H Protect - Sony WEGA KV27FV15 Training Manual

H Protect

Overview
The H protect circuit is used to shut down the horizontal deflection circuit
if the high voltage should rise above the acceptable level or if the pincush-
ion drive circuit. Pulses from the flyback transformer's heater winding are
compared against a reference voltage. The reference voltage can be
removed based on the condition of the Pin Out circuit. Loss of Pin Out will
cause horizontal protection to be indicated. When H protect is detected,
HP pulses will be removed from IC355/19 HP/Hoff. Video and HV will be
disabled, but the power supply will continue to function.
Circuit Description
When the horizontal circuit is operating normally, a signal is output from
T503/5 that is used to supply voltage to the filaments of the tube. At this
point R550, R529, C531 and C532 filter the signal to a DC voltage. This
DC voltage is input to IC501/3 non-inverting input. This voltage is com-
pared to a reference voltage of 8.2 volts. This reference voltage is derived
by a voltage divider consisting of R533, D520 and D521. As long as
IC501/2 is greater than IC501/3, the horizontal circuit is operating nor-
mally.
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IC501/2 is also connected to Q503/C. Q503 is OFF as long as the signal
from the Pin Out circuit is working normally. If the Pin Out signal were
missing, Q504, which is normally ON, would turn OFF. If Q504 were to
turn OFF, Q503 would turn ON. This would place IC501/2 at ground
potential. In that case, IC501/1 would output +12 volts that would turn
Q301 ON and ground the HP signal that is connected to Q301. If Q301
turns ON, IC355 would shut OFF the drive signal at IC355/19 HD. When
the drive signal is removed, the High Voltage is removed from the set.
This will cause a no video condition, but the Timer LED will not begin to
blink. If the set is turned on with a failure that causes H protect to occur,
High Voltage would begin to come up and then disappear. The timer LED
would blink to indicate that the AKB circuit did not unblank the video. The
Timer LED only indicates an AKB failure when the set is turned from OFF
to ON.