My iMac g4 20" power on problems

Hi,

After long time i try to power on my imac g4 20”(the last time the mac war perfect ,new hdd new os and perfectly worked)

I click and the mac start with sound i see the apple and i heard the hdd working , but i can’t see the os screen, After 1 minute i decided to click power off.i retry to power on but after 2 minutes automatically shut down

After this i retry to power on but now I hear the fan kick i see Red light on mouse for only 1 second but now the mac does't power on

I try to change new battery and click pmu reset button but nothing change

any idea what could it be the problem?

Aggiorna (30/06/2020)

Any solutions?something to do or i can try?

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