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CIRCUIT DESCRIPTIONS
PV154 - 923-03506 3-5 ZP26/28 - SERVICING
NOTE: If the 5.5V or the 3.3V regulators OPEN, the set
will not go into shut down, they must have a short to
produce a shut down input on PROTECT 1.
The set will shut down after the Screen go excessively
bright, but this is from the 115V over current sensor,
not PROTECT 1. When the power supply is in Stand By,
the Short Detection circuit could activate. By turning
off Q919, no accidental shut down operation can oc-
cur.
SUB POWER SUPPLY VISUAL LED
The Chassis has 1 Green LED on the Sub Power Supply
PWB. This chassis utilizes 1 Green LED in the power sup-
ply cold side. The power supply operates in two differ-
ent modes, Standby and Projection On mode. The LED is
lit in the Standby or Power On mode with the AC ap-
plied and the TV ON or OFF;
· SRS Audio Front 29V Regulator +29V indicated by
D912 Color GREEN
LED USAGE
The Visual LED is very useful in Trouble Shooting. With-
out removing the back cover, some diagnostics can be
made. By observing the operation of the Green LED, the
technician can determine if the Sub Power Supply is
running or not. The following will examine how the LED
is illuminated.
POWER ON ENERGIZE:
When the Microprocessor outputs a High from pin 53
when power is turned ON, the high is inverted by Q003
to a LOW. This low is inverted by Q004 to a HIGH and
routed through the PQS1 connector pin 2. This high is
routed through R954, R951, D926 to the base of Q914.
This transistor turns ON and it’s collector goes LOW.
This low is the Ground return for the Relay S901. The B+
for the primary of the relay is the SBY +5V generated by
the switching transformer pin 15, rectified by D920,
filtered by C935, L915, and C936.
When the relay S901 turns on, the contacts close and
AC is routed to the Deflection circuit power supply and
the Deflection Power Supply is Energized.
DEFLECTION HORIZONTAL DRIVE ACTIVATION
(IC01 Rainforest IC)
When the set has AC applied, one of the switch pulses
generated on the Sub Power PWB is the +11V pulse from
pin 10 of the Switching Transformer I901. This +11V
pulse is rectified by D919 and filtered by C929. This
+11V DC voltage is then routed to the +9V regulator
I906 pin 1. When the set is turned ON, the Horizontal
Drive Power supply I906 is activated. This happens when
pin 2 of I906 goes high with the Power On/Off control
line from PQS1 pin 2. The +9V is output via pin 3 to the
PQD2 connector pin 5. From here the +9V is routed
through the Deflection PWB to the Signal PWB via PSD3
connector pin 11, through the coil LC03 to pin 19 of
IC01 the Rainforest IC. When this voltage is applied to
pin 19, the horizontal circuit inside the Rainforest IC
is activated and a horizontal drive signal is output from
pin 26. This H. Drive signal is routed through the PSD2
connector pin 5 to the base of the horizontal drive
transistor Q709 on the Deflection PWB. The collector of
Q709 produces a drive signal routed through the drive
transformer T702 and output from pin 4 to the base of
the Deflection Horizontal Output Transistor Q777 to
begin driving the deflection circuit which in turn, ac-
tivates the High Voltage circuit.
The sub power supply in the chassis works very similar
to the previous models, with some very significant
exceptions. The use of the power supply creating the
SBY+11V supply eliminates the need for a Stand-By
transformer. The following explanation will describe the
Sub Power Supply Shut Down Circuit.
VOLTAGE LOSS OR EXCESSIVE LOAD DETECTION
The second most common circuit used is the Voltage
Loss Detection circuit. This is a very simple circuit that
detects a loss of a particular power supply and supplies
a Pull-Down path for the base of a PNP transistor.
This circuit consists of a diode connected by its
cathode to a positive B+ power supply. Under normal
conditions, the diode is reverse biased, which keeps
the base of Q1 pulled up, forcing it OFF. However, if
there is a short or excessive load on the B+ line, the
diode in effect will have a LOW on its cathode, turning
it ON. This will allow a current path for the base bias of
Q1, which will turn it ON and generate a Shutdown Signal.
B+ VOLTAGE TOO HIGH
In this circuit, a Zener diode is connected to a voltage
divider or, in some cases, directly to a B+ power supply.
If the B+ voltage increases, the voltage at the voltage
divider or the cathode of the zener diode will rise. If it
gets to a predetermined level, the zener will fire. This
action creates a Shutdown Signal.
SHUT DOWN CIRCUIT
Shut down occurs when the shutdown Photo Coupler
I903 is activated by pulling pin 2 low.
When I903 is activated by pulling pin 2 low it gives 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 base of Q902, turning it
on. This grounds pin (4) of I901, removing Vin, and
the power supply stops working.
The reason for the photo sensor I903 is to isolate hot
and cold ground.