Apple 21" Color Display Troubleshooting Guide

LEDs V125 and V810 on the main board (Apple Service Source)

Fault-finding for the Macintosh 21" Color Display. Apple's procedure for a dark screen reads six status LEDs; for everything else it uses symptom charts, with cures listed in order of likelihood. Refit the original module before trying the next cure if a replacement did not help. Rule out the location first: large displays bend near metal and jitter near fluorescent lights and appliances, and module swaps cannot cure that.

The CRT keeps a charge after switch-off. Follow the CRT Discharge Procedure before working inside, and do not wear a grounding wrist strap until it has been discharged. The LED checks below are made with power on and the shields removed: keep clear of the anode, flyback and high-voltage block.

Dark screen: the LED flowchart

Use this only for a no-raster fault. Switch off, remove the rear cover and the power supply and main board shields, switch on and look at the LEDs. All six are normally lit (steady, not flashing).

Status LEDs

LED	Board	Monitors
V125	Main board	Horizontal and vertical sweep
V810	Main board	High voltage
V683, V699	Power supply board	The power supply board
V684, V685	Power supply board	Several conditions; must be on steadily

1. If all four power supply LEDs are on and steady, check V125. If V125 is off, replace the main board. If it is on, check V810: off means the flyback transformer assembly; on means the video assembly.
2. If V683 or V699 is off, replace the power supply board.
3. If V684 or V685 is flashing, switch off and unplug the ribbon cable from X105 on the main board, then switch on. If the LED still flashes, replace the power supply board. If it stops, reconnect X105 and unplug X303 on the video board; if the flashing stops, replace the video assembly. If not, reconnect X303 and unplug X855 on the magnetometer degauss board; if the flashing stops, replace that board; if not, replace the flyback transformer assembly.
4. If all the LEDs are off or flashing, switch off and check the fuse on the on/off switch assembly. If it is intact, replace the power cord. If it has blown, replace it and switch on: if it blows again, replace the power supply board; if not, leave the display running for two hours and check again.

Apple's documents give the fuse no rating and no separate part number; it sits on the on/off switch filter assembly, part 076-0398.

Symptom charts

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

Symptom	Cures, in Apple's order
No raster, LED off	Check the power cord connection. Replace the power cord. Check all internal power connectors. Work through the LED flowchart
No raster, LED on	Adjust contrast and brightness. Check the video card. Check the connectors on the main board and power supply board. Work through the LED flowchart
Raster too short, tall, narrow or wide	Adjust V-SIZE or H-SIZE. Replace the main board
Raster not centred	Move the display. Adjust H-DC SHIFT or V-SHIFT. Replace the main board. Replace the magnetometer degauss board
Horizontal linearity bad	Adjust H-LIN. Replace the main board
Vertical linearity bad	Adjust V-LIN. Replace the main board
Raster tilted	Move the display. Do the geometry adjustments. Replace the magnetometer degauss board. Replace the position sensor board
Other distortion (corners not square, sides tilted or barrelled, top stretched or compressed)	Move the display. Check all cables. Work through the LED flowchart. Do the geometry adjustments. Replace the main board. Replace the magnetometer degauss board
Picture breaks into diagonal lines	Check the video signal with a known-good monitor. Replace the main board
Picture rolls vertically	Check the video card. Replace the main board. Replace the video assembly
Picture breaks and rolls horizontally	Check the video card. Adjust H-HOLD. Replace the main board. Replace the video assembly
Single vertical or horizontal line	Check the CRT video board is tight on the yoke connector. Replace the main board

Video and miscellaneous (Service Source, Symptom Charts pp. 16-18)

Symptom	Cures, in Apple's order
Predominant red, blue or green tint	Check the video card. Do the video adjustments. Replace the video assembly. Replace the video filter boards. CRT: international units replace it; domestic units call Apple to confirm a CRT failure
Picture too dark or too bright	Adjust contrast and brightness. Check the video card. Do the video adjustments. Replace the power supply board, then the video assembly, then the video filter boards. CRT as above
Out of focus	Adjust the focus controls. Replace the flyback transformer assembly. Replace the main board. CRT as above
Out of convergence	Check the video card. Use the convergence alignment chart and adjust the matching control. Do the convergence adjustments. CRT as above
Shuts down intermittently	Replace the flyback transformer assembly. Replace the main 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 board
Jitter or a wavy image at 1152 × 870 on a Power Macintosh 9600	Not the display: Apple/IMS video cards with serial numbers below Fx705xxx cause it on any monitor; Apple replaced them with part 661-1409
Black spots (burnt phosphor)	CRT as above
High-pitched noise	Replace the flyback transformer assembly
Does not degauss	Replace the magnetometer degauss board. Replace the position sensor board
ADB devices do not work or work erratically	Replace the keyboard, mouse or other ADB device. Replace the ADB cable in the stand

The adjustments named are on the general maintenance page, including the convergence alignment chart that matches each colour fringe to its control.

Rev. A and Rev. B

Rev. A CRTs need Rev. A main boards and Rev. B CRTs need Rev. B main boards. A Rev. B CRT has a small blue "NEW" label near the anode. A Rev. B main board has capacitor C153, fitted on either the component or the solder side; a Rev. A board has none, and Apple converts a Rev. B board for a Rev. A tube by removing C153.

Related pages

• Apple 21" Color Display
• Apple 21" Color Display General Maintenance
• Macintosh 21 Color Display Service Source
• CRT Discharge Procedure