Discharging a VT100 CRT

[Patrick Finnegan]

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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