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camera wont turn on corroded batteries cleaned it out and neutralized it put in new batteries but still wont turn on

Hi, You may have to dis-assemble the camera a bit to check that the corrosion has not gone further than the battery terminals. It may have affected the cable/wiring connections on the back of the terminals themselves. Here is a link to the Ifixit guides for your camera. The guides will help in the dis-assembly process. Nikon Coolpix L820 Repair