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From: barry@network.ucsd.edu (Barry Brown)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Discharging CRT - Attach lead where?
Keywords: CRT Discharge
Message-ID: <2773@network.ucsd.edu>
Date: 28 Aug 90 19:48:30 GMT
References: <28710@nigel.ee.udel.edu>
Distribution: usa
Organization: University of California, San Diego
Lines: 54

In article <28710@nigel.ee.udel.edu> cayz@udel.edu (James Cayz) writes:
>	In the Safety Techinques Chapter, Mr. Pina shows a picture of a CRT
>Discharge Tool (1/8" screwdriver, wire, gater clip, & 10Mohm resistor), with
>the tools attached to the Logic Board Rails (p. 11)...   Now, talking to a "Real
>Apple Technician", they said "No, attach it to the strap surrounding the tube.".
>Which (if not both) is more correct / safe?

You're right; Mr. Pina doesn't tell the reader how to use the discharging tool.
This is what I did (slight deviation from his diagrams):

Since I expect to me opening my Mac several times, I permanently attached the
10M resistor to the ground chassis.  If you look above the port connectors,
you'll see a strip of heavy metal with large round holes in it.  This is the
ground chassis.  Screwed into it will be a wire.  I loosened the screw,
attached one lead of the 10M resistor to it, and tighened it again.  Now, to
discharge my CRT, I merely clip one end of a test lead (the kind with aligator
clips at both ends) to the free lead of the 10M resistor and the other end
to the shaft of a flathead screwdriver.  Holding onto only the plastic handle
of the screwdriver, I worm the tip under the rubber "suction cup" until it
contacts the metal underneath.  Then I hold it there for several seconds until
I'm satisfied that all the charge is dissipated.  Then I unclip the test lead,
leaving the resistor attached for the next time I need to discharge the CRT.

>	Also, on page 131, "Analog Board - Removal and Installation", step 2 is
>a little bothersome for me.  It reads:
>"2. Physically disconnected the power cord from the wall outlet.  If you're the
>least bit hesitant about discharging the CRT, leave the set disconnected
>overnight.  By the next morning, most, if not all, of the high-voltage charge
>will have dissipated."
>
>	Huh?  Dissipated where?  I had even heard that since it is just a
>static charge, a CRT charged would not lose it, but even when discharged, would
>become (slightly) charged by sitting out unconnected in a low-humidity area,
>requiring another discharge before re-connection.

It's not a static charge.  It's an honest-to-God 15,000 volt charge, like that
contained in a very big capacitor.  It's not something you want to touch, nor
is it something you just want to short out.  That's why you need the 10M
resistor.  If you don't use the resistor and just short out the CRT, you'll get
a big spark and a couple of blown chips on the logic and analog boards.  As
Larry Pina puts it, that's COSTLY.

>	Last Question:  I'm slightly paranoid about being blown around the room
>by accidently discharging it across my hands.  Is there any _harm_ to
>discharging it, lets say, whenever I work on / inspect / clean the dust out of
>the Mac??

Nope.  In fact, it's a damn good idea.

-- 
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