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Direct Link To This Post Topic: The (hopefully) Definitive Guide to Discharging...
    Posted: 23 Apr 2019 at 9:06am
The (hopefully) Definitive Guide to Discharging Arcade Monitors Safely
This first appeared on the UKVAC mailing list in 2000? 
It has subsequently been bandied around the internet as text file.

Originally written by :
Andrew Welburn


Ok, I've seen a lot of people posting replies to n00b questions that are
not providing anything but babbling incoherent gibberish.


I feel it is my duty to step in and define it once and for all.


Monitor tubes are essentially giant capacitors, the 2 plates that make
up a capacitor in this instance are integral parts of the tube, one is
the coating on the INSIDE of the tube (that you cant see, and is
attatched to the magic bit under the rubber cap, the ANODE, the other is
the grey coating on the OUTSIDE of the tube, Aqua-DAG.

To discharge a monitor, all you need to do is to short the two plates

This involves clipping a crocodile test clip lead to the braid strap, or
to the ears of the monitor tube, and the other end to the metal bit of
any standard flat bladed screwdriver that has a plastic handle. The
metal braid on the back of all monitor tubes goes from ear to ear
somehow, this is so that the tube ears and implosion band have a decent
connection to the conductive coating on the outside of the tube (the
grey stuff, aquadag) and hence to one 'terminal' of the capacitor
arrangement. The crocodile test clip lead I mention is nothing fancy,
the wire bit is about a third the thickness of a single wire in a mains
cable 3-core wire. Its not substantial, and it doesn't need to be.

Anyhow, simply slip the blade of the screwdriver under the anode cap
(when the monitor is off of course (NEVER EVER EVER EVER WHILE IT IS ON)
and you'll hear a pop, or a crack, the volume and ferocity of this
pop/crack/flash depends on the tubes ability to hold the charge. If you
have just switched the monitor off, you'll get a very nice crack no
matter what. If the monitor has been off a few hours, it might be very
little. Some monitors have integral discharging circuits (later
hantarexes for example) and after 5 minutes you'll probably get nothing.
Some older monitors you can leave for weeks and come back to them and
they'll still have a crack that will make you jump.

That is ALL there is to it. Do NOT try this on a mono vector monitor,
you will blow the HV diode (expensive). This method is FINE for ALL
colour raster monitors. For vector monitors, get yourself a HV probe,
this has integrated resistors to trickle the voltage discharge and
prevent a big spike from killing the junction on the HV diode.

MYTH #1 : "The monitor has to be 'grounded' or 'earthed' somehow, like
to a radiator, or to the 3rd pin in a mains plug".. this is rubbish,
ignore EVERYTHING, ANYONE says about this. The monitor can be outside of
a cabinet, not plugged into anything, and discharging it will still
work. The electricity that you think is 'stored' like water, has to
actually 'go' someplace.. it doesn't, the mere act of shorting the two
plates out, makes it safe. Get a big fat capacitor out of a car, or a
big blue in an Atari game, stick a screwdriver over the terminals and
you will get a smaller, but identical effect. The capacitor is now safe
to handle, it wont zap you, the electricity has gone nowhere, you've
just equalized the charge between the two plates.

MYTH #2 :
- "You need a HUGE screwdriver with TONNES of insulation, and a big FAT
cable, and you should wear marigolds." All this is bollox too, if you
want to look like a prat, then fair enough. I use my regular flat bladed
screwdriver and a standard 10pence-from-maplins crocodile clip test lead
to discharge every monitor I have... yes it is true there is a large
charge there that can kill you, it's a huge voltage (up to 25,000 volts)
but the thing is, the act of discharging it requires very low current
carrying devices. Ie, screwdriver and croc clip lead. It might be at a
few amps for the first millionth of a second, but it decays to under an
amp within the next millionth of a second. Any old bit of wire and croc
leads, and tiny screwdriver can carry this sort of amperage for a
millionth of a second, yeh, it can't sustain it, but discharging a
monitor is an event that lasts for that millionth of a second. Remember
to use a screwdriver with a plastic handle at the very least!

MYTH #3 :
- "You should leave the monitor switched off for 3 days, then discharge
it" WTF ? have you got all the time in the world or something? This is
completely bogus too. Some monitors (as previously hinted) will have
lost their charge by this time anyway. Older monitors (G07, 4600 for
example) will have lost none of their biting capacity. You've just
wasted 3 days for no reason at all whatsoever. Once input power is
removed from the monitor, you can discharge it, as soon, or as late as
you like. I remember reading about someone who turned the game off,
unbolted and removed the monitor from the cabinet, put it on a shelf,
then discharged it 3 days later. And was really glad he had done it the
'right way' he told me. I went off on him for being a total dickhead.
What is the purpose of discharging? Its not for the monitors'
well-being, its so that you are safe to move it/handle it without fear
of being bitten. He had already done the dangerous bit by removing it
when it was live, defeating the whole point of discharging.

OK, This is not incredibly common and its not life-threatening, so its
not an integral part of discharging monitors, but its good to know, but
not essential that you carry out the following procedures every time you
discharge a monitor.

There is a strange 'ghost voltage' effect which means that a tube, once
discharged, will actually build up a small charge again.. you can get
zapped by this and its not that painful, about half as painful as mains
240v voltage. Obviously, its not that pleasant, so if you have a tube
that you've discharged, and removed the HT lead and anode clip, and you
come back to it 5 minutes later, discharge it again and see if it has a
charge by seeing if there is a spark. Once you've discharged it a second
time it should be super-safe then.

I wondered about this weird ghost voltage one day so I sat and
investigated. What seems to happen is that when you discharge a tube,
there is a crack a spark and bam, you're done. What has actually
happened is that the main voltage charge held in the tube
(19,000->25,000v) has been drastically reduced to around 1800v. This is
not enough to jump a gap and create a spark. So its not that dangerous.
Thing is, this ~1800v can increase as if by magic, something to do with
the resulting frantic electron cloud that has been kicked up inside the
tube after discharging. Anyhow, as the cloud 'settles' again, the
potential charge between the plates increases, therefore increasing the
voltage... and if the charge in the tube rises up to about 2500v, it has
the ability to jump a gap once again, ie, it will spark next time you
discharge it.. it also means it can give you a nip if you weren't paying
attention and touched it. But its no big deal really, its like the same
level as a static shock you get when stepping out of a car, or running
around the office dragging your plastic/rubber shoes on the cheap nylon
carpet, then touching a metal workstation that will have a significantly
different ground level to you. Those are annoying, but they don't hurt
or cause damage, and can't kill you, this is the same with the ghost
voltage on monitor tubes. Yes, you will be surprised how many volts you
can rack up from everyday movement.

If you want to prevent the ghost voltage syndrome, all you have to do is
discharge the tube like normal, remove the rubber anode clip/cap thing
and just prop your screwdriver in the anode hole and leave it for a few
minutes. This ensures a total, sustained short between the plates so
that the 1800v gradually falls, and falls, and falls, to about 200v..
this 200v residual charge is about as absolute zero as you can get, and
it reaches this after about 30 seconds of having a total sustained
short. This 200v will not climb up again like the ghost voltage
previously because you've settled any electron cloud that could
potentially increase it.. the monitor is now safe to LICK if you so fealt
the need to.

If you are really, really paranoid, always have your croc clip lead and
a screwdriver with you to just test-discharge every tube before you move
it, if you are that unsure.


Ok, now I think I've cleared everything up. Any questions?

In future, I suggest *some* people might want to think twice about
posting garbage replies to newbies on potentially dangerous topics if
they don't actually know the whole total truth, because someone is only
going to read it and end up with a glass monitor tube in their bum or
something. Imaginative and theatrical as some of the solutions and
explanations are, they are just not right, and are open to
interperetation in some potentially harmful ways.

I hope what I have written is an end to the eternal mystical question,
and should anybody need to know this in the future, it will be in the
UKVAC Archives..

Happy discharging..

Andy Welburn


Edited by DanP - 23 Apr 2019 at 9:16am
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