CIRCUIT DESCRIPTIONS
PV154 - 923-03506
3-1
ZP26/28 - SERVICING
SECTION 3
CIRCUIT DESCRIPTIONS
POWER ON AND OFF
The power supply runs all the time when AC is applied.
The use of the power supply creating Stand-By Voltage
supplies eliminates the need for a Stand-By transformer.
The following explanation will describe the Turning ON
and OFF of the projection television.
The Microprocessor I001 generates the ON-OFF control
signal from pin (53). The logic states of this pin are
High = On and Low = Off. When the set is turned On, the
high from pin (53) is routed to the Relay Drivers Q003
and Q004. This turns on Q003 and its collector goes low
which turns off Q004 and its collector goes High. This
On/Off from the Relay Drivers will perform the following:
· Turn on the Shut Down “Power Shorted” detection
circuit, Q917 and Q919.
· Turn on the Relay S901 through Q914 providing AC
to the Deflection Power Supply on the Power/Deflec-
tion PWB.
POWER SUPPLY SHUTDOWN
This chassis utilizes I901 as the Osc.\Driver \Switch for
the sub power supply, just as the previous chassis have
done. The Shutdown circuit, (cold ground side
detection) removes I901 B+. I903 (the Photo Coupler)
which isolates the Hot ground from the Cold ground
and couples the Shutdown signal to the Hot Ground
side, Q902 on the hot ground side and Q901 which
latches Q902 on. When Q902 is on, it removes B+ from
pin (4) of I901 (the Vin pin).
The Power Supply utilizes a Shutdown circuit that can
trigger Q902 from 5 input sources. (1 of these Short
Detection circuits are not operational in Stand By mode).
I903 is activated by a Low being applied to pin 2, which
forward biases the internal LED. The light from this
internal LED is then coupled to the receiver transistor.
The receiver transistor turns On and output a High from
pin 3. This high is routed to the base of Q902 turning
it On, which grounds out the Vin at pin (4) of I901,
disabling the power supply.
All of the Power Supply Shutdown circuitry can be broken
down into the following groups;
· Voltage Missing Detection
· Voltage Too High Detection
In the following explanation, the Shutdown circuits will
be grouped. This will assist the Service Technician with
trouble shooting the Chassis, by understanding these
circuits and having the associated circuit routes, the
technician can then “Divide and Conquer”.
STAND-BY MODE
As indicated in the Power On/Off circuit diagram expla-
nation, 3 of the 6 shut down inputs are not active when
the set is in standby.
· Shorted SW +2.5V (IC03) on Signal PWB through Pro-
tect 1 to (D940) on Sub Power Supply PWB
· Shorted SW +3.3V (IC04) on Signal PWB through Pro-
tect 1 to (D940) on Sub Power Supply PWB
· Shorted SW +5V (IC02) on Signal PWB through Pro-
tect 1 to (D940) on Sub Power Supply PWB
These SW voltage loss sensing circuits are defeated be-
cause the Shorted SW (Switched) power supply detec-
tion circuits are turned off in stand-by to prevent faults
triggering of the shutdown circuit.
Q919 supplies the high for shutdown if any of the volt-
age loss circuits become activated. Q919 requires emit-
ter voltage to operated. Emitter voltage is supplied from
the emitter of Q917. Q917’s base is connected to the
power on/off line. When the set is not on or turned off,
the power on/off line goes Low. This Low pulls the cath-
ode of D927 low, removing the base voltage of Q917
turning it OFF. This removes the emitter voltage from
Q919 and this circuit can not function.
HOT SHUT DOWN SENSING CIRCUITS
(INTERNAL TO I901)
LATCHED SHUT DOWN MONITORS: (AC must be removed
to recover).
1.
Pin 4 is monitored for Over Voltage at pin 4 of
I901.
2.
I901 itself is monitored for Excessive Heat. This
block is labeled T.P.O. (Thermal Protection Overload).
RECOVERING SHUT DOWN INPUT: (Driver IC will recover
on its own when trouble is removed.)
3.
Pin 5, besides being used as a regulation input,
also monitors the low ohm resistors R910, R911,
and R912. If these resistors have an excessive cur-
rent condition caused by monitoring the current
through the internal Switch MOS FET, the voltage
will rise and pin 5 has an internal Over Voltage de-
tection op-amp. If this voltage rises enough to trig-
ger this op-amp, the IC will stop producing a drive
signal.
COLD SHUT DOWN SENSING CIRCUITS
(AC must be removed to recover)
All of the Cold Ground side Shutdown detection circuits
can be categorized as either Voltage shorted or Voltage
too high.
VOLTAGE SHORTED DETECTION
· Shorted STBY +3.3V (Q025) on Signal PWB through
PROTECT 2 to (D938) on Sub Power Supply PWB
· Shorted SW+2.5V (IC03) on Signal PWB through PRO-
TECT 1 to (D940) on Sub Power Supply PWB
· Shorted SW+3.3V (IC04) on Signal PWB through PRO-
TECT 1 to (D940) on Sub Power Supply PWB
