Symptoms may be the iMac does not turn on at all or turns on once in awhile after spamming the power button.
Solutions
Recap the power supply
Disassembly Notes
I recommend following a guide from online like iFixit. One tip is if the hard drive/dvd drive unit doesn’t come out, unscrew the DVD drive and slide it back and they should both slide out easily.
Original Parts
iMac G4 20" Power Supply
Important Note: I did not record the capacitor locations on the power supply, I also did not replace them all, but have their values and sizes, so please confirm where each capacitor goes when you are replacing yours. So take note what value of capacitor goes where when you are removing them.
