Complex sentence: Guitar playing took a major role in his life while he was doing his military service in the army, during 1964, but it wasn't until 1967, with his first television experience, that he started to become well known and influential among Cuban revolutionary youth.
Split into simpler sentences: Guitar playing took a major role in his life while he was doing his military service in the army, during 1964. But it wasn't until 1967, with his first television experience, that he started to become well known and influential among Cuban revolutionary youth.