NotTaR of Television Sets : Part of picture cut off                 
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Part of picture cut off

The following applies if the part of the picture is missing but not otherwise squashed or distorted. For example, 85% is missing but the portion still visible is normal size.

Wow! That's an interesting one, more so than the typical run-of-the-mill "my TV just up and died on me". Or, "my pet orangutan just put a hole in the CRT, what should I do"?

Since the size of the picture fragment is correct but 85% is missing, my first thought would be to check waveforms going into the vertical output stage. The supply voltage is probably correct since that often determines the size. It almost sounds like the waveform rather than being mostly on (active video) and off for the short blanking period is somehow only on during the last part of the active video thus giving you just the bottom of the picture. If there is a vertical output IC, it may be defective or the blanking input to it may be corrupted. The problem may be as far back as the sync separator. Then again who knows, maybe wait for the schematics.