RetroTechCollection

Apple 12" RGB Display General Maintenance

Adjustment of the Macintosh 12" RGB Display, following Apple's Service Source.[1] The display has three groups of adjustments: the CRT screen (G2) voltage, geometry, and video (white balance and luminance). All of them are made with the display running.

Geometry adjustment controls on the main deflection board, rear cover removed (Apple Service Source)

Safety

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The CRT holds a charge after the display is switched off. Follow the CRT Discharge Procedure before touching anything inside, and do not wear a grounding wrist strap until the CRT has been discharged.[2] Apple's rules for live work on its monitors: do not work alone, remove jewellery, wear safety goggles, never use a wrist strap or grounded mat during live adjustments, and keep one hand away from the chassis.[3] Adjustments are made from the rear: watch the screen in a mirror and do not reach round the cabinet.[1]

Apple says not to touch the yoke adjustments, which are set at the factory. The one exception in its symptom charts is a tilted raster, corrected by loosening the frontmost screw on the CRT neck, twisting the yoke and retightening without overtightening, since the neck can break.[1][4]

Screen voltage

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Apple calls for this only if the main deflection board has been replaced, the screen control has been moved, or a CRT/yoke of a different revision has been fitted. It must be done before any other adjustment after a main deflection board change.[1]

  1. Use Apple's high-voltage probe 076-0392, or one with a 40 kV DC rating and 1000 MΩ input impedance; Apple warns that other probes can damage the monitor. Set the voltmeter to its 2 V DC range.
  2. Switch on and let the display warm up for at least 10 minutes. Clip the probe's ground to the metal chassis.
  3. Set contrast to maximum and brightness to the centre detent. Display the Full Black Screen and turn SUB-BRIGHT (VR202) fully anticlockwise.
  4. Read the CRT type from the label on the side of the tube.
  5. Touch the probe to the test point marked SCREEN (US version) or PIN 4 (international version) and adjust the screen control for 500 V on a type AT12A9SL tube, or 400 V on a type M29JMN097X13 tube. Keep the probe from shorting to neighbouring parts.
  6. Remove the probe, display Gray Bars and set SUB-BRIGHT so the first bar is black and the second barely visible.
  7. On the All-White Screen set SUB-CONTRAST (VR201) for 30 foot-lamberts; if that cannot be reached, do the white balance adjustment.[1]

Geometry

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Geometry may be needed after replacing the main deflection board, the CRT or the video board.[1]

Geometry controls and targets (Service Source, Adjustments pp. 16-28)[1]
Adjustment Control Pattern Target
Vertical size V-HEIGHT (VR404) All-White Screen 153 mm (6 1/16 in) high, ± 2 mm
Vertical centre V-CENT (VR403) All-White Screen Centred top to bottom; recheck height
Horizontal centre H-CENT (VR503) All-White Screen Centred side to side. If it will not centre, set H-CENT to mid-range, set H-PHASE (VR501) to the midpoint of its range with the cover off, then readjust H-CENT
Horizontal size H-WIDTH (L501), hex-head insulated tool, cover off All-White Screen 205 mm (8 3/32 in) wide, ± 2 mm
Vertical linearity V-LIN Crosshatch I Line spacing equal at top and bottom
Vertical hold V-HOLD (VR401) All-White Screen Raster stable; it should lock at once when the display is switched off and on
Horizontal hold H-HOLD (VR502) All-White Screen As for vertical hold
Focus FOCUS (on the flyback transformer) Focus As clear as possible

V-HEIGHT, V-CENT and H-CENT can be reached without removing the rear cover.[1]

White balance and luminance

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Apple says to do this only if the colour is unacceptable, after any geometry adjustment, with the rear cover, EMI shield and video board shield off and the display warmed up for 10 minutes.[1]

  1. Display the Full Black Screen and set brightness and contrast to maximum.
  2. Preset the GREEN, RED and BLUE CUTOFF controls to minimum, DRIVE-RED and DRIVE-BLUE to mid-range and SUB-CONTRAST (VR201) to mid-range.
  3. Turn SUB-BRIGHT (VR202) down until the raster disappears, then up until it is just visible.
  4. Adjust GREEN CUTOFF until green is the predominant colour; the screen should be dark with an obvious green tint.
  5. Display Gray Bars, set brightness to its detent, and adjust RED and BLUE CUTOFF alternately until the three left bars show no predominant colour.
  6. Check that the leftmost bar is as black as the border; if not, turn SUB-BRIGHT anticlockwise until it is. If SUB-BRIGHT cannot be set, Apple treats the monitor as needing repair.
  7. If the right-hand bars show a colour, adjust DRIVE-RED and DRIVE-BLUE until they do not, then recheck the left bars.
  8. Display the All-White Screen and set SUB-CONTRAST for 30 foot-lamberts ± 3: 23 on the bottom scale of the R77 meter, the middle of the 10 scale on the L-248, or 23 on the red scale of the 246.[1]

Apple's R77 light meter is part number 076-0310; remove its 10X multiplier plate and read the bottom (10-50 fc) scale with the lens against the middle of the screen.[1]

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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Apple Computer, Macintosh 12" RGB Display (Service Source), hosted as File:12 rgb display.pdf. Adjustments: screen voltage pp. 1-15 (PDF pp. 67-81), geometry pp. 16-28 (PDF pp. 82-94), video pp. 29-39 (PDF pp. 95-105), light meter set-up pp. 40-45 (PDF pp. 106-111).
  2. ↑ Macintosh 12" RGB Display Service Source, Take Apart, Rear Cover and CRT discharge, pp. 1-6 (PDF pp. 20-25).
  3. ↑ Apple Computer, Macintosh Displays and Video Cards service guide, September 1996, hosted as File:Apple Macintosh Displays and Video Cards Service Guide (September 1996).pdf. Ten rules to CRT safety and discharging the CRT, pp. 2-3 (PDF pp. 20-21).
  4. ↑ Macintosh 12" RGB Display Service Source, Troubleshooting, Symptom Charts, pp. 2-10 (PDF pp. 10-18).


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