Discharging a VT100 CRT

[Tony Duell]

> Yeah, I've replaced CRTs and flybacks before.

As, of course, have I. 

> 
> 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.


Agreed. Actually, I have a Heathkit EHT probe. It's just an 800M resistor 
and a microammeter in series. I connect the ground lead of that to the 
CRT earth, then hold the probe end on the anode connector for a minute 
after the meter has got to 0. That seems pretty reliable.

> 
> 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 

The semiconductor rectifiers used in computer monitors, etc will leak 
enough to discharge the CRT. Watch out if you work on old TVs with valve 
rectifiers, they are much less leaky.

> 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.  

I once had the 30kV EHT in a colour monitor arc over to my fingers. 
Suffice it to say I then used langauge that I would not repeat on this 
list :-)

If you're working on a 'live' unit, don't work alone. Have somebody  
nearby who knows how to turn off the power if things go wrong. 

And rememebr that (at least in Europe), the mains is a lot more dangerous 
than the RHT due to the former's much lower impedance. Don't take great 
care to avoid the (generally well insulated) EHT and then put your hand 
on a live heatsink in the SMPSU.

> 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.

Actually, in a VT100 the presets are all on the video bard. It's still 
worth rememebring there's about 800V on some of them, so use a 
well-insulated screwdriver to tweak them.

-tony