There could be something jamming the disc door, or one of the gears are cracked. If it's the gear, you'll need to get it replaced. Luckily, some online shops sell it cheap.
I have a playstation 2 that when you push the power button, it turns on for a second then right back...
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I have a playstation 2 that when you push the power button, it turns on for a second then right back off