servicing terminals/monitors

Tony Duell ard at p850ug1.demon.co.uk
Mon Feb 23 14:58:01 CST 2009


> 
> I've got a quick question about power safety here.
> 
> My ADM-3A has needed fiddling for a while, since what I believe is the
> flyback is making a very high-pitched whine that gives me a splitting
> headache and makes it hard to see straight after 5 minutes. I want to
> take a poke around, make sure everything is connected firmly, make
> sure none of the components are visibly bad, that kind of thing. How
> long does it take an unplugged CRT to discharge? This hasn't been

A are CRT will hold its charge for a very long time. The capacitor is the 
inner and outer aquadage coatings with the glass of the CRT flare as the 
dielectric, and glass is an excellent insulator.

But you don't ahve a bare CRT. In a number of cases there's actually a 
leeder resisoptr inside rhe flyback (more common in colour monitors, 
where it forms a potential divider to get some other CRT electrode 
voltages). In which case the thing will discharge in well under a minute 
in my expeirence.

Also, the CRT beam current in nromaly operation will tend to discharge 
the CRT at switch-off. If the terminal/monitor was displaying a normal 
image when you turend it off, it's unlikely much charge remains. But if 
the screen was blnak due to a fault (video amplifier, CRT heater, etc) 
and there was no beam current, some charge could well remain.

> plugged in for over a month, so I figure it's probably safe right now,
> but if I test it and then want to check something else, how long do I
> need to leave it sit? Is there a safe way to discharge a CRT when
> you're working on it in the living room? I'm a little leery of just
> shorting connections with a screwdriver randomly, although that's

I would certainly recoemnd against doing that, for all some manuals seem 
to recomend it. Apart from a slight risk of damaging the CRT itself (it's 
posisble to damave the internal connection between the inner aquadag 
coating and the anode connector), it's much more likely that the current 
spick will indirectly cause damage to ICs all over the terminal.

I use my EHT meter to discharge CRTs. It's got an intenral resistance of 
800M, and thus discharges them safely and fairly quickly. But I guess not 
everyone has that sort of instrument.

> almost exactly what I did the first time I worked with a CRT :) (it
> was 5th grade, and I had just had it on, unplugged it, opened it, and
> started poking around with wires. BIG SNAP)

I would be suprised if what you discharged was the EHT. The only place 
you'l l find that is  on the final anode connecotr on the side of the CRT 
flare. It's not on any PCB tracks, etc. 

Unless you're actually working on the CRT/flyback, there's little risk of 
getting zapped y the EHT itself.

-tony



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