This chapter is from the book
This chapter is from the book
This chapter is from the book
CRT Safety Procedures
Over the last few years, cathode-ray tube (CRT) displays have been used in fewer and fewer computers. In the Macintosh systems, only one model, the eMac, has been available. From June 2002 until mid-2006, the eMac was sold to schools and some private parties. While CRT-based Macintosh systems are no longer sold, they will be a part of the supported product line for years to come. At some point, you may have to service a CRT system.
Between the changing and improving technology and the lowering of prices, attempting to fix a CRT unit should rarely be your first option. If you are not an Apple Authorized Service Provider (AASP) or in a certified training course, you should not attempt to do so at all.
Risks
The CRT is one of the most dangerous pieces of equipment you’ll repair. Happily, there are very few occasions when you should have to open a display and expose a CRT. Yet because of the risks inherent in working with CRTs, it is extremely important that you know what to do and what not to do when troubleshooting and servicing Macintosh computers that contain them.
CRTs are glass vessels that have the air pumped out of them. They have very thick glass in the screen area and thinner glass in the narrow neck area. This makes a CRT fairly fragile when it is not encased in a computer or bezel. The neck area is particularly easy to break or crack.
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The vacuum tube in a CRT can implode if it is broken or punctured. The surrounding air will rush violently into the unsealed vacuum in the CRT, spraying broken glass in every direction.
Color CRTs may contain mercury or other potentially toxic materials. If the CRT is broken or cracked, these materials may be released and pose a risk of toxic exposure.
A charged CRT carries high voltage—about 27,000 volts in a color unit. You could electrocute yourself unless you handle the display using the appropriate safety procedures.
Safety
If you handle a CRT properly, neither you nor the display will come to any harm. There are several CRT safety procedures that can keep you safe.
Handling CRTs correctly consists of not placing stress on the neck portion of the CRT assembly. Since the neck has thinner glass, you should never lift a CRT by the neck. It is also important to handle CRT modules carefully when lifting them or putting them down.
If you must transport a CRT module, always make sure that it is in a shipping package or installed in the computer.
These areas of a CRT can present a shock hazard:
- Anode cap and connector
- High-voltage cable
- Yoke assembly
- Flyback transformer
- Any exposed soldered connections
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Dangerously high voltages flow through these parts until the display is disconnected from its power source and properly discharged. Do not touch any of these parts inside the product housing until after the display is disconnected from its power source and properly discharged.
In typical repair situations, you should always follow ESD precautions while working inside a Macintosh. This means setting up an ESD-compliant workstation and consistently following all ESD rules.
However, working around a CRT inside a Macintosh, or working around the inside of any Macintosh while it is powered on, can bring parts of your body dangerously close to hazardous voltages. Thus doing so requires an exception to the ESD rules.
Being grounded in these situations is extremely dangerous because your grounded ESD wrist or heel strap and grounded ESD workbench mat create a path through your body to ground. If you accidentally contact high voltage, the current has a clear path through your body and can electrocute you.
To work safely on a CRT inside a Macintosh, follow these safety rules every time:
- Never work alone. Having someone nearby in case of an accident could save your life.
- Turn off the power and disconnect the AC power cord before you remove the CRT cover.
- Remove any metal jewelry.
- Remove the grounding wrist or heel strap until the CRT has been discharged.
- Disconnect the snap fastener on the grounded workbench mat until the CRT has been discharged.
- Wear safety goggles.
- Discharge the CRT immediately after removing the case and before touching anything inside the system or display. (The CRT-discharge procedure is discussed next.)
- After discharging the CRT and turning off the CRT power, reconnect and wear a grounding wrist or heel strap.
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Discharge
By now you shouldn’t have to be reminded that CRTs carry a high voltage and can be dangerous. But you still have to work on them sometimes. So how do you do that without electrocuting yourself?
Newer Apple CRT displays are equipped with a bleeder resistor (contained in the flyback transformer) 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. For that reason, Apple requires all service technicians to discharge all CRTs before performing repairs.
The Apple discharge procedure is a precautionary measure to confirm that the CRT has been discharged prior to working on it.
After completing this section, you will be able to safely discharge the high voltage from a CRT.
When discharging a CRT, you need the following equipment:
- Safety glasses
- Ungrounded foam pad
- Needlenose pliers
- Wire lead with alligator clips at both ends
- CRT discharge tool
To ensure your safety, follow Apple-recommended CRT discharge procedures. Search the Knowledge Base and Service Source for the display or Macintosh you are servicing.
Before you do anything, including discharging a CRT, turn off and unplug the display or Macintosh. Then follow these steps:
- Follow the first six CRT safety rules, listed in the preceding section, to prepare to discharge the CRT.
- Remove the housing.
If you have access to them, refer to the Take Apart instructions in the appropriate service manual for your Macintosh or display.
- Put one hand behind your back.
- Using the Apple CRT discharge tool shown below, connect the alligator clip from the lead to the ground lug on all-in-one systems.
- Slide the discharge tool probe under the anode cap and into the anode aperture. As soon as you can feel the metal of the probe touching the metal of the aperture, the CRT is discharged.
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If a discharged CRT must remain exposed for any length of time, establish an ongoing lead between the anode and ground.
When discharging a CRT, use only the ground lug to make your ground connection on a Macintosh to prevent damage to the logic board. Any high voltage that may be present is safely discharged to ground circuits (on the power/sweep assembly), which are designed to handle such voltage.
Disposal
Use the following instructions for returning color CRTs, whether in-warranty or out-of-warranty.
Some dead CRT assemblies, specifically color CRT assemblies, cannot be thrown away with regular trash because they have the potential of becoming hazardous waste.
As with dead lithium, lead-acid, nickel-hydride, and nickel-cadmium batteries, AASPs should return dead Apple color CRT assemblies directly to Apple if the original packaging is available.
When returning dead color CRT assemblies:
- Do not release the vacuum.
- Enclose them in the packaging in which they were originally shipped.
If you no longer have the original packaging, do not return color CRTs to Apple. Instead, dispose of CRT assemblies according to your local hazardous waste ordinances.
Similarly, broken CRTs (for example, monitors with cracked glass) must not be returned to Apple. Dispose of any broken color monitor CRT assemblies according to your local hazardous waste ordinances.
Remember that CRT displays present these basic dangers:
- CRT displays may implode if mishandled.
- CRT displays may contain hazardous materials.
CRT Safety Quiz
- Name the major risks of working on CRTs.
- Is carrying CRTs by the neck recommended?
- Name one toxic material that can be found inside CRTs.
- You are troubleshooting an eMac for a no-video issue. You want to open the system to check internal cabling. What is the first recommended step you take?
- What are the eight CRT safety rules?
- You have an Apple CRT that is cracked and you do not have the original packing for the part. How do you dispose of it?
Answer Key
1. Implosion (flying glass), hazardous materials if CRT is cracked or broken, lethal shock hazard; 2. No; 3. Mercury; 4. Make sure someone is in the room with you; 5. a) Never work alone; b) Turn off the power and disconnect the AC power cord before you remove the CRT cover; c) Remove any metal jewelry; d) Remove the grounding wrist or heel strap until the CRT has been discharged; e) Disconnect the snap fastener on the grounded workbench mat until the CRT has been discharged; f) Wear safety goggles; g) Discharge the CRT immediately after the case has been removed and before touching anything inside the system or display; h) After you have discharged the CRT and turned off the CRT power, reconnect and wear a grounding wrist or heel strap; 6. According to your local hazardous waste ordinances