Discharging CRT anode and capacitors on a VT52 DECscope

From: John Lawson <jpl15_at_panix.com>
Date: Wed May 26 21:35:31 2004

On Wed, 26 May 2004, Ashley Carder wrote:

> Can an "expert" here provide some proper directions
> on how to discharge the anode and capacitors? How
> long do these things on a charge?

Before starting: power OFF and power cord DISCONNECTED and in your sight -
be sure that the terminal is NOT energized.

   First, identify a metal part of the inside electronics that is
definitely at 'ground' potential, ie. connected somehow to the 'ground'
blade of the 3-prong wall plug. This could be the metal chassis support,
other metal support structures, or the metal case of the power supply if
it's a seperate unit. The ide is to have your wire hooked directly to the
'ground' ('earth') side of the terminal. If it's inadvertantly connected
to something that is not ground, when you discharge the CRT residual
potential, you'll zap whatever is connected...

  Use a smallish, thin screwdriver, having a 5" to 7" blade with a beefy
insulated (repeat insulated, as in plastic and/or rubber) handle. Using
an "alligator clip lead" of sufficient length, or a piece of thin,
insulated wire - attach one end of the wire to the grounded part of the
chassis that you identified, and attach the other end to the blade of the
screwdriver up near the (insulated!) handle.

  Find the "anode" cap where it clips into the side of the CRT - this will
have a thickish insulated wire going down to the high-voltage supply of
the terminal. Sometimes this will have a rubber cap over it, sometimes it
will just be a metal, paperclip-looking thing snapped into a small well on
the wall of the CRT.

  Being very sure the grounding wire is attached to Ground, work the tip
of the screwdriver blade up under the rubber cap (or into the anode
connection well) until the blade tip contacts the metal part of the HV
lead. Count to "3" and remove the screwdriver, then, if you need to remove
the CRT for any reason, pull the wire off the side of the CRT - this may
take a bit of force, or 'working' the clip back and forth until it
releases from the CRT. You can proceed from there in relative safety.

 A well designed CRT circuit can retain a charge for a long time, days and
weekes sometimes, especially if the outside of the CRT is clean and the HV
supply is well isolated. Unconnected CRTs can actually acquire a hefty
charge from particle bombardment and atmospheric charges - especially if
you're in a dry climate. I've been popped by this phenomena myself.



> Ashley
Received on Wed May 26 2004 - 21:35:31 BST

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