The Cosmic Computer by H. Beam Piper

Conn Maxwell returns from Terra to his poverty-stricken home planet of Poictesme, “The Junkyard Planet”, with news of the possible location of Merlin, a military super-computer rumored to have been abandoned there after the last war. The inhabitants hope to find Merlin, which they think will be their ticket to wealth and prosperity. But is Merlin real, or just an old rumor? And if they find it will it save them, or tear them apart?