Discharging a VT100 CRT

[Tony Duell]

> 
> 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.

Well, glass [1] is an excellent insulator and could hold a charge 
for 3 months _but_ it is very likely the CRT has discharged either 
through leakage in the EHT rectifier diode (built into the flyback in 
this unit), or through the beam current when the thing was last used.

The worst case for the CRT to hold a charge is if the EHT side (flyback, 
etc) keeps working and there is no beam current due to the grid being 
very -eve or the heater going out, or... There is then nothing to 
discharge the CRT. 

[1] The capacitor you are discharging consists of aquadag layers on the 
inside aod outside of the CRT flare. The CRT glass envelope is the 
dielectric.

> 
>  
> 
> This is what I've learned so far from reading:
> 
>  
> 
> Put one hand in your back pocket.  Use an insulated screwdriver with

More particularly, work one haned, keep the other hand well away from the 
unit and anything that's earthed. The most dangerous current path is up 
one arm and own the other.

It is normal to keep your left hand in your pocket. Some say it's because 
your heart is on the ledt side, I personally do it because I am right-handed.

> 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

You should connect to the CRT earth. I seem to rmemeber on a VT100, 
that's a spring contact on the yoke with a white wire/faston clip.  The 
idea is to ensure that the discharge current doesn't flow through 
anything it shouldn't

> 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?

It's safe for _you_, but I wouldn't do it that way. The high initial 
current could damage the CRT (burn the conenction between the anode 
conneector and the internal aquadag layer), and more particularly could 
damage IC's all over the terminal.

The first thing I'd do is take out the logic board(s) and PSU. That gets 
all the delicate ICs out of the way.

I would also use a discharge resistor. Take a 1M resistor (or if you 
prefer 10 100k resistors in series if you're worried about the voltage 
rating), connect one end to the CRT earth, the other to the screwdriver 
blade [2]. Carry on as above, but hold the screwdriver in contact with 
the anode conenctor for a minute or so. If you're worried, repeat the 
process with no resistor just to be sure.

[2] Don't do what I did once when inebriated [3] and connect both the 
chassis and screwdriver to the same end of the resistor, thus not having 
it in-circuit

[3] Why was I working on a monitor when drunk? Well, it was my 21st 
birthday, one of the presents was a non-working FTS-88 computer. Of 
course I started to repair it ASAP.

The charge stored in a normal-size CRT -- particularly a monochrome one 
-- is not likely to be dangerous in itself to anyone without heart 
problems. But it will make you jump, you might then drop the CRT with 
very unpleasant results.

-tony