This article provides safety precautions for working on a cathode-ray tube (CRT.)

This article covers repair procedures for authorized service providers on selected Apple products, but we have published them in the Knowledge Base for the general public for educational purposes. At this time, there are a limited number of repairs that a customer can perform and these are covered under the customer installable parts program http://www.info.apple.com/installparts . If you have not been authorized to perform these procedures, please contact Apple (800-APL-CARE in the USA) or your local authorized service provider to schedule a repair.

DANGER! This procedure exposes you to high voltages that can result in death or serious injury. DO NOT ATTEMPT THIS PROCEDURE IF YOU HAVE NOT BEEN CERTIFIED AS AN APPLE-AUTHORIZED TECHNICIAN.

CRT Safe Electrical Setup Precautions

A cathode-ray tube (CRT) provides the video display for the computer. The CRT is a picture tube that operates at very high voltages and contains a high vacuum. If cracked or broken, the CRT can implode (collapse into itself) and scatter fragments of glass. When working on or near a CRT, follow all safety rules and take every precaution against breaking the tube, especially at the neck where the tube is thinnest.

The following precautions must be taken to ensure your safety, especially when you are making adjustments on a live CRT.

General Setup Rules

Electric Outlet - Be sure your outlet is correctly wired and properly grounded. Polarity and ground testers are available from most electronics stores. Test all outlets in your service shop before working on any electrical equipment. If you have any doubts about the wiring in your building, consult a qualified electrician.

Adapter Plug - Never use an adapter plug to connect a monitors three-pronged power plug to a two-pronged wall outlet. Adapters defeat the ground pin, which is a safety feature.

Buddies - Do not work on a CRT alone. If there is an accident, having someone else nearby could save your life. Apple recommends that your staff be trained in cardiopulmonary resuscitation (CPR).

Metal Jewelry - Remove rings, wristwatches, hanging necklaces, and other jewelry before performing repairs on a CRT. Metal jewelry is an excellent conductor of electricity. Removing jewelry will reduce the possibility of electrical shock.

Grounding Devices - Never use a grounding wriststrap or heelstrap or work on a grounded workbench mat when discharging a CRT or when performing live adjustments. Even though the straps and mats contain one-megohm resistors and conduct only small electrical charges, Apple recommends that straps and mats be used only when working on dead (uncharged) equipment.

Safety Goggles - Wear safety goggles when working with a CRT. The CRT contains a high vacuum. If cracked or broken, the CRT can implode (collapse into itself) and scatter fragments of glass.

Power - Before working inside a monitor or a Macintosh containing a CRT, turn off the power and disconnect the AC power cord. Certain parts of a monitor or Macintosh chassis are hot (electrified) when the unit is under power. Except when you must have the power on (for example, when making live adjustments), never work on a plugged-in monitor or Macintosh with video.

One Hand - Keep one hand in your pocket or behind your back when working on a live monitor or Macintosh with video. Working with only one hand reduces the risk of current passing through your heart, should you accidentally contact high voltage.

Anode - Discharge the anode before working inside the unit. See Discharging the Cathode Ray Tube (CRT) in this manual. Some monitors and some Macintosh computers containing CRTs have a bleeder resistor on the anode that drains the charge when the power is turned off. Nevertheless, in case the resistor fails and leaves the anode fully charged, you must perform the discharge procedure.

Isolation Transformer - When performing live adjustments, use an isolation transformer between the monitor power cord plug and the power outlet. This setup decreases the hazard of electric shock in case you were to accidently touch a hot component or chassis. Isolation transformers are available at some stores that specialize in electronics components. Select one that is rated to handle the products you service.

Anode Connector and Aperture - Warning: Never touch the anode connector or the anode aperture. Normally, the anode aperture has a connector plugged into it. When a CRT is replaced, the anode connector is removed, exposing the anode aperture. If the bleeder resistor fails, the anode can retain a charge of several thousand volts (even when power is off) and can regain some charge even after it has been discharged.

CRT Neck - Warning: Do not pick up or handle a CRT by its neck, where the tube is the thinnest. To prevent an implosion, take every precaution against breaking the CRT.

CRT Live Adjustment Rules

Plastic Adjustment Tool - When performing live adjustments, use a plastic adjustment tool or insulated screwdriver only.

Live Components - When adjusting a live monitor, never touch the components listed below.

Warning: Serious injury could result if you touch any of these components with the power on:

• High Voltage Cable
• Anode Cap
• Yoke Cable/Connector
• Flyback Transformer

Discharging the Cathode Ray Tube (CRT)

In the following procedures, you will discharge the high voltage from a cathode-ray tube (CRT). Discharging the tube before working on it lessens the chance of an electric shock.

Most later Macintosh video monitors are equipped with a bleeder resistor that automatically drains the charge from the CRT when the power is shut off. However, if the resistor fails, the anode may retain a charge. Thus, to ensure your safety, perform the following discharge procedure.

Materials Required:

• Safety goggles
• Ungrounded foam pad
• Needlenose pliers
• Alligator lead with clips at both ends
• CRT discharge tool - There are three types of discharge tools:
  • Apple CRT Discharge Tool
  • Older Apple CRT Discharge Tool
  • Screwdriver and wire lead

Apple CRT Discharge Tool

Apple recommends the version of the Apple CRT Discharge Tool (Part No. 076-0381). This version features a stainless steel alligator clip, thicker insulation, and a hand guard. No internal resistor is installed so the tool gives auditory feedback (the electrostatic crack) to assure the technician, when discharging any monitor without a bleeder resistor, that the CRT is indeed discharged.

Older Apple CRT Discharge Tool

The older version of the Apple CRT Discharge Tool (Part No. 076-0243) has a copper alligator clip but no hand guard. It has a built-in 100-megohm resistor, which means you will not hear an electrostatic crack when the CRT discharges, even if the monitor has no bleeder resistor. This older tool was designed for the original Macintosh or Macintosh Plus logic board. A resistor was installed to diminish the impact of the electrostatic discharge if a technician accidentally discharged the CRT to the metal chassis instead of to the ground lug. (The Macintosh logic board circuitry is grounded to the chassis.) This older tool is safer for use on the Macintosh or Macintosh Plus CRT.

Screwdriver and Wire Lead

If you do not have a discharge tool, you can use an insulated screwdriver attached to a wire lead with alligator clips on both ends as shown here. You should also use this discharge method as a follow-up after using the older Apple CRT discharge tool to ensure that the CRT is discharged.

CRT Discharge Procedure Overview

These are the major steps for discharging a CRT:

1. Set up a CRT-safe electrical area.

2. Remove the housing.

3. Attach the discharge tool to the ground lug on a built-in monitor. Attach the discharge tool to the metal chassis on a stand-alone monitor.

4. Touch the discharge tool probe to the anode aperture.

1. Set Up CRT-Safe Electrical Area

2. Read the CRT Safe Electrical Setup Precautions section in this manual before you proceed!

3. Turn off the power and disconnect the AC power cord.

Warning: To prevent serious injury, do not touch the yoke assembly, yoke cable/connector, high voltage cable, anode connector, flyback transformer, the inside of the AC power switch, the primary fuse, any soldered connections or exposed uninsulated wires. If you have questions as to the location of these parts in the monitor or computer on which you are working, refer to the manual for that product.

4. Remove any metal jewelry and grounding wriststrap.

5. Disconnect the snap fastener on the grounded workbench mat.

6. Put on safety goggles.

Remove the cover and any internal shielding (refer to the Take Apart chapter for the monitor or Macintosh computer on which you are working).

Set the monitor or Macintosh upright on the ungrounded foam pad, with the back facing you.

Attach Discharge Tool to Ground

Ground for Computer with Built-in Monitor

Caution: If you are working on a compact Macintosh that contains a CRT and incorrectly discharge the CRT to the metal chassis, you may destroy the logic board. You must discharge to the ground lug!

Attach the clip of the CRT discharge tool to the ground lug of the computer.

Ground for Stand-Alone Monitor

If you are working on a stand-alone monitor, attach the clip of the CRT discharge tool to the monitor chassis.

Touch Probe to Anode Aperture

1. Put one hand behind your back, and grasp the handle of the discharge tool with your other hand.

2. Hold the CRT discharge tool to the tube surface, and slide the tool probe under the anode cap until the probe touches the anode aperture.

3. Remove the probe of the CRT discharge tool from under the anode cap.

4. Detach the discharge tools alligator clip from the metal chassis or Macintosh ground lug.

Removing the CRT/Video Board

Do this only if you are replacing the board or removing the CRT.

Warning: Be sure to discharge the CRT before attempting to remove the CRT/video board.

Caution : Twisting, bending, or applying force to the CRT/video board could damage the neck of the CRT.

Remove the CRT/video board assembly from the neck of the CRT. Refer to the Take Apart chapter of the monitor or computer for steps to remove the CRT/video board.

Establishing an Ongoing Lead to Ground

A discharged CRT can build up a charge. If a discharged CRT must remain exposed for any length of time, you must establish an ongoing lead to ground.

Warning: Be sure to discharge the CRT before attempting to establish an ongoing lead to ground.

1. Using needlenose pliers, compress the anode cap to free it from the anode aperture.

2. Connect one end of an alligator lead to ground.

3. Connect the other end of the alligator lead to the anode aperture.