Live Adjustment Rules

Some hardware failures obviously point to the need to replace the display or its components. In such cases, there's no need to supply power to the display. However, many times you can correct a display anomaly through adjustments to internal controls. In such cases, it's necessary to expose the internal controls while still leaving the display or Macintosh operational and turned on so that you can see the effects of the adjustments in real time. When making such live adjustments, be sure to use diagnostic tools such as the Display Service Utility to provide reliable test patterns.

Adjustments to CRTs vary depending on the display; however, the following basic adjustment rules apply to all models:

• If the display isn't broken, don't fix it.

• Unless screen size or alignment is seriously off, don't adjust it.

• If color values are not seriously out of line, don't adjust them.

• Use a mirror so you can see the screen image while you adjust internal controls from the rear of the unit. It will be easier for you to keep away from danger areas.

Not all displays have internal controls. Refer to the service manual on Service Source for the adjustment procedures for the specific display you are servicing.

If the display you are servicing requires live internal adjustments, follow these adjustment rules:

• Before working on any live equipment, make sure that the equipment is properly connected and grounded, and that the building's electrical outlets are properly wired and grounded.

• Make sure you are not grounded.

  During Take Apart procedures, you normally wear a grounding ESD wrist or heel strap, to prevent ESD damage to computer components. But when you wear your grounding ESD wrist or heel strap, your body has an electrical path to ground. This could be very dangerous if you were to accidentally touch high voltage, because the high voltage would go through your body, through the grounded wrist or heel strap, and to ground, electrocuting you. Before you perform any live adjustments, remove your grounding wrist or heel strap, and disconnect the ground fastener from your ESD mat. Doing this significantly reduces the chance that any high voltage will find a path through your body to ground.

• Always use a grounded three-prong outlet.

  Apple equipment comes with three-prong electrical plugs. Don't use two-prong adapters. Using a two-prong adapter to make a three-prong plug fit in a two-prong outlet could reverse the two power lines of the current going into the system, causing the hot line to be fed into the circuit intended for the neutral line. If you plug the cord in the wrong way, the entire display chassis could be made “hot.”

• Use a polarity tester to test the outlets in your service area.

  A polarity tester tests that the hot, neutral, and ground lines are wired into the plug properly. Inexpensive polarity testers are available at most electronics stores.

• Use an isolation transformer.

  The isolation transformer insulates the technician from coming between the power source and ground, decreasing shock hazard. Isolation transformers are available at most electronics stores.