Discharging a VT100 CRT

[Julian Wolfe]

You were dead on about this Pat.  Absolutely NOTHING happened.  I sat there
with the screwdriver for about 3 or 5 minutes and nothing happened, then I
took the anode cap off.  However, I had my vet tech friend there in case I
needed CPR or any other sort of medical assistance (she's trained in basic
human medicals too)

And Pat, you're also dead on about the controls, I had to readjust those
twice before when the flyback was going in a hurry, and had to do it live,
but I was a lot less scared of that - I knew as long as I stayed way from
the CRT I would be fine.

It's funny, my particular life's experiences have pretty much told me that
if I have some sort of safety net with me, I never need it ;)

Thanks again to everyone who offered assistance.

Julian

-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Patrick Finnegan
Sent: Wednesday, November 02, 2005 10:13 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Discharging a VT100 CRT

Wolfe, Julian declared on Wednesday 02 November 2005 22:52:
> Okay, I'm going to try and take another crack at replacing the flyback
> on my VT100. A few months ago I almost did, and chickened out.   I'm
> very very afraid to do so, as the idea of trying to discharge the CRT
> scares me very much.  The terminal has been off and disconnected from
> power for at least 2-3 months now.
>
>
>
> This is what I've learned so far from reading:
>
>
>
> Put one hand in your back pocket.  Use an insulated screwdriver with
> alligator clips on either end, one to the screwdriver blade, one to
> the frame (VT100 service manual says the frame right above the CRT is
> where you should ground to).  Place the screwdriver under the anode
> cap and touch the metal connection underneath.  Wait till the
> crackling stops.  Remove the anode cap.
>
>
>
> Can anyone reassure me that this is a semi-safe procedure?  Has anyone
> done this sort of thing before?

Yeah, I've replaced CRTs and flybacks before.

A better idea is to place a current-limiting resistor in series with the 
alligator clip to ground (twist one lead around the screw driver, and 
tape it on, attach the allegator clip to the other lead).  100k to 1M 
ohm should do the trick, and hold it on the metal part of the anode cap 
for 5 seconds or so.  Not using the resistor will still discharge the 
tube, but it runs the risk of damaging it.

If you have the tube powered off for a couple days, the charge will most 
likely be quite dissipated.  In fact, having it off that long means it's 
unlikely to have any charge on the tube at all, but it's still best to 
still discharge it, just in case.

Of course, I've worked on monitors live before.  Trick there is to stay 
clear of the PSU and HV parts alltogether, and use insulated tools.  
Once you do it once or twice, you'll start feeling a bit more 
comfortable doing it.  It isn't a good idea to *replace* the flyback 
with the terminal live, but you'll probably need to have the power on 
when you adjust the screen/focus controls on the new flyback you 
install.

Pat
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