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Author Topic: Discharging a CRT.  (Read 1567 times)

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Slice

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Discharging a CRT.
« on: July 24, 2002, 07:24:15 am »
Hi all,
I'm taking my Arcade Monitor from my Cabinet and replacing it with a TV, which I plan to mount in the cage from the old monitor.
Now I'm a service technician by trade and I know that CRT's are EXTREMLY Dangerous!! But I've never discharged one.
I've visited a couple of sites for advice and I'm wondering has anyone ever built a discharge circuit? ( I assume a high resistance chain going to ground is enough? )
Also on removing the Anode - Are insulated pliers the way to do this or does it need to be prised off - with a screwdriver?

MameFan

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Re:Discharging a CRT.
« Reply #1 on: July 24, 2002, 12:12:49 pm »
Check www.therealbobroberts.com, then go to his "bulletin board" ( a place with his tech info, not a BBS like this one). He has details on discharging monitors.  I believe he claims you do not need to "slowly disappate" it, just do it 2-3 times, then remove the anode cap using a screwdriver and pinching it.

(I'd still wear some gloves when doing this just in case... better safe than sorry.)

Slice

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Re:Discharging a CRT.
« Reply #2 on: July 29, 2002, 04:25:09 am »
Checked that out, thanks very much! One question though! It says to unplug everything before you short the Anode to ground, if this is the case how does the charge dissipate, Would it now not just sit on the chassis?
I don't know, this may be a bit of a stupid question - But when it comes to my personal safety I don't mind asking!!!!!!!

;)

MameFan

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Re:Discharging a CRT.
« Reply #3 on: July 29, 2002, 11:30:03 am »
When discharging to the chassis frame, you're "shorting" the capacitance built up within the tube.

Basically, you're shorting the + anode inside to the - cathode outside.

You're NOT shorting it to "ground" or "neutral" with respect to the mains (AC) outlet..

In fact, since all arcade monitors are isolated from ground & neutral through the use of an isolation transformer, even if you DO have it plugged in, you still don't have a "straight line" to ground/neutral.

[And think about it... even if you could short it to ground/neutral... do you want a 22 Kilovolt zap travelling through you're home's neutral line on the AC until it hits ground outside your house on the transformer?????  Probably not!!!! :) ]

Basically, just think of the picture tube as a big massive electrolytic capacitor.

One lead is + (the anode cap, that leads to the capacitance charge plate on the inside of the tube), and the other is - (the outter glass plate on the tube that *should* have a ground wire wrapped around it!).

When you short it, you remove the capacitance built up between the plates, just like shorting 2 wires of an electrolytic capacitor. You don't dissappate an EC by connecting the + lead to your AC wiring ground or neutral... you just disappate it to the - on the capacitor itself.

If you search google you will find some people claiming that if you dont slowly discharge (through a resistor of some type), you will ruin the tube.  Others claim that's hogwash and leaves you more susceptable to a lingering charge that never disappated, and can still hurt you.

The choice is yours... believe who you want.  I always have just grouned it out to the frame (that is electrically connected to the outside of the tube--always checked with an Ohmmeter before I do it), but never had a zap, as the discharge circuitry was already working and no charge was still in the tubes, but you need to always be careful and never assume.  I trust what Bob says so I go with the quick zap route.

Be safe!

Slice

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Re:Discharging a CRT.
« Reply #4 on: July 31, 2002, 04:10:08 am »
Thanks MameFan, I've forgotten so much basic stuff it's not funny - But I know how to go about it now!!!!!

  
 

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