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Hazards inside of Monitors
Date: Wed, 24 Jan 90 21:17 EST
From: "David A. Moon" <Moon@stony-brook.scrc.symbolics.com>
Date: Wed, 24 Jan 90 15:26 PST
From: TYSON@Warbucks.AI.SRI.COM (Mabry Tyson)
Remember you are near high voltages and can get fried if you slip.
Thanks for pointing that out. And the rest of you, pay attention.
Right off the bat I can think of three different ways to suffer serious
personal injury or death while trying to adjust a display from the
inside (any brand, not just Symbolics). Only one of them involves
Don't be so mysterious about this, please. I'm not sure what you
picked for the third one, but one point which should be warned about
is the danger posed by any CRT from flying glass if it is mechanically
damaged. The amount of force on a Symbolics monitor's CRT is about
3 tons...from atmospheric pressure against the outside of the tube.
On the inside is vacuum. If something should happen to that glass
envelope, it can implode quite violently.
When I used to repair televisions, I used to dispose of dead
CRT's by taking them to the dump and shooting them from a
good distance with a rifle. I used a .22 rifle which won't
do much more than collapse a window, but the glass fragments
from a CRT fly in all directions a good long way from their
own stored energy. It is not something you want to happen
in your face.
I'm going to assume that some of you will consider yourselves
qualified to work with this stuff, and unlike lawyers, I feel
it's your decision. But here's some safety precautions you
should remember. (If you didn't already know these, I think
you shouldn't consider yourself qualified. I DO NOT guarentee
this list of precautions is complete. It's intended as a
REMINDER for those who know, and to discourage those who don't
1) Wear safety goggles when working with CRT's. Eyes are
2) Remember that in the event of an accidental shock, you
may move uncontrollably, and this may cause additional
accidents, either damaging the CRT or exposing you to other
3) CRT's have the curious property of "recharging" themselves
even after you've discharged them. This can happen many
times! Charge buildups inside the tube can take time to
migrate from place to place. Consider that touching any part
of the high-voltage circuitry, or any part of the black
surface of the CRT, may be a source of shock, at any time,
even after having been turned off for a substantial time.
3a) The high voltages used on CRTs may appear in places you
don't expect, or may jump significant distances. Stay
*AWAY* from the CRT.
4) The most vulnerable part of a CRT is the neck. Do not
apply any force to it. I would recommend leaving any
adjustments of the deflection yoke (the coils around the
neck) to Symbolics Field Service. Screwups here can be a
pain to fix.
4a) I would recommend to Symbolics Field Service personnel that
they wear goggles and gloves and long-sleeve shirts before
working with the CRT itself. This can both protect the skin
in the case of implosion, and can help prevent or reduce
shock, which may in turn lead to implosion.
5) Avoid jewelry, loose long hair, etc. when working around
6) *DO NOT* leave the top off consoles. This is important
to prevent accidental shock, accidental damage (resulting
in implosion), and to contain any flying glass from implosion.
7) I almost forgot to mention the clasic rule: Work with one
hand in your pocket to avoid accidentally bridging your heart
between voltage and ground.
(Note: CRTs are specially constructed to avoid sending glass
out the front when the break. I've heard of very old CRT's
tossing the neck through the front, but I've never seen this
happen in the ones I've imploded).
This hazards are quite real. Blindness or death are real
consequences of mistakes when working in these areas.
Adjustments are particularly hazardous because you must work
on the equipment while it's live. I don't know the voltage
used in Symbolics monitors, but I have seen color televisions
where it approches 30,000 Volts. This is a lot different
from working with house current, and THAT'S quite capable of
killing you. Even if it doesn't kill you, it hurts like the
blazes, and your muscles *JERK*. You tend to end up on the
floor. It is *NOT* fun, and you do *NOT* want it to happen
to your heart.
I have been poking my nose into TV's and other CRT's for over
20 years, and I STILL get real uneasy when it comes time to
make adjustments around live CRT's. I still remember my
mistakes from 20 years ago.
[The third way of suffering personal injury that comes to *MY*
mind is to injure your back while bending over it or lifing it.
You can guess why I think of this one, but I'm not sure it's
the one Moon had in mind...
You could burn yourself, too, but that involves electricity by