Apple 21" Color Display General Maintenance

Exploded view with Apple part numbers (Apple Service Source)

Adjustment of the Macintosh 21" Color Display to Apple's figures. Everything here comes from Apple's Service Source. The procedure needs a compass for magnetic alignment, Apple's diagnostic overlay for geometry and a voltmeter for the horizontal hold, and the focus and convergence controls are spread over the high-voltage block, the main board, the magnetometer degauss board and the yoke.

Safety

The CRT keeps a charge after switch-off. Follow the CRT Discharge Procedure before reaching inside, and do not wear a grounding wrist strap until the CRT has been discharged. All of the adjustments are made live with access panels removed; Apple marks the anode connector and wires, the yoke, the flyback transformer and the high-voltage block as parts not to touch, and uses insulated screwdrivers. Apple's general rules for live work: do not work alone, remove jewellery, wear safety goggles, keep one hand behind your back, and do not use a wrist strap or grounded mat.

Aligning to magnetic north or south

The degaussing circuitry and magnetometer make the picture depend on direction, so Apple aligns the display before any adjustment.

1. Keep the display at least three feet from metal objects such as steel filing cabinets.
2. Find magnetic north/south with a compass and put the display on a protective pad with the bezel facing north or south.
3. Put the compass on the bezel and swivel the display until the needle points to north (360 degrees).

Apple also says to rule out the location before adjusting at all: if the picture changes when the display is moved or turned, the cause is the environment.

Geometry

Remove the rear cover and the main board access panel, switch on, let the display warm up for 15 minutes and align it north/south. You need a compass, a voltmeter and Apple's diagnostic overlay (part 076-0444). Tilt the display up slightly, tape the overlay against the inside left and bottom edges of the bezel, and take every measurement with the same eye held directly above the point being measured, because straight lines on a screen this size look bent.

Geometric alignment controls (Service Source, Troubleshooting pp. 7-9 and Adjustments pp. 7-23)

Symptom	Control
Raster too tall or too short	V-SIZE (R52): height 280 mm ± 3 mm
Raster too wide or too narrow	H-SIZE (R93): width 371 mm ± 3 mm
Raster shifted left or right	H-PHASE (R262) and H-DC SHIFT (R117)
Raster shifted up or down	V-SHIFT (R75)
Rectangles larger at one side than the other	H-LIN (R101)
Rectangles larger at top than bottom	V-LIN (R26)
Raster not straight at top or bottom	N/S PHASE and N/S TRAP (R69), then N/S (R34) and N/S BAL (R28)
Centre line not horizontal	N/S TILT (R74)
Centre line not straight	N/S BAL (R28)
Sides not straight	E/W AMP (R89)
Sides not vertical	E/W TRAP (R80)

Horizontal hold

Display the All-White Screen. Cut a plastic alignment tool down to 3 in long, set the voltmeter to its 200 V DC range with its ground on the chassis, and adjust H-HOLD until the meter reads 4.1 V ± 0.2 V at test point TP254. Remove the overlay afterwards.

Convergence

Apple says some misconvergence is normal on a large screen and that the yoke convergence controls are set at the factory and should never need readjusting. Before going through the whole procedure, use the Convergence Alignment chart below to pick the one control that matches the fault.

Convergence alignment (Service Source, Troubleshooting pp. 10-15)

Red and blue visible in...	Control
Vertical lines (centre)	H-STATIC on the magnetometer degauss board
Horizontal lines (centre)	V-STATIC on the magnetometer degauss board
Top 1/4 in (5 mm), 2 in (50 mm) or 4 in (100 mm) of vertical lines	Yoke controls VR2, VR3 or VR5 respectively
Bottom 2 in (50 mm) or 4 in (100 mm) of vertical lines	Yoke controls VR4 or VR6
Top 2 in of the leftmost and rightmost vertical lines	AMP-T, then TILT-T (TILT-T affects the left more than the right)
Bottom 2 in of the leftmost and rightmost vertical lines	AMP-B, then TILT-B
Vertical lines at the left and right edges	H-RIGHT, then H-LEFT1
Vertical lines 3-5 in (75-125 mm) from the edges	H-LEFT2
Vertical lines on the left third	1/2W
Horizontal lines at the top and bottom edges	V-NS, then V-N
Centre horizontal lines at the left and right edges	V-EW
Left end of the centre horizontal line	V-W

Focus

With the CRT access cover off, display Crosshatch.

1. Turn V.DYNAMIC FOCUS (R155) on the main board fully clockwise.
2. Adjust FOCUS2 on the high-voltage block until the centre vertical line is as thin as possible.
3. Adjust FOCUS1 on the high-voltage block until the centre horizontal line is as thin as possible.
4. If the horizontal lines at the top and bottom are not sharp, turn V.DYNAMIC FOCUS back anticlockwise until they are as thin as possible.

Video

Checking the picture

Apple adjusts video only if the picture is unacceptable. Warm up for 15 minutes and align north/south. Set brightness to its detent and contrast to maximum and display Gray Bars: there should be no tint, the brightest bar should read 27 foot-lamberts ± 3 (between 21 and 22 on the bottom scale of the R77 meter, the low end of the 10 scale on the L-248, 21.5 on the red scale of the 246), and the three darkest bars should be black, barely distinguishable from black, and dark grey.

Adjusting

Remove the rear cover and the CRT and video board access panels. You need a compass, light meter and voltmeter.

1. Display the All-Black Screen. Set the external brightness to maximum and contrast to minimum.
2. Preset SCREEN (on the high-voltage block), C-LIM, BRT PRESET, the B, G and R GAIN and the B, G and R LEVEL controls (on the video assembly) to minimum.
3. With the voltmeter on its 200 V DC range and grounded to the chassis, adjust BRT PRESET until the voltage at G1 rises by 4 V ± 0.2 V. G1 is negative, so −61 V would become −57 V.
4. Adjust SCREEN until the raster just appears; the whole screen should be dark with an obvious tint.
5. Set brightness to its detent and contrast to maximum. Set the three GAIN controls to mid-range and BRT PRESET back to minimum.
6. Display the All-White Screen and adjust C-LIM for 27 foot-lamberts at the centre. If that cannot be reached, turn the three GAIN controls a quarter turn clockwise and try again.
7. Display Gray Bars. Adjust the red and blue LEVEL controls until the five darkest bars have no tint, using green LEVEL only if red and blue cannot do it. Then adjust the three GAIN controls until the five brightest bars have no tint, and repeat both until all the bars are neutral.
8. Adjust SCREEN until the darkest bar is as black as the border and the next is barely distinguishable from it, recheck the tint, and finally recheck the luminance on the All-White Screen with C-LIM.

Related pages

• Apple 21" Color Display
• Apple 21" Color Display Troubleshooting Guide
• Macintosh 21 Color Display Service Source
• CRT Discharge Procedure
• Recommended Tools