.

Hope that helps, but please be carefull if you are unsure then leave it
alone and get a competent person to look at it for you. 

Dave

-- 
David Bottrill <david at bottrill.org>