Summary Of Shut Down Circuits - Toshiba 1997 G Series Troubleshooting Manual

1. Summary of Shut down circuits.

Figure 14 is a block diagram of the eleven shut down
circuits. When any of the shut down conditions occur,
the latch in Z801 holds the power relay off as long as the
set is plugged in. Neither the front panel power switch,
nor the remote power switch will restore power to the
set. The front panel power LED will blink at approxi-
mately 1/2 second intervals. To reset the shut down con-
dition, the AC main power cord must be unplugged.
Plug the AC cord back in. While listening carefully for
the click of the power relay activating, turn the power
on. If the power relay clicks on, then immediately off,
and the power LED starts blinking, then a shut down
condition has occured.
If the relay doesn't energize at all, check the AC input
circuits, fuse F801, and the standby power supply.
If the relay clicks on and the power LED remains steady
on, the set is not in shut down.
When the set does go into shut down, the problem be-
comes how to determine which conditioned caused it.
Peak Response Meter.
Each shut down circuit has a "Trigger Voltage", which
will cause shut down. The trigger voltage lasts for just a
brief moment before the power supply shuts off. A Peak-
Response Meter is required to troubleshoot the shut
down circuits. A peak response meter measures and
holds the highest voltage that occurs at a test point.
Nine circuits are connected to pin 14 of Z801. Each of
these circuits operate in a similar manner. Normally, all
"triggers" are approximately zero volts. When an ab-
normal condition occurs, the appropriate shut down cir-
cuit triggers a "high" to pin 14 of Z801. Z801 sets and
holds pin 16 low (0 volts). This causes relay SR81 to de
energize, dropping all power in the TV, except the
standby power. This means the trigger voltage also goes
away. There is now no means to determine which cir-
cuit caused the shut down. The only way to reset Z801
is to pull the AC plug.
To determine which cicuit triggered the shut down, con-
nect the peak response meter to each shut down circuit
Trigger monitor point in turn, while applying power to
the set. Any voltage at a monitor point above
aproximately 2 volts is a clear indication that the circuit
being monitored is in shut down, narrowing the search
for the actual fault.
X-ray Protect.
The X-ray protect is connected to pin 13 of Z801. Z801
compares this voltage to the reference voltage on pin 11
(aproximately 25 volts). If the voltage on pin 13 rises to
a value GREATER than pin 11, shut down occurs and
all power goes away, except the standby supply.
A peak response meter connected at pin 13 of Z801,
while power is applied to the set, will capture a voltage
greater than the reference voltage, indicating excessive
High Voltage.
+125V Over Current Protect.
The peak hold meter is connected across resistor R470,
and NOT referenced to ground to monitor for a momen-
tary increase in voltage. The normal voltage across R470
is about 0.4Volts. An increase in current will cause the
voltage across R470 to increase. When Z801 detects
this voltage increase, it puts the set into shut down.
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JRS 12-97