It has been exhibiting some worrying symptoms. The screen occasionally flicks off for a fraction of a second, with a click sound like it's regaussing. It's also spontaneously switched off a few times - in fact it did it just now when I was writing this post. Each time it starts up again fine when I hit the power button. Sometimes, the power button stays lit, and once the external monitor continued to show a scrambled screen, even thought the iMac itself seemed to be dead.

Question: If I run with an external monitor, can I switch off or sleep the internal display? If so, will this take some of the strain off the power supply and/or reduce the internal temperature?

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After a bit of searching, I found out this is impossible. The G3 CRT iMacs are not supported by Screen Spanning Doctor, and one forum post indicated that the VGA port is hardwired to mirror the internal CRT.

It's hooked up now to my 21" Compaq, which is certainly a lot easier on the eyes at 1024x768 than the 15" internal. However neither SwitchResX or ... the other one ... support changing resolutions on the CRT iMacs, either.

One forum post suggested turning the trimpot marked "SCREEN" on the flyback all the way down to dim the internal CRT. I've taken another approach (which doesn't involve opening the son-of-a-camel case again). I've reduced the screen height and width as far as they'll go (ie lower voltage on the deflection coils) in the Display prefpane, and turned the brightness and contrast all the way down too. I think that should reduce the strain on the internal components somewhat, fingers crossed.