Discharging CRT anode and capacitors on a VT52 DECscope

From: jpero_at_sympatico.ca <(jpero_at_sympatico.ca)>
Date: Wed May 26 18:27:51 2004

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

Good question! This is safer to tell and ask.

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

This is cardial rule here at work, I always make sure I can see the
plug is visible and is clearly goes to that that unit I'm working on.

Also there is few capacitors still charged by the way.

My discharge "tool" is basically two clip cables and sand block
resistor of big wattage like 10W to 20W, 180 to 220 ohms is PERFECT
for both charged caps and HV discharges.

I usually discharge any caps that are marked 35V or higher by
touching and holding for few seconds with pair of that clips via that
sand block resistor, there is few caps that is so BIG it has heavy
pins, I leave them clipped for few minutes, they leak away so long.

Reason I use that resistor for HV discharge is to cut down on current
(damage can happen because the internal dag coatings can be burnt off
around the anode button or blow a component.

The dag ground is dead easy to find. It is the bare braid cable
sketched across the CRT's bell touching the dark grey dull coating
(dag). I always clip to that ground strap across the back side of
CRT bell via same resistor and clip that to a small worn flat head
screwdriver with plastic handle. Carefully wiggle this under that
HV suction cup and you're done the instant screw driver tip touches
that HV prongs. You will hear few crackles or quiet snap using
resistor. Without it, you get a loud scary SNAP!

Keep the discharge tool away the board or wires. HV can jump
anywhere where there's easier path to take. Highest HV typically
seen is about 30KV in projectors, big CRT TVs (32, 36 and even 40").,
somewhat less in monitors.

> 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.

Finding the dag ground is MOST easiest and safe than sorry.

I don't hook that ground end to the chassis ever. this is not
always direct connect to the dag ground. hard to tell which is hot
or cold even it is not that direct. Also another reason to be safer
because components are protected only using dag ground to leak that
HV.

I always do this to any CRTs every time if I must get HV suction cup
off. After discharge grab hold of the edge and fold back, gently
push one tine of the anode clip to get it over the lip of the anode
button.

Clean both surfaces surrouding anode button (windex and alochol
because the red "paint" comes off with actone and beyond for few
inches and the suction cup (actone) then apply thin line in a circle
around the button where suction cup will touch with dielectric
silcone grease before clipping that HV lead back on, auto shops
stocks this slimy grease in a tube of any size.

I've seen HV sneak past the layer between suction cup and the CRT
bell surface when a guy touched the edge of suction cup with
screwdriver clipped to the dag ground. Blue arc with a some "crack!"
He looked surprised and that one didn't have grease, was original
dry install (typical in many CRT stuff and I don't like this way
because of this HV leaks that could occur.

Also HV burns through materials when it breaks through. Often looks
like a faint pinhole (found by flexing and pullng) in flexible
insulation. Even a tiny pinhole (like a period of a sharp pencil) in
the flyback, the worst ones usually looks like someone blew out a
side out and charred.

Once saw a wisp of "hair" those actual super fine magnet wires for
the HV winding got blown out of the flyback's casing giving it a bit
of a tuft sticking out of axial crack running around the flyback's
case.

HV can be destructive if left loose to run amok in the chassis. :-)
Snap crackle pop zzzzzzzzzzt or touch you. OW! or fling you off
(secondary injury).

For 2 and half years, I have YET to be bitten by HV hit but someday I
may! But I have been buzzt'ed or shocked several times by the
charged caps or high voltage stuff like 100Vto 300V (to me any
shock is like a buzzt than a "zap") Had got on high freq
1.2KV ac (15.7KHz) kinda gives you a burning (stinky) fine trickle
but still a fair "ow" rather like a brush of soldering burn.

I was told HV hit is like mule kick. But I wonder what others who
did get that hit felt like? Oh, I had been "shocked" by a
experiment low level HV generator (felt like powerful static zap)
but not that powerful at electronic class years ago.

At science museum once saw a very tickish student girl walk off lexan
platform by the big sphere HV generator depsite tech's warnings as
she stepped off a looong 2 feet long arc hit her but she only did
make short yelp without jerking or jump. Wow but I wondered why she
didn't "bounce" or jerk or fling a arm or leg when HV hit her when
her muscles spasmed. That 2 feet long HV arc must be really packing
of power. :-O

Cheers,

Wizard
Received on Wed May 26 2004 - 18:27:51 BST