CRT Discharge Procedure

CRT inside an MM12 monitor

A cathode ray tube (CRT) stores a charge at its final anode and holds it after the equipment is unplugged. Sam Goldwasser's monitor and television repair notes put the retention plainly: the high voltage of a CRT "will retain a dangerous or at least painful charge for days or longer". Apple's own service documentation is equally blunt: the anode "can retain a charge of several thousand volts even when power is off and can regain some charge even after being discharged".

There are two ways a CRT can hurt you, and discharging only deals with one of them. The second is the vacuum: the envelope is under external atmospheric pressure and can implode if it is cracked or struck, which is why eye protection is not optional. A third hazard sits next to the tube rather than in it — the mains-derived primary side of the analogue board, which is a separate job with its own precautions.

Safety rules

Apple published ten rules for CRT work. They are reproduced here in substance because every one of them addresses a way people have actually been hurt.

1. Do not work on a monitor alone. Someone nearby — ideally someone trained in CPR — could save your life.
2. Remove jewellery before working on a CRT. Rings, watches and bracelets are conductors.
3. Never wear a grounding wrist strap or heel strap, and never work on a grounded mat, while discharging a CRT or making live adjustments. Anti-static grounding is for working on dead, uncharged equipment. Wearing it while discharging makes you the discharge path.
4. Wear safety goggles. The tube contains a high vacuum and can implode.
5. Switch off and unplug before working inside. Parts of a monitor chassis are live when it is under power.
6. Keep one hand in your pocket or behind your back when anything is live. This keeps current from crossing your chest.
7. Always discharge the anode before touching anything inside. High voltage can be present on the anode and elsewhere even with the power off.
8. Never touch the anode connector or the anode aperture.
9. Do not pick up or handle a CRT by its neck. The neck is the thinnest part of the envelope.
10. Never touch the back of the power switch, the yoke wires, the anode connector, the anode wire or the flyback transformer while the set is live.

Rules 3 and 6 are the two most often ignored by hobbyists, and both of them are the difference between a shock and a fatal shock.

There is no safe waiting period

Leaving a set unplugged overnight is not a substitute for discharging it. Charge on the tube can survive days, and Apple warns that the anode can build voltage back up over time after it has been discharged. Treat every tube as charged, every time, including one you discharged an hour ago.

Why the charge persists

The tube is itself a capacitor. The inside and the outside of the glass bell are both coated with Aquadag, a conductive graphite paint; the glass between them is the dielectric. The resulting capacitor is of the order of 0.005–0.01 µF, which sounds trivial until you multiply it by the anode potential. The outer coating is returned to chassis ground through the springs, clips and wire that run round the tube, and the inner coating is connected to the final anode, so the charge sits between the anode button and that earth return.

That is also why the earth return point matters so much: you are discharging one plate of a capacitor into the other, and the second plate is the outside of the tube — not the logic board.

Anode potential

Do not work from a generic figure. The anode potential depends on the tube and is stated in the machine's own service manual. Two sourced examples:

Equipment	Stated anode potential	Source
Compact Macintosh (9-inch monochrome: 128K, 512K, Plus, SE, SE/30, Classic)	up to 12,000 V DC	Apple Service Guide, 1990
Larger colour televisions and monitors	25,000 V and above	Goldwasser, CRT Information

A figure quoted without a source should be treated as unverified until you have checked the service manual for the machine in front of you. The practical point is unaffected: any of these potentials will hurt you, and the correct procedure is identical.

Discharge tooling

You need three things: an insulated probe, a series resistor rated for the voltage, and an earth lead with a crocodile clip.

• A purpose-built CRT discharge tool is the right answer. Apple's service procedure is written around its own CRT discharge tool — an insulated probe with a flying earth lead.
• A series resistor of a few megohms is what makes an improvised tool safe to use. Goldwasser recommends discharging a CRT through a resistor of a few megohms rather than shorting it: it avoids the current pulse, it stops the tool and the anode button being arc-damaged, and it removes the bang that makes people flinch into something else. The discharge is still essentially instantaneous — the time constant is of the order of a millisecond.
• The resistor must be rated for the voltage, not just the power. Ordinary resistors, including 5 W parts, are typically rated for only a few hundred volts and will flash over across their own body at CRT potentials. Use a high-voltage resistor, a series string, a high-voltage probe, or a known-good focus/screen divider network.
• An earth lead with crocodile clips at both ends, long enough to reach comfortably.
• Safety goggles. Not optional.

Apple's procedure does sanction a flat-blade screwdriver with an insulated handle for the confirming pass after the discharge tool has been used. It is not a substitute for the tool on the first pass, and repeatedly forcing a screwdriver under the anode cap risks scratching the glass envelope, which is exactly the damage you do not want on a tube you cannot replace.

What improvised methods actually get wrong

• A bare screwdriver bonded to a clip lead is a dead short. It works, but it arc-welds the tip to the anode button, pits both, and produces a flash and a crack that has put more than one person's hand into a live part of the chassis by reflex. A series resistor removes all of that at no cost in discharge time.
• Earthing to the wrong place destroys the machine. See below.
• Wearing anti-static grounding while discharging puts you in series with the discharge.
• Using a resistor rated only for power invites a flashover across the resistor body at 12–25 kV.
• Discharging once and walking away ignores the fact that the anode can build charge back up.

Identifying the earth return point

Correct points:

• The metal ground lug on the chassis that the CRT's earth braid terminates at. On a compact Macintosh this is the point Apple names.
• The earthing spring, clip or braid that runs round the funnel of the tube and bonds the outer Aquadag coating to the chassis.

Wrong points:

• The logic board ground plane, or any signal ground.
• Any screening can or bracket whose bond to the tube's earth return you have not verified with a meter.

If you cannot identify the tube's earth return with certainty, find it on the machine's service manual diagram before going any further.

Discharge procedure

CRT anode cap

This is Apple's procedure, which it states may be used on any Macintosh monitor and which generalises to other manufacturers' sets.

1. Switch off and disconnect the mains lead.
2. Remove your grounding wrist strap and any jewellery, and put on safety goggles.
3. Attach the crocodile clip of the discharge tool to the metal part of the ground lug (or the tube's earthing spring).
4. Put one hand in your pocket or behind your back. With the other hand, slide the tip of the discharge tool under the rubber anode cap until it touches the anode ring.
5. You may hear a crack and see a flash. With a resistive tool you normally will not.
6. Withdraw the tool. Repeat the procedure to confirm the tube is discharged — Apple suggests the second pass may be made with an insulated flat-blade screwdriver.
7. Only then remove the anode cap, by pinching the rubber and easing it clear. Do not twist it.

Keep it discharged while you work

Apple's instruction after discharge is to establish an ongoing ground: fasten one end of a crocodile lead to the ground lug and the other to the anode aperture, and leave it there. This drains any charge that builds back up while you are working. Goldwasser makes the same recommendation, noting that people are regularly caught out by charge that has reappeared by the time the anode lead is reconnected.

Do not replace this with a timer. "Discharge again after fifteen minutes" is worse than a shorting lead in every respect.

The neck board holds charge too

The focus and screen (G2) voltages on the small board on the neck of the tube are derived from the CRT high voltage. If you are going to touch that board, discharge the tube first — even if you have no intention of removing the anode lead.

The other hazard on the analogue board

Discharging the tube does nothing for the mains side of the analogue board or power supply. The primary-side reservoir capacitors sit at the rectified mains potential — roughly 325 V DC from a 230 V supply — and hold it after the set is unplugged. Recapping a monitor or an all-in-one machine is ordinary, legitimate restoration work, and this wiki documents it; the point is that it is a second, separate hazard with its own procedure. Confirm each large capacitor is at a safe voltage with a meter before you touch the board, discharge any that are not through a suitable resistor, and re-check. Goldwasser's rule of thumb for these is a resistor of about 100 Ω per volt of the capacitor's working voltage, which discharges the part in a couple of seconds without an arc. See Capacitor Failure Symptoms for what these parts look like when they have failed.

After discharge

• Keep the discharge tool and the shorting lead within reach for the whole job.
• Re-discharge before touching the anode area again after any period away from the bench.
• When refitting the anode cap, seat it so both spring contacts engage the anode button and the rubber skirt sits flat. Do not twist it into place.
• Remove the shorting lead before reconnecting the anode lead and restoring power.

Disposing of a dead tube

A scrapped CRT still holds a vacuum and can implode if it is crushed in a bin lorry. Apple's service documentation includes a devacuuming procedure for exactly this reason, carried out with the tube inside a closed box, wearing goggles and gloves, by clipping the connector pins off the end of the neck to admit air. Discharge the tube first: the Aquadag capacitance is still there. Many local authority recycling centres take CRTs intact and will do this themselves, which is the easier route.

Related pages

• Recommended Tools
• Capacitor Failure Symptoms
• Battery Explosion, Capacitor or Corrosion Damage