Imac G3 CRT Takes long time to show up

Welcome!

I suspect the great killer of G3 iMacs: the failed PAV board. "PAV" stands for "power/analog/video" and its location in the case combined with that model's poorly-conceived convection ("fanless") cooling meant the PAV board was exposed to more heat than it could tolerate for long-term survivability. Your description sounds like the power side of the PAV board is not getting enough power to the monitor.

Understand that there are virtually no "new old-stock" PAV boards out there. Everything is going to be used/pulled and thus have already been exposed to life-sapping heat. Even accessing that board is a chore and makes you work in the part of the CRT iMac where deadly residual voltage can lurk. I take apart a lot of computers but will not go that far for a G3 iMac.

Rather than try to fix it, I'd recommend staying with the external monitor if possible. You are lucky--I believe that model was the first imac to feature an external monitor port.

Beyond soldering capabilities, the point Allan was making involves the hazard with repairing a CRT-type TV or display. Unless you know how to properly discharge the stored high voltage within the CRT before you begin working near it, that voltage might find a path to ground through your body, seriously injuring or killing you. Years ago, there were plenty of individuals wanting to save money, who attempted to repair their televisions by pulling and replacing bad tubes. Unfortunately, some were electrocuted, because they didn't know that having the power cord unplugged didn't make a safe environment for probing about. Buying and installing a used P-A-V board in an almost 20 year-old computer is probably a waste of time and money. As Allan pointed out, a used part has been subjected to the same overheating that caused your board to fail, so the service life you'll get from it is anyone's guess. I have a lime green iMac DV 400 MHz that met the same fate. As cool and different as it was when new, putting it on life support by installing equally-old and abused parts wasn't something I was willing to do. At some point, it's better to cut one's losses.

>Is it always the same component that has broke in this case?

The fact that there is a change during 30 minutes or so appears to indicate a temperature-related drift, perhaps related to an earlier heat-damaged component. One can only guess here, but capacitors are always prime suspects. Old electrolytic capacitors are known to create difficulties (they can leak and dry out). Another example could possibly be a heat-sensitive trimmer potentiometer (oxide?). Also, among many other things, one should not rule out a bad solder joint as the culprit.

Anyway, as has been mentioned by Allan Jones and Jeff, all work in the power-analogue-video section of an iMac like this one is dangerous. Repairs must be performed by qualified personnel only.