iMac G3 - Screen Discoloration

In the PDF repair guide from iFixit site (I've one in english) there is an image

of the iMac G3 logic board and the button to reset power management is

labelled in the picture. Partially disassembly is required to access the button.

And as Jeff mentioned, some degaussing may help; though that doesn't

reset the PMU, or whatever official name that era used in this reference.

So to study the required steps to access the 'power reset button' & try

to safely re-set the circuit, is something you should be able to do OK.

Also, in order to fix the clock setting between shutdowns where no power

cord is attached, you may choose to replace the clock/PRAM battery while

poking around inside. The part: 1- 3.6V 1/2AA Lithium; a replacement fits in

just the same way as the old one comes out. Polarity is important.

Oh, if you choose to keep and use this G3 iMac, before attempting to upgrade

(or even start up from a later OS X than 10.2.8 on installer disc) you would need

to see about updating the Firmware to perhaps version 4.1.9. ~ Depending on

which iMac G3 you have, the version may vary. Older models cannot use 4.1.9.

• iMac Firmware Update 4.1.9: Information and Download - Apple Support

If the firmware is not updated to a later version there may be a major problem

or even a hardware failure where the display may remain black on startup.

Good luck & happy computing! 🙂

Another possibility is this may be a visual symptom of failure

in the video analog board or other component that supports

the display. ~ Of course good parts are harder to locate.

Stuff with odd names such as flyback transformer, PAV,

and the like. And old sketchy solder joints to consider.

And may not be cost effective unless you locate extra iMacs

of similar vintage.... Then carefully don't electrocute yourself

in the process of learning how to trade parts between them

to rebuild the perfect beast.

Perhaps there is a reset procedure that may help see if the

display is really a problem; such as PRAM reset or a power

management reset. An original service manual may show

a hidden button on the logic board.

Several power-management reset methods were used among

different vintage Apple computers; a few are covered here

http://thexlab.com/faqs/resetpmchip.html (reset power chips)

but does not appear to have an easy keyboard shortcut for an

iMac G3. ~ Some had a reset button on the outside the case,

by the ports; others had one on the logic board by the battery.

So your quest to restore an early edition iMac G3 may be

fraught with mystery, intrigue, headache, & maybe shocks.

You may be able to locate an original PDF service manual.

To identify the exact build model can help locate the guide.

In any event...

Good luck & happy computing! 🙂

Color CRT (cathode ray tube) picture tubes traditionally had 3 separate color guns (red, blue, and green) and a shadow mask to aim the colors on the screen. Sony had a better design - the Trinitron CRT. It contained a single gun combining all 3 colors and used an aperture grille of fine wires instead of a shadow mask. A pair of very fine, white horizontal lines, about 1/3 and 2/3 down the screen, are visible on single-gun CRTs. Both the shadow mask and aperture grille can be affected by external magnetic interference, causing a rainbow-like distortion of the screen. Your iMac's screen's imperfection is so evenly distributed in a single hue, that degaussing may have no effect. When you power ON the computer, a self-degaussing feature occurs to eliminate acquired color impurities. After powering OFF the computer and letting it sit for at least 15 minutes, you should power it ON to activate the degaussing feature again. If there is no change in the screen's appearance after several cycles of this, the CRT itself may have an internal problem. New parts aren't available and used ones are typically pulled, high-mileage parts with indeterminate remaining service life. At some point, the P-A-V board goes in those iMacs and that's when it's a good time to retire the 16/17 year-old computer.

"I'll try to replace the faulty part without hurting myself too much."

The stored voltage at the CRT, the P-A-V board, and flyback transformer is enough to electrocute you, which is the definition of hurting oneself too much. If you're not experienced with the CRT discharging procedure, don't tinker blindly in those areas. The label warnings affixed on the interior of older TVs or displays were reminders to experienced technicians, as well as do-it-yourself repairman. The other problem with opening up those iMacs is related to the plastic housings and the inflexible nature of the old/stiff, invisible snap-catches that secure the pieces with a snug fit. Inserting a small tool to pop them free often causes them to break off, making it impossible to reassemble the housing as designed.

It will most likely require that the CRT in that iMac be replaced.

Here is a disassembly guide if you need to take it apart. iMac G3 disassembly guide - iFixit