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Protection Circuit 1
Power ON
The protection circuitry is connected to the power ON circuit to shut off the
TV set. The power ON path is mapped out in the chart below listing the
purpose of the parts:
P o w e r O N P a r t s
N a m e
I n p u t
M a s t e r
M e c h a n ic a l
p o w e r
p u s h b u t t o n – s e lf
O N /O F F
la t c h in g
s w it c h
M a in
M a s t e r p o w e r s w
M ic r o
S 4 3 0 1 in t o p in 4 6 .
I C 3 2 5 1
I R c o m m a n d fr o m
I C 4 3 0 1 in t o p in 7 .
Q 5 5 0 2
0 . 6 V p o w e r O N
f r o m I C 3 2 5 1 / p in
7 1
D 5 5 1 5 ,
L o w f r o m Q 5 5 0 2 / C
Q 5 5 0 5
a t t u r n O N
When the TV power button is pressed, the Main Micro IC3251/pin 71 out-
puts +5V to turn on relay driver transistor Q5502. Q5502 turns on the AC
relay, as well as a second relay driver transistor Q5505. Q5505 energizes
the surge relay. Therefore at power ON, the AC and surge relay turn ON.
In another stage, a third (degaussing coil) relay that is not shown also
turns ON. Only the first two relay clicks are heard. The degaussing coil
relay clicks ON at the same time as the AC relay and turns OFF approxi-
mately 9.5 seconds later (just about the same time as picture unmuting
occurs).
Protection Latch Circuitry
The protection circuit uses a latch formed by Q5503 and Q5504 to divert
power ON current from IC3251/pin 71 to ground. This starves the AC
relay driver transistor Q5502 by removing its base bias so the TV shuts
down.
O u t p u t
P u r p o s e
S w it c h e s t h e
+ 1 2 V f o r A C
r e la y
+ 1 2 V A C r e la y
c o il v o lt a g e .
O n c o m m a n d t o
M a in M ic r o
I n f o r m s I C 3 2 5 1 o f
I C 3 2 5 1 / 4 6
t h e s w p o s it io n .
P o w e r O N d r iv e
T V p o w e r O N
s ig n a l f r o m p in
c o n t r o l
7 1
E n e r g iz e s A C
A m p lif ie r / r e la y
r e la y R Y 5 5 0 1 t o
d r iv e r
p o w e r t h e T V
s e t .
E n e r g iz e s s u r g e
R e d u c e in r u s h
r e la y R Y 5 5 0 2 t o
c u r r e n t e x t e n d in g
s h u n t s u r g e
r e la y c o n t a c t lif e .
r e s is t o r R 5 5 0 2 .
73
The protection latch remains active as long as the Main Micro IC3251/pin
71 is outputting a power ON command. Turning the TV OFF (IC3251/pin
71 then goes low) resets the Q5503 / Q5504 latch.
The latch can be triggered by any of 12 inputs (see the Protection Block).
In this diagram, only two items on the G board are monitored:
¨ +15 OVP and
¨ Surge relay RY5502 not energized.
+15 OVP (Over Voltage Protection)
+15 volts from Converter 1 is monitored by zener D5025 for excessive
voltage. If Converter 1's regulation fails, the +15 volt line may increase to
22V and trip the latch. However, Converter 1 also outputs the +135V B+
that is monitored for OVP and OCP and is connected to the failure indica-
tor circuitry. Since the B+ is more closely monitored for OVP and OCP, it
is more likely that the B+ will trip the latch before this zener will.
Surge relay RY5502 not energized
The surge relay RY5502 must engage shortly after the AC relay to bypass
the surge resistor R5502 or the resistor will over dissipate. To prevent
this, the TV will shut down if R5502's coil is not energized.
Normally when RY5502's coil energizes, 4 volts is developed across R5512.
This is more than sufficient to back bias D5510 and keep the latch from
tripping. The latch trips when the set is powered and the surge relay coil is
not energized. Both ends of R5512 are now at ground potential and that
turns ON the latch by bringing Q5504/base to ground via D5510.
Zener diode D5511 shuts down the TV if the surge relay coil shorts. The
short will place 17Vdc at D5512's anode. This avalanches D5511, in-
creasing the voltage at the base of latch transistor Q5503. When it turns
on, the latch trips and the AC relay is de-energized, shutting off the TV.
Other inputs to this latch on the G board are shown in the Protection Cir-
cuit 2 diagram.
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