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CIRCUIT DESCRIPTION
· SW+5V (D943)
· SW+12V (D944)
· Stby+7V (D955)
· Stby+11V (D952)
Negative Voltage Loss Detection
· SW-12V Loss Detection (D939, D940)
Excessive Current Detection
· Not used in the Sub Power Supply.
Voltage Too High Detection
· SW+12V (D935, D936)
· SW+5V (D932, D933)
· Stby+11V (D941)
· Stby+7V (D938)
If any one of these circuits activate the power supply will STOP, and create a Power Supply Shutdown
Condition.
SOME SHUTDOWN CIRCUITS ARE DEFEATED IN STANDBY MODE. (Set Off).
As indicated in the Power On/Off circuit diagram explanation, 6 of the 16 shut down inputs are not active
when the set is in standby.
· Shorted SW+2.5V on Signal PWB through Protect 1 to (D957) on Sub Power Supply PWB
· Shorted SW+9V (D015) on Signal PWB through Protect 1 to (D959) on Sub Power Supply PWB
· Shorted SW+5V (D014) on Signal PWB through Protect 1 to (D959) on Sub Power Supply PWB
· Shorted SW+3.3V (D016) on Signal PWB through Protect to (D959) on Sub Power Supply PWB
· SW+5V (D943)
· SW+12V (D944)
These SW voltage loss-sensing circuits are defeated because the SW (Switched) power supplies are turned
off in standby to prevent erroneous of the shutdown circuit. Q909 supplies the high for shutdown if any of
the voltage loss circuits become activated. Q909 requires emitter voltage to operate. Emitter voltage is
supplied from the emitter of Q908. Q908s base is connected to Q903, which in turn is connected to the
power on/off line. When the set is not on or turned off, the power on/off line goes high. This high is inverted
to a low by Q903 and pulls the cathode of D945 low, removing the base voltage of Q908 turning it OFF.
This removes the emitter voltage from Q909 and this circuit cant function.
SHUT DOWN CIRCUIT:
Shut down occurs when the shutdown SCR Q905 is activated by gate voltage. When Q905 receives gate
voltage of 0.6V, the SCR fires and give a ground path for the emitter of the LED inside I903. The light
produced by turning on this LED turns on the internal photo receiver and generates a high out of pin (3).
This high is routed to the gate of Q901 turning it on. This grounds pin (4) of I901 removing Vcc and the
power supply stops working. The reason for the photo sensor I903 is to isolate hot and cold ground.