iMac G4 Flat Panel 1Ghz - 1 beep when turning it on.

There may be some indication of the beep's cause after some thorough
testing of the computer with some utility software; perhaps even the basic
Apple Hardware Test which would have accompanied the computer when
new could be helpful to some degree, if the machine were run in that, in a
long continuous loop to see if at some point, it may generate some codes.

However, the startup sequence itself is where such a hopeful bit of information
would more likely be generated; and this is the kicker, in that some of the info
at start-up is too early in the beginning of the run-cycle to be recorded or read.
But, you may find evidence of the issue in the Console error message reports
in that utility; and there are a few different sub-titles of places in Console to look.
Also, you may be able to find some error or crash messages, or other runtime
messages and glitches, in the System Profiler> Software> progression path.

There may be another source of the single-beep on startup tone. Not sure of
any other, though; since the Power-on self-test is generally what checks out
those computers on start-up. And the 'single beep' suggests there is a RAM
issue, as this Beep would replace the normal startup chime.

So, that early in the startup, and without a booted system, the error logs or
crash and failure reports would not be generated to be read later.

Is the computer you have really a 20" LCD and does it truly have a 1GHz
CPU? Or, does it have a 1.25GHz PPC processor, as it should? Did the
company you obtained the replacement logic board from, have anything
to say about the kinds of issues you've noted the computer displaying?
There may have been a limited duration warranty on the rebuilt board.

There have been a few of the old iMac G4 17" & 20" 1.25GHz models
generating an odd, single 'buzz' sound after the normal startup tone.

Is your's making a 'single beep' or a 'buzz' sometime after booting?

Do you have any other RAM chips of correct supported version to
replace one or both RAM slots, of reliable quality product? How was
the OS installed into the computer, or have you been unable to do so?
Once in a great while, new correct RAM could be bad on arrival. Rare.

{This is a lot like chasing gremlins or setting a trapline for gnomes.}

Maybe someone, such as the contributor alias 'spudnutty' may know of
some other factor in this matter that he has found, that may cause that.

Good luck & happy computing! 🙂

Since the iMac G4 20" computer shipped with a 1.25GHz logic board and nothing less,
the actual replacement would have to be the same for it to function. A 1.0GHz board
probably would create more problems than it could solve; and video support, power
supply output & other details would likely not match the original demands of hardware.

(If the computer you have, originally shipped with a 1.0GHz processor on its board, the
model did not have a 20" LCD display; so that may be part of the issue, if you have
one part and it does not really belong with the others. Those do not interchange.)

And a 1.0GHz board likely requires a older, different kind of RAM than a 1.25GHz
needs to have, to run. This is why the question(s) and answer(s) are detailed. If you
have a download copy of MacTracker for use in a computer offline, you can look up
the general specs and some upgrade info, plus OS version information, within it. See
the site and http://mactracker.ca to get a version of this for the sake of comparison.
Also, the web site http://everymac.com/ has info and lists of resources, that may help.

+{These forum pages are generally supported by Apple, but the replies and content to+
+the user questions are the results of persons with varying amounts of experience, or+
+knowledge, or skills, or time on their hands and a fair grasp of using a search engine;+
+or, maybe some are actual technicians and persons who have training in their day job,+
+they stop by on occasion to help out. Regular Mac users, none receive compensation.}+

Hopefully the issue in your computer can be resolved without much additional expense.
Good luck & happy computing! 🙂

The 20" iMac G4 also sports USB2.0 ports (3) and a newer AirPort wi-fi card support.
The older model iMac G4 uses the same card as the oldest iMac G3 with CRT; and
the later/last G4 models use the AirPort Extreme card. They are not intercompatible.

So, the issue goes beyond the RAM specifications, the wires to the displays are also
different as are some of the support devices for other aspects of those different years
of LCD iMac G4 models. I've owned several of them, outside of two 17" USB2.0 models.

And generally the USB2.0 models use the same RAM type, too. Both the 17" & 20" iMac
G4 USB2.0 computers sport the 1.25GHz processor; but the logic boards don't swap.

If you can read it, there are some applicable part numbers good for looking online in a
search for a correct logic board; but note there sometimes are more than one "correct"
part number, so be wary of why there may appear to be more than 1 listed in a search.

• G4 iMac USB 2.0 Parts - note the expanded view and part numbered image here:
http://www.mac-pro.com/Mac-Pro-Online-Store/G4-iMac-USB-2-0-Parts

The part numbers can be entered into a new search engine field to see what or where
similar items appear. Some are resellers or other incidental results; some not valid.

Good luck in this matter & best regards! 🙂

The main page is a fine 'parts-number reference' you can check to be sure
correct part number(s) are used in a secondary search to get a better price
and also to double or triple check the actual item from more than one poss-
ible source before committing to the purchase.

And who ever you end up buying the correct logic board from, be sure
to check well into their return and exchange policies, & any guarantee.

The other links provided can also be helpful in finding upgrade specs
and limits on various models of older Macs; systems, hardware, etc.

Hopefully those links to resources and a judicious use of google or
another search engine can help sort out a viable path in this matter.

Good luck & happy computing! 🙂