Complex sentence: He deduced that where the existing turntable design failed was that the cartridge must be held still wherever the groove goes, and thus the mean line traced by the groove relative to the body of the cartridge needed to be constant.
Split into simpler sentences: He deduced that the existing turntable design failed because the cartridge was not being held still wherever the groove goes. He believed that the mean line traced by the groove relative to the body of the cartridge needed to be constant.