CRT Discharge Procedure

Schneider MM12 monochrome monitor with its case removed. The anode lead (red) enters the cup on the bell of the tube.

A cathode ray tube (CRT) keeps a high-voltage charge on its anode after the equipment is switched off and unplugged. Apple's service guide for the compact Macintosh puts the anode at up to 12,000 V DC and warns that it "can retain a charge of several thousand volts even when power is off and can regain some charge even after being discharged". IBM's 8513 display manual warns that high voltage "may remain for a number of days, even if the display is switched off". Sam Goldwasser's CRT notes say the same: the tube "will retain a dangerous or at least painful charge for days or longer".

Discharging the tube deals with that charge only. The tube is also under high vacuum and can implode if it is cracked or struck, and the mains-side capacitors on the same chassis hold their own charge (see below).

The same procedure applies to every machine with a built-in tube: the compact Macintoshes from the Macintosh 128K to the Macintosh Color Classic II, the Macintosh LC 520 family and Macintosh TV, the Apple eMac, the Apple Lisa and Macintosh XL, the Commodore PET 2001 and Commodore PET 4000 Series, the Osborne 1, Osborne Executive and Osborne Vixen, and the Tandy TRS 80. It also applies to separate monitors: Apple's displays, the IBM 5151 and IBM 5153, IBM's PS/2 displays, and the Amstrad monitors that power the Amstrad CPC 464, Amstrad CPC 664 and Amstrad 6128.

Safety rules

Apple's service guide for the compact Macintoshes gives ten rules for CRT work:

1. Do not work on a monitor alone. Someone nearby, preferably someone trained in CPR, could save your life.
2. Remove all jewellery before working on a CRT.
3. Never use a grounding wrist strap or heel strap, or work on a grounded bench mat, when discharging a monitor or making live adjustments. Anti-static grounding is for "dead" (uncharged) equipment only.
4. Wear safety goggles. The tube contains a high vacuum and can implode if cracked or broken.
5. Switch off and disconnect the mains lead before working inside a monitor. Parts of the chassis are live while it is powered.
6. Keep one hand in your pocket or behind your back when working on a live monitor.
7. Always discharge the anode before touching anything inside the monitor. High voltage can be present on the anode and other components even when the power is off.
8. Never touch the anode connector or the anode aperture.
9. Do not pick up or handle a CRT by its neck, where the glass is thinnest.
10. When adjusting a live CRT, never touch the back of the power switch, the yoke wires, the anode connector, the anode wire or the flyback transformer.

IBM's display manuals add rules for handling the tube itself: wear safety glasses and a long-sleeved garment, do not scratch or bump any part of the tube, handle it with two hands and never by the neck alone, support it at the large end when fitting or removing it, and do not leave it anywhere it could roll. IBM's 8513 manual also asks for non-essential people to leave the area while a tube is being exchanged. The Lisa manual adds that screwdrivers used for video work should have insulated handles and, apart from the one used for discharging, insulated shafts.

Why the charge persists

The tube is itself a capacitor. The inside and outside of the bell are coated with Aquadag, a conductive graphite paint, and the glass between them is the dielectric. Goldwasser gives the capacitance as between 0.005 and 0.01 µF. The outer coating is connected to chassis ground through the springs, clips and wires around the tube; the inner coating is connected to the anode. The discharge path is therefore from the anode to the earth return of the outer coating, not to the logic board.

The charge comes back because the glass, held at high voltage for years, stores charge within itself (dielectric absorption). Goldwasser's notes quote Joseph Gwinn: a CRT shorted for a week can show voltage again when the short is removed, "so leave it shorted". IBM's manuals say that "a second capacitive charge" builds up after the first discharge and tell technicians to discharge each tube a second time immediately before removing it. Apple's Lisa manual warns that the tube "can continue to recharge itself for several hours after being discharged".

Anode voltages

The anode voltage depends on the tube and its supply. Use the figure from the service manual for the machine on the bench.

Anode voltages stated in manufacturers' and service documents

Equipment	Anode voltage	Source
Commodore PET 2001 display assembly	About +10 kV (from flyback T2)	Commodore display circuit schematic
Compact Macintosh: 128K, 512K, Plus, SE, SE/30	Up to 12,000 V DC	Apple Service Guide, 1990
Apple Lisa (later sold as the Macintosh XL)	Over 15,000 V DC	Lisa Level 1 Technical Procedures, 1985
IBM 5153 Color Display	20.5–21.5 kV	Sams Computerfacts
IBM 8513 Color Display	25 kV at minimum brightness	IBM 8513 manual
IBM 8517 and 9517 Color Displays	Over 25 kV	IBM Hardware Maintenance Reference, 1992
AppleVision 1710 and 1710AV	26 kV	Apple Service Source
Colour televisions and monitors in general	25 kV or more	Goldwasser

Any of these voltages can injure or kill, and the procedure below is the same for all of them.

Discharge tools

Manufacturers' procedures use one of two tools:

• Apple's compact Macintosh procedure uses Apple's CRT discharge tool, an insulated probe with an earth lead and crocodile clip. Apple says the same procedure and tool can be used on any Macintosh monitor.
• Apple's Lisa procedure and IBM's 8517/9517 procedure use a flat-bladed screwdriver with an insulated handle, connected to ground by a jumper lead with crocodile clips. Apple attaches the lead to the screwdriver shaft three inches from the handle; IBM lists jumper 7838690 with crocodile clips 7838688, or meter lead kit 6428104.

Goldwasser recommends discharging through a resistor of a few megohms rather than a direct short. The time constant is still about a millisecond, and the resistor avoids damage to the screwdriver tip and the anode button from the arc. The resistor must withstand the voltage: most ordinary resistors, "even 5 W jobs", are rated for only a few hundred volts and will flash over at CRT voltages. Goldwasser suggests a long string of ordinary resistors, a high-voltage probe or a known-good focus/screen divider network; with the last two the time constant may be a few seconds.

You also need safety goggles and a second crocodile lead for the ongoing ground described below. Goldwasser warns that repeatedly forcing a screwdriver under the anode cap can scratch the envelope.

Earth return point

The point to use differs between manufacturers:

• Compact Macintoshes: the metal ground lug shown in Apple's service guide.
• Apple Lisa: the metal chassis.
• IBM 8517 and 9517: the ITC (integrated tube component) shield.
• Any tube: the earthing spring, clip or braid that bonds the outer Aquadag coating to the chassis.

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

Discharge procedure

Anode cap on the tube of a Macintosh Classic II

This is the order of Apple's compact Macintosh procedure, with the points where IBM's and the Lisa procedure differ.

1. Switch off and disconnect the mains lead and any signal cables. IBM says to turn the display off before unplugging it and wait about five seconds, because a static charge can remain on the power connector if the cord is pulled first.
2. Remove the covers needed to reach the anode cap.
3. Remove any grounding wrist strap and all jewellery, and put on safety goggles.
4. Clip the earth lead of the discharge tool, or the jumper on the screwdriver shaft, to the earth return point.
5. Put one hand in your pocket or behind your back. With the other hand, holding only the insulated handle, slide the tip under the anode cap until it touches the anode ring. Do not use force. The Lisa manual says to free a cap stuck by suction with a smaller screwdriver first, still with one hand behind your back.
6. Withdraw the tool and repeat. Apple says the repeat can be made with an insulated flat-blade screwdriver; IBM says to discharge "several times".
7. Remove the anode cap straight away if the job needs it removed. IBM says to remove the cap immediately after discharge so that the tube does not recharge, and to discharge again if delayed by more than a minute.

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 to drain any charge that builds up. Goldwasser also recommends a shorting lead while working, because of the number of people caught by charge that has returned by the time the anode lead is reconnected. Remove the shorting lead before refitting the anode cap and restoring power, and discharge again before touching the anode area after any break.

The neck board

The focus and screen (G2) voltages on the small board on the neck of the tube are derived from the anode supply. Discharge the tube before touching that board, even if you are not removing the anode lead.

Manufacturer notes

• Apple: the AppleVision 1710 Service Source says never to put on a grounding wrist strap until after the CRT has been discharged. Its high-voltage check starts by waiting at least two minutes after switching off for the CRT to discharge, then connects Apple's high-voltage probe (076-0392) under the anode cap with the power off.
• IBM: the 8517/9517 procedure is on pp. 15–16 of its Hardware Maintenance Reference, and the CRT safety rules on pp. viii–x. The 8513 manual repeats the warning at each removal step that exposes the anode.
• Osborne: the Field Service Manual removes the Osborne 1 video monitor as a complete unit and gives no anode discharge step. Its caution in that section is about the power supply, which "can hold an electrical charge for long periods of time". Discharge the tube before working on the monitor board.
• Amstrad: Amstrad's service manuals for the CPC monitors (CTM640/GT64, and the GT65/CTM644 amendment) consist of parts lists, circuit diagrams, board layouts and fault-finding flowcharts, with no discharge procedure. Use the general procedure above.

Mains-side capacitors

Discharging the tube does nothing for the mains side of the analogue board or power supply. The primary reservoir capacitors charge to the peak of the mains voltage, about 325 V DC from 230 V and about 340 V from 240 V, and can hold it after the set is unplugged. Recapping a monitor or an all-in-one machine is routine restoration work, and this is a separate hazard from the tube. Measure each large capacitor with a meter before touching the board, discharge any that hold voltage, and measure again. Goldwasser suggests a resistor of about 100 Ω per volt of the capacitor's working voltage, which brings it to a low voltage within a few seconds without an arc; for a 100 µF 350 V capacitor that is 5 kΩ. See Capacitor Failure Symptoms for what failed parts look like.

Disposing of a dead tube

A scrapped CRT still holds its vacuum. Apple's service guide gives a devacuuming procedure: with the tube in a closed cardboard box and only the tip of the neck protruding through a hole, wearing goggles and gloves, clip off the connector pins, then reach under a cloth veil taped over the hole and snip off the tip of the neck with large pliers to let air in. IBM's instruction to its field engineers is the opposite: "Field personnel must not release the vacuum in CRTs", and tubes are to be disposed of under existing safe working procedures. Discharge the tube first in either case; the Aquadag capacitance is still there.

Related pages

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