Discharging CRT anode and capacitors on a VT52 DECscope

From: John Lawson <jpl15_at_panix.com>
Date: Wed May 26 22:54:44 2004

On Wed, 26 May 2004, Ashley Carder wrote:

> John, thanks for the detailed info.


  No prob - been working on the damn things (TVs etc) for more than 40
years now.... and I've been hit a few times with Ultor HV - like most
high voltage/low current supplies - there is usually not enough power
there to do much real damage, what usually happens is you get zapped, and
then damage the device / hurt yourself in the ensuing reaction - it's
quite startling and painful so one tends to 'jump' - once I put my butt
right through the neighbor's drywall in their living room when I was in
Jr. High - took all my summer money (including my labor charges to them)
to have it repaired and painted... ;{} maybe *that's* why I hate TVs so
much!

 'Wizard' has responded also to this thread with much more detailed info -
I don't think you'll have to worry about discharging the power supply caps
in a VT52 - there's no 'medium' voltages until you get to the HV supply.

  When he refers to 'dag' he means the outside coating (a dull black matte
finish) called "aquadag" that is the conductive outer layer of the CRT -
the front periphery of the CRT ought to be clamped to a frame to hold it
in place - you can use this frame/clamp assembly to ground your
discharging screwdriver. Somtimes there will be a thick uninsulated metal
braid going from this peripheral clamp to the chassis somewhere - this can
be used as a conveient place to attach your discharging wire.

  The ground prong of the power cord (the longest, round pin of the three)
is always the green, or green/yellow wire in the power cordset, and will
always be attched to the main ground point in the device.

  You may or may not hear a snap, or see a small blue flash when you get
the tip of the screwdriver close enough to the anode connection - it
depends on how well the CRT in the terminal is retaining it's charge.

  So combine both our narratives and you should be just fine.

  Only please do remember: Power cord UNPLUGGED and visible to you before
starting the work.



 Cheers

John
Received on Wed May 26 2004 - 22:54:44 BST