CRT 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 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 cathode-ray tube.
NOTE
The CRT discharge component of this lesson is intended for service technicians working under the direct supervision of an AASP. Do not attempt the CRT discharge part of this lesson if you are an individual studying AppleCare Technician Training.
Required Tools and Equipment
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
CRT Discharge Procedure
To ensure your safety, you should follow Apple-recommended CRT discharge procedures. Search the Knowledge Base and Service Source for the display or Mac you are servicing.
Before you do anything (including discharging a CRT), turn off and unplug the display or Mac.
1. | Set up an ESD-safe electrical area and follow the Apple-recommended CRT safety rules, as covered in Lesson 4, “Safe Working Procedures and General Maintenance.” |
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3. | Put one hand behind your back or in your pocket. This helps to prevent current from passing through your heart if you touch a high-voltage area. |
4. | Using the Apple CRT discharge tool shown below, connect the alligator clip from the lead to any metal part of the chassis on standalone displays or to the ground lug on all-in-one systems. The length of the tool's handle helps prevent you from accidentally touching parts inside the display.
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5. | Slide the discharge tool probe under the anode cap and into the anode aperture. 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) that are designed to handle such voltage. |
