iMac G3 Original (1998) seems to have broken CRT.

That's good, and good to know (about the old Apple display working directly).

The flyback transformer part itself is a common part, so new replacement parts should still available. It's the complete board (which is only used in those iMacs) that is impossible to find new; even if it's unused, it will still be old. So if you are getting the flyback transformer replaced on the existing board by someone who knows how, that is probably the best approach.

Why don't you give it a try...?

800x600 should be a supported resolution, since you can use the iMac's built-in display at 800x600.

If it's stuck at 1024x768, try doing a reset PRAM at startup.

http://support.apple.com/kb/HT1379

Screen resolution is stored in PRAM, so resetting it should clear the current resolution. Whether it will pick up the 800x600 resolution and know to use it, I do not know. The best way to get the answer is to try it. 🙂

That's usually a symptom of the analog video board becoming faulty, specifically a part on it called a flyback transformer. The analog video board is along the side of the CRT (in the user-inaccessible portion of the case), and it is the component that powers and drives the CRT.

During the startup self-test, the iMac is not passing, so it powers OFF at the point where the CRT would normally power ON. Unfortunately, it is not a economical fix for most people.

You can do a test to more-or-less confirm this is the problem, if this is a tray-loader iMac (not a slot-loader). Open the bottom panel as if you were removing the internal chassis to upgrade RAM. You don't need to remove anything, just disconnect the internal video cable that connects the chassis to the rest of the iMac; it's the one that looks like an old-style Apple video connector, with the two screw-on fasteners.

With it disconnected, attempt to start up the iMac. Obviously, there will be no video. However, if the problem was on the analog video board, it will go through startup. You won't see anything on the screen, but you can tell by the sound. You can shut down without seeing the video by pressing (and releasing) the power button (on the iMac or keyboard) and then pressing the Return key.

So if it goes through the startup sequence with no video, the problem is probably the analog video board. If it still does not startup, the problem may be elsewhere, such as the power supply board.

Note: If it starts up with the video disconnected, you can connect an external VGA display to the internal video connector through an adapter. Please post back if you want more details.

It's probably not the CRT itself. It's the board that provides the high voltage needed to operate CRTs. The board is placed vertically along the side of the CRT (I can't remember if it's the left or right side). On the other side is the power supply board. Because of the high voltage that is present (even if the iMac is disconnected from power), it is dangerous poking around in there.

If it's so, can I send it to an apple service center to get the CRT replaced?

It will probably cost more to have it fixed than the value of that model working. A business that repairs CRT TV sets may be able to replace that one part on the analog video board that is usually faulty - the flyback transformer - and do it for less cost. If you replace the whole board, the replacement will be old (and maybe used) and may also go bad soon.

I would save your money and get a newer Mac. For the time being, if you have an old VGA display or can get one cheaply, you can connect it to the connector on the internal chassis and keep using that iMac as is. You need one of these adapters (or something like it).

[Apple Mac to VGA Video Adapter TVCS-183|http://cgi.ebay.com/New-Switchable-Apple-Mac-to-VGA-Video-Adapter-TVCS -183 W0QQitemZ6739937687QQcmdZViewItemQQptZPCA_Cables_Adapters?hash=item191bb4997&trksid=p3286.m20.l1116]

Connect it to the video port on the internal chassis. Connect the VGA cable that goes to the external display to the adapter. Just set the switch on the adapter to the side that says VGA, and it works. An interesting benefit is that, if your external display can handle higher resolutions, you can go as high as 1600x1200 (thousands of colors) using the iMac's built-in video (if it has the full 6mb VRAM). 1280x1024 at millions of colors. The best you can do with the built-in display is 1024x768. You can consider an upgrade (instead of a broken iMac). 🙂

Apple itself will not fix it; way too old. An Apple-certified repair business probably will, if you pay them enough (or have a neighbor friend in your case). If you know how to safely work around the exposed back end of the CRT, it's actually a fairly simple fix. The hardest part is figuring out how to remove the plastic parts; it fits together like a puzzle. The other hard part at this point may be getting a reliable replacement board. After doing those two things, it's just a matter of removing the old analog video board and putting in the replacement.

Because the replacement boards will also be old (even if they are not used), a better fix may be to have the board removed and have a replacement (new) flyback transformer installed (involves de-soldering and re-soldering) on the existing board by someone who had the equipment and skill to do such things.

Here's more relevant info

http://www.avnr55.dsl.pipex.com/dotcom/panthereatsimac/tray.htm

Yes, but just to clarify again, the problem is probably not the CRT itself, but the board the provides the power and high voltage needed to run the CRT.

Only one more question, can I use my 1996 Macintosh color display with the iMac?

Actually, those old Apple monitors use the same shape connector that is on the iMac's internal chassis. Therefore, you can connect it without any adapter. Whether the pins in the connector are connected the same way, I do not know for sure. If you have the display, you can give it a try, since you don't even need that adapter. One issue may be the resolution. For using Mac OS X, it should be at least 800x600. I believe those old Apple displays are fixed resolution (not multi-sync like VGA displays), so hopefully its resolution is one that works with the iMac's video.

Glad to hear that another old iMac is back in business...!