Do you have a schematic?, there should be part of the circuit specifically designed to prevent that, as it will fairly rapidly burn a mark in the middle of the screen.
To be honest, it's so many years since I did CRT TV's, I can't remember where it might be on a Sony? - but you might try looking round the beam limiter (ABL) circuit, it 'may' be associated with that. Have a look at R816, R856 and R857 which set the operating point.
TVTech might come along?, he's still repairing CRT sets.