CRT Discharge

FixaMAC · Nov 24, 2004

In the following Apple manual:

http://www.usd465.com/staff/classes/service_training/display_foundations/Safety/safety_2.html

it clearly states:

"Before you do anything (including discharging a CRT), turn the monitor off and unplug it."

It is my understanding that CRT discharge is generally run to earth and yet this cannot happen if the power lead (with its earth in the UK) is unplugged and no other earths are introduced.

So, should the power lead be IN or OUT when performing the discharge and if it should be left out where does the charge dissapate to?

Thanks

Sun Baked · Nov 24, 2004

It's a big charged capacitor holding tens on thousands of Volts, because of the design it doesn't have a way to dissipate the charge by just leaving it turned off and plugged in.

And like any capacitor, a quick way to discharge it is to short across the leads.

You can do this with a discharge tool (or your body if you don't discharge it and just happen to touch the right spot.)

sgarringer · Nov 24, 2004

Sun Baked said:
It's a big charged capacitor holding tens on thousands of Volts, because of the design it doesn't have a way to dissipate the charge by just leaving it turned off and plugged in.

And like any capacitor, a quick way to discharge it is to short across the leads.

You can do this with a discharge tool (or your body if you don't discharge it and just happen to touch the right spot.)

Or a screwdriver with a long shaft and insulated handle.

I always thought that was a discharge tool =]

pncc · Nov 24, 2004

FixaMAC said:
In the following Apple manual:

http://www.usd465.com/staff/classes/service_training/display_foundations/Safety/safety_2.html

it clearly states:

"Before you do anything (including discharging a CRT), turn the monitor off and unplug it."

It is my understanding that CRT discharge is generally run to earth and yet this cannot happen if the power lead (with its earth in the UK) is unplugged and no other earths are introduced.

So, should the power lead be IN or OUT when performing the discharge and if it should be left out where does the charge dissapate to?

Thanks

There is a special discharge tool that has a ground lead you plug in and a 100MOhm resistor so the charge is discharged in a controlled way.

Never Disassemble a CRT w/out discharging it first.
Never work on a CRT with both hands. Put one hand in your back pocket.
The voltage stored in a CRT is DEADLY. Don't mess with it.

FixaMAC · Nov 24, 2004

Thanks

I'm clear about the use of the tool (or screwdriver

). What I'm not sure of is whether the power lead should be IN or OUT when using it.

The instructions in the Apple guide say the lead should be attached to the "Ground Lug" but I can't see where the charge will dissapate to unless the earth lead is plugged in while the instructions say clearly it must be OUT.

sgarringer · Nov 24, 2004

FixaMAC said:
Thanks

I'm clear about the use of the tool (or screwdriver ). What I'm not sure of is whether the power lead should be IN or OUT when using it.

The instructions in the Apple guide say the lead should be attached to the "Ground Lug" but I can't see where the charge will dissapate to unless the earth lead is plugged in while the instructions say clearly it must be OUT.

The power lead should be OUT.

Never, ever, ever work on electronics with it connected to mains. EVER!

It disapates into the air.

pncc · Nov 24, 2004

FixaMAC said:
Thanks

I'm clear about the use of the tool (or screwdriver ). What I'm not sure of is whether the power lead should be IN or OUT when using it.

The instructions in the Apple guide say the lead should be attached to the "Ground Lug" but I can't see where the charge will dissapate to unless the earth lead is plugged in while the instructions say clearly it must be OUT.

In the USA, grounded AC outlets have three pins, + - and ground.

Unplug the computer!!!
Plug the CRT discharge tool into the ground pin on the AC outlet. You can buy plugs that have plastic + and - pins and a ground pin with a cable connected to the ground pin so you can clip onto that and be grounded.

FixaMAC · Nov 24, 2004

Thanks scarringer and pncc.

scarringer, how does it dissapate into the air?

pncc, your suggestion makes sense to me but is it possible that Apple would miss grounding instructions on their manual?

On something safety critical are they not liable to have paid a lot of attention to detail?

Have we missed something about the way the charge is lost?

I appreciate your help

DanTekGeek · Nov 24, 2004

the charge is big enough to jump. meaning, you dont have to actually touch it to get a hell of a jolt. im not sure of the actual equation to figure out how far a spark will travel in air, but id assume its at least a few centemitres.

Sun Baked · Nov 24, 2004

It's just a difference in the number of electrons stored on two plates, place a wire between the two plates and the electrons equal themselves out instantly.

No need for a ground, just a path between the charged plates.

Sort of like a static charge, you don't need to touch a ground just somebody who isn't charged like you are -- touch them and you equal yourselves out.

CanadaRAM · Nov 24, 2004

CRT Discharge

Simple answer:
If you are not a trained technician, there is no g#%-d$*@ed way you should take the hood off the monitor.

Second answer:
The service training manual page in the URL states clearly to unplug the monitor then use a proper discharging method. If you are wanting to question that advice there is no g#%-d$*@ed way you should take the hood off the monitor.

Third Answer:
Death by electro-cardial arrest is presumed to be painful, fast, and permanent.

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