Re: Monitor Discharging

On Thu, 9 Dec 1999, ddhumphr wrote:
> John Robertson wrote:
> > ... After the HV lead is removed, I like to leave a jumper lead
> > connecting the anode cup to the chassis for a while to clean up any
> > residual charge.
>
> One note I would make, however, is that a jumper to the anode cup
> doesn't really make a lot of sense to me. I *believe* that the charge
> is kept in the tube itself, not in the HV flyback circuitry, so the
> jumper would have to be somehow connected to the tube, and not the
> anode cup...

I'm curious about this too - the tube is the capacitor, and you discharge
it by shorting it out across its "leads", which are presumably on opposite
sides of the glass.

Now that I think about it more carefully, the bit about the screwdriver
under the anode cap *really* means "make sure the metal bit from the HV
supply is still touching the inside of the tube when you touch the metal
bit with the screwdriver".

If you grab the anode cap and break contact with the tube before you
get in with the screwdriver, you're just tapping a piece of metal to a
transformer winding (i.e. another piece of metal), while the tube remains
charged.

Though I suppose if there's residual charge in the caps in the low-voltage
power supply, there *might* be enough to "run" the HV supply for a few
milliseconds if the HV supply is connected to something grounded. (I'm
guessing here, someone with clue please help me out :)

My 2-bits worth on "the chassis is where you discharge to":

If you look at other monitors, you'll often see the DAG (conductive
graphite coating on the back of the tube) connected directly to the
monitor chassis by a springy piece of metal.

(Does anyone know what DAG actually stands for? Lots of words that begin
with "A" and "G" come to mind when I'm in a guessing mood. Guess this is
monitor trivia day!)

> PS - The first monitor I discharged was a 19V2000 for my Battlezone
> and I was sweatin' bullets! The thing had been off for 2 weeks so
> there was no spark. I felt like I hadn't gotten my reward for my fear.

Yeah, I'm still laughing at my first tube discharge. Screwdriver in hand,
monitor on toilet seat, big grounding strap going from screwdriver to
faucet. And no spark, so I discharged it two or three times because I
expected to see one before giving up and "hoping for the best". I've
gotten considerably less careful since then. :)

My preferred method is to ground the discharging tool to monitor chassis
and have the monitor unplugged.

Three general rules I use:

1) If working on anything with a shock hazard, and this includes a
   discharge, use one hand only. If current finds its way to ground
   through you, better that it go through your arm and leg than from
   one hand to the other through your heart.

   The easiest way to remember this is to always have one hand in your
   back pocket when fiddling around.

   A side effect of rule #1, by the way, is that if you end up with
   a shock that causes your hand to clench up onto whatever is causing
   the problem, you *may* have enough presence of mind to pull your
   hand out of your back pocket and slap your clenched hand away.

2) Be aware of where your hand is in relation to sharp or fragile
   items. Don't put it where, if you jerk it in a random direction,
   it's likely to break something or cut itself on metal sheeting.

3) Don't carry monitors with the glass resting against your stomach.
   
   The story about the guy who eviscerated himself this way is
   probably an urban legend, but hey, what harm can it do to have
   a layer of monitor chassis between yourself and the tube?

And while we're at it, two "background knowledge" rules. While most
folks won't ever break them, us hardware tinkerers are an inquisitive
bunch, and curiosity combined with not-knowing-how-monitors-work is a
recipe for disaster. Two Really Dumb Things That You Should Never Do:

4) The black stuff on the back of a monitor is there for a reason.
   Cleaning dust off a monitor is good, but your goal is *not* to
   be able to eat your lunch off the back of the tube.

   I'd imagine that the resulting failure mode is "interesting".
   (Read: sparks, flames, other pyrotechnics as 20000V has nowhere
   to go and starts inducing other failures around it...)

5) The metal band around the front of the tube is there for an even
   more important reason. It holds the tube together and provides
   limited protection against implosion.

   I've never heard of the failure mode from loosening this band,
   probably because nobody dumb enough to do it has lived to post
   about it. I'd imagine it involves large chunks of glass flying
   everywhere, and would be roughly equivalent to sitting on top of
   a hand grenade. Do not taunt happy fun band.

Later,
Doug.

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