Adjusting iMac CRT colour guns


I've just had an iMac in for repair and I thought I'd share how it was done here.

Here's the problem...

This slot loading iMac came in with the wrong colours on the screen. Didn't look too bad - white was white but there was too much red and not enough green. This was evident in the startup screen which should have been grey on power up but was purple instead.

Now, in the good ol' days of CRT monitor repair it was just a case of popping the cover and adjusting the POTs inside which controlled the Red, Green and Blue colour guns, but the iMacs are cleverer than that. The Monitors control panel wasn't enough for the job and connecting an external monitor confirmed the video hardware was okay (as the external screen looked fine), it was just the internal CRT that was bad.

You will need two things if you want to carry out your own colour gun adjustments....

1. A handy bit of OS9 software called Display Adjustment Utility
2. A monitor adjustment jumper cable (Apple part #922-4165)

This article used to link to a location where the Display Adjustment Utility could be downloaded from but that link has died and I have been unable to find a replacement. Apple are pretty protective about software such as this and R3UK Limited can't distribute it so if you don't have a copy you may be scuppered (a visit to your nearest Apple repair centre will be needed). Unfortunately R3UK Limited cannot distribute the DAU software.

Good news if you can't get hold of the official Apple adjustment cable though as I'll show you how to make one (and where to stick it !)

Important disclaimer:
Unless you're a dab-hand with a screwdriver, don't attempt any of this. Messing around inside your computer could cause damage and in certain cases can be hazardous to your health. This page is just a guide - you follow these instructions at your own risk. You should be careful handling the internals of a computer and follow antistatic precautions or you may cause damage. This is a dangerous procedure. You will come into close contact with high voltage electronics if you follow these instructions. Only proceed if you are qualified to do so, otherwise call in a suitable technician.

The purpose of the jumper cable is to allow the Display Adjustment Utility to save your changes as factory default. Without it you will be able to play with your iMacs CRT settings but you cannot save changes. The jumper cable is nothing more than a bit of wire with suitable connectors at each end. You can make one from an old PC power supply as below:

1. Start with a standard molex power connector on an old power supply such as that pictured below...

2. Use a Dremel to cut the two end connectors off. Cut the wires (giving yourself a few centimetres to play with) and join them. I soldered the wires together and covered the join with heat shrink. The finished product should look as below...

That's the cable made. Now for insertion.

Switch off and disassemble your iMac as per our instructions here. Take it as far as step 11 (you can skip step 5). Now disconnect the left ground wire from the back of the tube (arrowed green in the picture below) and fit one end of your jumper to one of the posts. Connect the other end to the post in the hole below the ground connector (arrowed red).

Switch on the iMac and run the Display Adjustment Utility. With this jumper in place you can now save your settings as factory default (assuming you didn't kill yourself by touching the wrong thing!). Here's a screenshot of the utility in action....

A couple more points to note...

If you turn down your display settings too far, you won't be able to see the controls to turn them up again (Doh!). Use the "Command-R" key combination to revert settings.

Making adjustments may knock out your focus. This can be adjusted with a POT on the side of the flyback transformer (big thing on the left side when facing the iMac), as can overall screen brightness (be careful not to turn this up too high as it might overload).



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