IMac crashes at start-up

Question

Hi,
So this is my problem:
I turn my iMac on, then it chimes and the grey screen with the Apple logo appears.
Everything fine till now.
Then suddenly, it just switches off.
So I turn it on again, but while it chimes, it makes a loud noise like a propeller is spinning or something. The apple logo appears, and, it turns itself off again.
So I leave it. But after 5 minutes, it turns itself on (by itself), makes a loud, long beep and then a grey screen appears, welcoming me to "Apple Open Firmware" and telling me to either type "mac-boot" or "shutdown".
I typed "shutdown" and plugged it out of electricity, because I don't know what to do!
Does someone have any idea?
Thanks a lot,
Ferdinand

Answer 1

The computer probably needs to have some hardware testing
done to see the extent of the possible situation and to get an
idea of an estimate of the problem and costs of a cure.
Could be something relatively simple for the age of the computer;
since hard disk drives do wear out. Or it may be something else.
There is a reset button for the Power Management Unit, under
the metal disc on the bottom of the computer. Finding the button
may be a trick, since it is sort of hidden in plain sight...
"Resetting the iMac (G4 flat panel) Power Management Unit - PMU"
http://docs.info.apple.com/article.html?artnum=95165
An expert, an authorized tech with experience in these matters
should be able to give you an idea; and depending on your own
options, a repair may be one of several possible paths. Some
resellers and small shops have restored computers for sale; and
then there are also some good products in both Apple Certified
refurbished (online store) and in the newest Apple products, too.
Some times, the retail store in some towns may have older items
for sale at a price not reflecting the latest values; so an all new
item could sometimes be bought for a great price, with warranty.
And in some cases, a company may specialize in repairing the
compact and portable computers and have some on hand for
a price comparable to the repair estimate; for a newer computer.
I'd suspect the computer's hard disk drive, first. Then maybe other
possible items. A good tech could troubleshoot the iMac in maybe
an hour of shop time and give you an estimate; you'd have to be
able to leave the computer where this could be done. Some of the
certified repair techs have been less than interested in repairing
the iMac G4; and independent shops vary across the country.
In any event...
Good luck & happy computing!