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POWER ON/OFF and PROTECTION CIRCUITS
Power On/Off
The CPU performs the On/Off function through pin 27. In the Power On mode
pin 27 changes from Low to High, forward biasing Q681. When Q681
switches On, the base of Q627 will become Low, forward biasing Q627.
When Q627 (+12V Sw.) switches On, forward bias will be applied to Q486
(+9V Reg.). When Q627 switches On, current will flow through relay RL601,
closing the contacts and applying AC power to the degaussing circuit.
In the Power Off mode, pin 27 of the CPU will become Low. Q681 will now
be switched Off, switching Off Q627. Then Q486 (+9V Reg.) will switch Off.
With Q627 Off, current will cease to flow through RL601, opening the
contacts.
Power Supply Protection
The C-003 CPU provides a power source protection function to automatically
switch Off the power if an abnormal condition occurs in the chassis power
supplies to help prevent secondary damage.
Detection of an abnormal condition is accomplished by monitoring the +4.8V
(IC301 V
DD
), +7.6V and the +9V DC supplies at pin 3 of the CPU.
Pin 3 is normally High, approximately 5V (Vcc). If, while the power is On, pin
3 becomes Low (0.4 V
CC
or less) for a continuous period of approximately 1.5
seconds, pin 27 (power) of the CPU will be switched Low, shutting Off the
power.
If, while the power is Off, the power is switched On and pin 3 remains Low for
a period of approximately 3.0 seconds, pin 27 will be switched Low, shutting
Off the power.
In circuit operation, if all +4.8V, +7.6V and +9V supplies are their normal
potential, the diodes D312, D801, and D489 will be reversed biased. With all
diodes Off, a High of 5V will be input to pin 3 of the CPU. Should 4.8V,
+7.6V, or +9V supply become 2V or less, one of the diodes will switch On,
forcing a Low at pin 3 of the CPU. The input to pin 3 is evaluated every 20
ms.
Note
: The C-003 CPU provides a Power Surge Protection feature. If power
failures occur three times within 15 minutes, the CPU will automatically
stop functioning to help prevent secondary damage. (TV will not turn
On by pressing the POWER key.) To reset the operating programs
within the CPU, disconnect the AC power cord for at least 10 seconds.
Auto Shut Off Function
The Auto Shut Off feature operates to switch the TV Off if no video signal is
received for a certain period of time. The Time Base signal is used within the
CPU to determine the presence of an active channel. If pin 26 of the CPU
goes Low for a continuous period of approximately 15 minutes, the CPU
program will determine that no active channel is present and activate the Auto
Shut Off feature. The Auto Shut Off has priority over the Sleep Timer function,
however, it is inactive when the TV is in the AV (Video) mode.
Anode Current Leak Protection
The VB7C chassis provides an anode current leak protection circuit to
automatically switch Off the power to help prevent the risk of fire if an
abnormal condition such as high voltage arcing occurs on the picture tube
anode or in the high voltage circuits due to accumulated dust or liquid spilled
into the TV cabinet.
Detection of an abnormal condition is accomplished by monitoring the Heater
voltage supply at pin 3 of the CPU. Since the high voltage arcing on the
picture tube anode or in the high voltage circuits causes excessive current
flows and lower voltage supplies from the secondary windings of the Flyback
Transformer (T402), the excessive current flows are effectively monitored.
In circuit operation, the normal potential Heater voltage, approximately
22VDC higher than the zener voltage (15V) of D428, is applied to the cathode
of D428, diode D429 will be reversed biased.
With D429 Off, a High of 5V will be input to pin 3 of the CPU. Should the
cathode voltage of D428 become 15.2V or less, D429 will, switch On, forcing
a Low at pin 3 of the CPU.
