JAKARTA, Indonesia—Investigators searching for a black box of a crashed Lion Air jet believe its locator beacon is broken and are preparing new equipment to visually scour the muddy floor of the Java Sea.

Investigators are no longer detecting pings from the cockpit voice recorder, or CVR, which they believe is covered in mud at a depth of about 100 feet. A search area extends about 80 feet in all directions from a point where teams detected what they believed to be its pings in the days just after Flight 610 crashed after...