I have a lime grape /green/ imac that chimes once at
start up and then switches off seconds later,
On the logic card it has Kingston SO DIMM RAM and
by the CD-ROM a HD. There's an APPLE sticker on th
CD that says model CR-175-0 Manufactured APR 99
if thats relevent, The one chime may mean it found
the memory and then for some reason it turns itself
off Any suggestions? eg replacing parts etc.

Maybe you have to reset the PRAM and/or the PMU

you can do this when you hear the startup tone

just press:

Option-alt-P-U (to reset PMU)

or

Option-Alt-P-R (to reset PRAM)

press this combination until you hear again the startup tone.

Tellme if that worked for you...

This faulty imac came with no keyboard. I have a
spare general USB keyboard I could try after puting
the logic card back in.
I have been loking for information about this logic
cars in an original imac without any luck. On an
Apple sticker on the board it says Model CR-175-0
Man Apr 99 if that is relevent. Also after the chime it
turns itself off immediatly that is why I suspect either
the AC power adaptor or SODIMM memory or lack of
proper HD.

If you don't have an iMac usb keyboard then you can reset the PMU manually by pressing the PMU reset button on the logic board. PRESS THE BUTTON ONLY ONCE. Pressing it multible times can crash the PMU chip. If resetting the PMU does not work then there are several diagnositic checks that you will have to go through to trace the problem. This is a rather involved process and if you do not know how to dissassemble your iMac chances are you will not want to try it. The problem that I have found is usually a bad circuit on the logic board.