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From: stever@videovax.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Popping monitor
Message-ID: <4067@videovax.Tek.COM>
Date: Wed, 26-Nov-86 16:32:22 EST
Article-I.D.: videovax.4067
Posted: Wed Nov 26 16:32:22 1986
Date-Received: Fri, 28-Nov-86 01:45:01 EST
References: <685@husc6.UUCP> <4344@amdahl.UUCP>
Reply-To: stever@videovax.Tek.COM (Steven E. Rice, P.E.)
Distribution: net
Organization: Tektronix Television Systems, Beaverton, Oregon
Lines: 61
Summary: BE CAREFUL!!!

In article <4344@amdahl.UUCP>, Kim DeVaughn (kim@amdahl.UUCP) writes:

[ Description of how to find and fix a high-voltage arc. ] 

> P.S.  If you try this make VERY SURE that the H-V terminal at the CRT gets
>       fully discharged!  That circuit can hold onea helleva big charge
>       (20-30 kV) for a LONG time (hours).

Kim's description of how to find and fix the arcing is a very good one.
Let me add a word of caution, however!

If you discharge the HV terminal of the CRT,

  !!! *DO*  *NOT*  *EXPECT*  *IT*  *TO*  *REMAIN*  *DISCHARGED* !!!

There are two phenomena that combine to cause a voltage to reappear on
the HV terminal minutes, hours, or even days after it is discharged.
The first is dielectric absorption -- the molecules that make up the
dielectric (in this case, the glass of the tube) of what is basically
a big capacitor are stressed by the continual application of high
voltage.  When the voltage is removed by discharging the tube, the 
molecules slowly relax.  As they relax, they rearrange their internal
charges, causing a voltage to reappear at the HV terminal.

The second phenomenon stems from characteristics of the material used
on the inside of the CRT to connect the faceplate to the high-voltage
terminal.  A slurry called "Aquadag" is sprayed on the inside of the
funnel as the tube is being built.  The Aquadag is basically finely
divided carbon in a binder.  It has a very high resistance (and there
can be some sections of it that are almost -- but not quite -- isolated
from the rest).  Thus, when you short the HV terminal to ground, you
are discharging the capacitance of the tube through the resistance of
the Aquadag.  This may leave quite a bit of charge on parts of the
tube.  When the ground wire is removed, the remaining charge will
gradually redistribute itself around the tube, causing a voltage to
appear at the HV terminal over a period of hours or days.

The moral:

   1. Always treat the HV terminal with respect.  Don't get your
      fingers near the terminal or anything connected to it (for
      example, the high-voltage rectifier) if the terminal is not
      grounded  *AT*  *THAT*  *MOMENT*.

   2. Discharge the HV terminal  *EVERY*  time you start to work around
      the CRT, even if the set has been off for days.  Even better,
      ground the terminal when you start to work on it, and keep it
      grounded until you are done.  Don't trust a ground wire, though!
      Before you get close to any part of the high voltage system,
      make sure the grounding wire is making good contact at both ends.

   3. !!! *DON'T*  *WORK*  *ALONE* !!!  Have someone in the same room
      who can call for help if you should have an accident.

If you follow these precautions, your chances of being zapped will be very
small, and you can safely work on your monitor.

					Steve Rice

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