iMac g4 won't boot up and weird screen when it cans

G’Day Correia ;

You are on the right track , HOWEVER , to be really successful one should purchase a decent ESR meter to check any electrolytic caps in the power supplies .

An ESR tool (Effective Series Resistance , I could be wrong , been a long time for that definition ) will test capacitors ‘in circuit’ . No need to de-solder . HOWEVER there can be a gang of caps in parallel and some meters will give a pass . If any have swollen tops or stuff leaking they are bad FOR sure . If any are in a parallel gang I would swap them all . Hope you have a heavy duty motorized solder sucker to get caps out of multi layer boards ( heat from most hand irons will get pulled away into large copper ground planes )

Sorry I have not had any G4 experience , but Apple makes such good quality I doubt caps are bad , but , I have done so many other devices and brands that I would not be surprised .

That ‘clicking’ you mention could now be a shorted MOSFET causing switching power supply to constant reboot . Use DVM on semiconductor junction test to look for bad power MOSFET or transistors .

Less likely is you are hearing the hard drive smacking the heads against the frame . Shed the hard disk for boot test maybe .

Just some thoughts , Good luck

Hugh