Slowly blowing a CRT?

From: Geoffrey G. Rochat <geoff_at_pkworks.com>
Date: Thu Sep 28 19:50:58 2000

>>
>> Good thing you didn't take the cap off the side of the tube, as
you're
>> looking at something like 18KV there. DON'T EVER GO INSIDE A RUNNING
OR
>> RECENTLY POWERED OFF MONITOR OR TV WITH A CRT UNLESS YOU KNOW WHAT
>> YOU'RE DOING. You could easily be electrocuted, and that would be,
as
>> they say, "bad."
>
>IMHO the EHT (the high voltage to the final anode in the CRT) is about
>the least likely supply in a monitor to give you a dangerous shock.
There
>are 2 good reasons for this :

The mains supply is obviously the most powerful voltage source in a
VT320, but it is entirely wrong to dismiss the anode voltage, for two
reasons.

1. My intention was to serve a warning, not to debate the merits of
which and what. Somebody who admits to being in over their head, and
who managed to blow the vacuum in a CRT and who doesn't recognize the
symptoms, may well not know about the possible effect of anode voltage
on the human organism - specifically, their own organism. If you've
ever gotten across a CRT's anode voltage - as I have - it will knock you
right on your ass. 'Hurts like hell, too. Yes, I know it's a
high-impedance source whose voltage drops as the current rises, but if
it does 100uA at 18KV, how much does it do at 100V? Making several
assumptions, 18mA at 100V is quite enough to kill you if you just happen
to do it right. And are you any less dead than if you'd plugged
yourself into mains current? If you get run over by an automobile are
you any less dead than if you got run over by a bus? *ANY* voltage
source is worthy of respect. Anything over about 40V or so can be fatal
in the right circumstances and I've gotten nasty tickles off 48V phone
lines on hot days when I was sweaty and foolishly not being attentive to
what I was doing. I've seen people weld themselves to 5V busses with
their wedding rings, and suffer serious burns in consequence.

2. Unlike the mains supply, which "goes away" when you unplug the
VT320, the CRT anode voltage takes a long time to discharge. I've had
TVs take a day to do so. This is often surprising to novices, who
sometimes learn the fact the hard way.

>
>2) The EHT is pretty hard to get at in modern monitors. It's produced
>inside the potted flyback transformer/rectifier assembly and sent to
the
>CRT connector. Everything is well insulated. You're not going to touch
it
>by accident.

Agreed, but here was a poster talking about removing the anode connector
from the CRT, presumably soon after powering the unit down. A classic
invitation to "surprise" as ever I've heard one. Also, insulation on
flybacks and the anode voltage lead is in an electrically stressful and
hot environment, and sometimes gets friable with age. (I've got an old
flyback in a junk box in my basement that amply demonstrates the
phenomenon.) Even without attempting to remove the anode connector
touching the wrong "insulated" thing in an old terminal without first
discharging things can be very unpleasant.

If you feel that getting across the anode voltage in a VT320 terminal is
safe, that dying is unlikely, and that you're adamant enough about it to
write to a newsgroup with your views, I ask you please not demonstrate
your claims to attempt to prove your point.

My whole purpose in making the capitalized comments in my original post
was to save lives, not take them. Unless you've got a damned good
reason to do otherwise, *ALWAYS* power off what you're working on before
poking around inside, and make *CERTAIN* all the high-voltage nodes are
discharged. And if for some damn-fool reason you can't do that,
whenever you're poking around in circuitry with even a chance of getting
a shock, take precautions. (And if you're uncertain, presume there is a
shock hazard, and it is high.) In particular, keep one hand in your
pocket and wear well insulated shoes on a dry floor. That way, even if
you do touch something nasty the current path is less likely to be
through your heart. This is *NOT* a sufficient rule, it is merely the
first of many, but it is very easy to do and has saved many lives.
Everybody, take care out there, OK?
Received on Thu Sep 28 2000 - 19:50:58 BST