My imac (G3 400 MHz) screen goes black 6 out of 10 times when I switch it on. It isn't freezing becuase I could hear it loading to desktop, the screen just simply goes black. It turns to normal when I restart it (sometime requires more than twice) I tried to reset the PRAM but didn't help. I was also told that by changing the clock battery might solve the problem. But which one is the clock battery? Does it the one near the left- top of Memory stick and pink color (about 1inch long)? But I don't seem able to reach it. Also, how to open up the imac cover (if there is more than one , apart from the Memory stick place)

You can't replace the battery without opening the machine. I would strongly suggest you consult a professional - there are high voltages inside an iMac, just like there are in a TV. If you don't know what you're doing, you can screw up the iMac and/or yourself very easily.

And besides that, your problem most likely is NOT the battery. Unplug your iMac from power and let it sit for 20 minutes. Plug it back in and turn it on. If the date and time are correct, your battery is fine.

Every piece of HV equipment Apple has produced since about 1988 has self-discharging circuitry so that technicians won't get shocked. Besides, the battery isn't anywhere near the analogue board. p

So you don't discharge the CRT when you work on an iMac? You're a braver man than I, Gunga Din. That's the first thing I do whenever I have to remove the top rear cover. Yeah, I replace batteries without discharging the CRT, but I know what I'm doing. Once the bottom cover is off, it exposes several openings into the CRT area where an unwary person could stick their fingers. Are you really advocating people who obviously don't know what they're doing should try to work on their machines themselves? That's sure what it sounds like. Perhaps you need to preface your advice with a disclaimer, to avoid potential liability if and when someone screws something up following your advice.

And have you ever succeeded in producing the telltale "snap" when you try to discharge the CRT? Didn't think so. p