Apple 12" RGB Display Troubleshooting Guide

Exploded view with the Rev. A and Rev. B part numbers (Apple Service Source)

Fault-finding for the Macintosh 12" RGB Display: Apple's symptom charts first, then faults that owners have found and repaired at component level. Apple lists cures in order of likelihood; refit the original module before the next cure if a replacement did not help. Boards and tubes come in Rev. A and Rev. B versions that must not be mixed; see Revisions.

Discharge the CRT before working inside; see CRT Discharge Procedure. Apple's take-apart procedures start by discharging the CRT and removing the anode cap, and warn never to wear a grounding wrist strap until it has been discharged.

Apple's symptom charts

No raster and geometry (Service Source, Symptom Charts pp. 2-6)

Symptom	Cures, in Apple's order
No raster, LED off	Check the video cable. Check the power cord. Check the internal power connectors. Replace a blown fuse. Replace the external power cable assembly. Replace the main deflection board
No raster, LED on	Check the video cable. Adjust contrast and brightness. Check the video card. Check the main deflection board and CRT/video board connectors. Replace a blown fuse. Do the screen voltage adjustment. Replace the main deflection board. Replace the CRT/video board. Replace the CRT
Raster short, tall, narrow or wide	Adjust V-HEIGHT or H-WIDTH on the main deflection board. Replace the main deflection board. Replace the CRT
Raster not centred	Move the display to rule out the environment. Adjust H-CENT or V-CENT. Replace the main deflection board
Horizontal linearity bad	Replace the main deflection board
Vertical linearity bad	Adjust V-LIN. Replace the main deflection board
Raster bows	Move the display. Check V-HEIGHT, V-CENT, H-WIDTH and H-CENT. Some bowing from the environment is within tolerance. Replace the main deflection board. Replace the CRT
Whole raster tilted	Move the display. Switch off, remove the rear cover, loosen the frontmost screw on the CRT neck, twist the yoke and retighten (not too tight, or the neck can break). Replace the main deflection board
Other distortion	Move the display. Check all cables. Do the geometry adjustments. Replace the main deflection board, then the CRT/video board, then the CRT

Synchronisation, video and miscellaneous (Service Source, Symptom Charts pp. 7-10)

Symptom	Cures, in Apple's order
Picture breaks into diagonal lines	Try another monitor to check the video signal. Adjust H-HOLD. Replace the main deflection board
Picture rolls vertically	Check the video card. Adjust V-HOLD. Replace the main deflection board
Single vertical or horizontal line	Check yoke connector DY. Replace the main deflection board. Replace the CRT
Predominant red, blue or green tint	Check the video cable. Check the video card. Do the white balance adjustment. Replace the CRT/video board. Replace the CRT
Picture too dark or too bright	Adjust contrast and brightness. Check the video card. Do the cutoff and white balance adjustments. Replace the main deflection board, then the CRT/video board, then the CRT
Cannot adjust brightness, contrast or colour	Replace the contrast/brightness assembly. Replace the main deflection board. Replace the CRT/video board
Out of focus	Adjust the focus control on the flyback transformer. Do the screen voltage adjustment. Replace the main deflection board. Replace the CRT
Shuts down intermittently	Check the video cable. Replace the main deflection board
Picture jitters or flashes	Check the ground cables. Check nearby equipment is grounded, move electrical devices away and switch off fluorescent lights. Replace the main deflection board
Flashing or wavy screen	Crimp the metal tabs on the video connector. Replace the main deflection board
Black spots (burnt phosphor)	Replace the CRT

The adjustments named are on the general maintenance page.

Fuse

The fuse is F901 on the main deflection board. Remove the rear cover, discharge the CRT and remove the anode cap, then lever the fuse out of its holder with a small flat-blade screwdriver. Apple lists it only as part 941-5222, without a rating.

Faults found by repairers

Apple treated the main deflection board as a module. Owners repairing these boards today report the following; they are individual repairs, not Apple data.

• A collector who keeps many of these displays reports that distortion in the upper part of the picture is cured by replacing capacitors on the main deflection board, and that focus which goes soft after about 30 minutes points to the flyback transformer, which has the focus control built in and is replaced as a unit.
• No picture, no power light and a high-pitched whine after a recap: in one repair the board had been corroded by electrolyte leaking from the capacitors around C418 and C419, which sit next to the large resistors R525 and R534 near the board edge. Transistor Q501 had failed (it tested as a resistor). Before the repair the 80 V test point read 13 V, the 12 V test point 1.5 V, and the +12 V feed to the flyback 0 V; replacing Q501 with a KSC2383 restored all three.

The same repairer replaced R525 and R534, bridged the thin underside trace between them with solder, bent the resistors away from the capacitors and shielded the capacitors with Kapton tape, because the resistors run hot and cook the capacitors beside them. Scrape any corrosion back to bright copper and seal the area afterwards.

When swapping boards between displays, one collector matches donor boards to a display made in the same month, because the CRT and board pairings changed during production.

Related pages

• Apple 12" RGB Display
• Apple 12" RGB Display General Maintenance
• Macintosh 12 RGB Display Service Source
• Capacitor Failure Symptoms
• CRT Discharge Procedure