TV_SUB_ON/TUNER_SUB_ON
The STB 5V at pin 5 of connector A7 and the TV SUB ON command at pin 6 are used to activate the PA
SOS detect circuit on the A board. This circuit monitors the F+15V, DT9V, and SUB5V for abnormality.
Under normal operation, the TUNER_SUB_ON voltage from pin 1 of the main CPU (IC1101) turns on
Q3504. When Q3504 is on, a low is applied to the base of Q3502 turning it on. When Q3502 is on, 5V is
applied to the emitter and base of Q3503. The transistor remains off until a short circuit or excessive
voltage is developed in the supply lines connected to its base. When Q3503 is off, a low is output to pin
114 of IC1100.
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