1985 NEC TV has sound, no picture / vertical collapse!?

How do I fix a vertical collapse? I don't really want to pay to get it fixed... don't feel like paying a buttload for labor fee. Anyway, the sound is fine, but the screen would be on sometimes, and all of a sudden, it gets the horizontal shaky lines, and then goes dark, and stays dark for a while, until the picture comes back on. All the while, the sound is fine. How do I fix this???