Above circuit is used for protection against screen & focus grounding. Basicly the base of PNP transistor Q609 is connected to screen&focus potentiometer of FBT, in another words FBT focus pack is connected to ground via R654. The voltage at the base of that transistor is normally higher then the zener voltage at the emitter. When any short circuit occurs at focus or screen the base voltage is decreased to a value lower then zener voltage, that forces transistor to conduction and trigers another circuit which will force UC to protection mode. In protection mode LED becomes RED and remote control does not work.
