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Protection Circuit 3
The shutdown latch and the general sensing circuitry are located on the G
board. These circuits sense general failures such as +5 and +135 exces-
sive voltage and current. When a general failure occurs, the Main Micro
IC3251 causes the standby light to blink, indicating one of these failures.
See the Overall Protection Block.
The remainder of the sensing circuits are found on the D board. They
monitor specific power handling circuits on this board. Although the D
board sensing circuitry also shuts down the TV when there is a problem,
there is no communications to the Main Micro. There is no visual (standby
light blink) indication of the problem area. Therefore, if the TV shuts down
without a visual indication, the problem is likely to be on the D board.
The sensing circuitry on the D board monitors the following:
¨ Driver supply voltage OVP
¨ H. Output OCP
¨ H. Output Supply OVP
¨ FBT Output OVP
¨ H Centering IC4003 OCP
The items in bold are shown in this simplified Protection Circuit 3 diagram.
The remaining ones are in Protection Circuit 4 diagram.
Driver supply voltage OVP Circuit Operation
The horizontal driver stage receives supply voltage from a PWM circuit.
This PWM circuit not only regulates its own output voltage, but also changes
the voltage to increase picture width when required. Since a failure in the
PWM circuit would result in excessive scan current, the PWM voltage is
monitored.
D r iv e r S u p p ly V o lta g e O V P S e n s in g C ir c u it
P a r ts
N o r m a l C o n d itio n
R 4 0 1 1 , R 4 0 0 8 ,
R 4 0 1 2 v o lta g e d iv id e r
Q 4 0 0 1
O N
Q 4 0 0 5
O N
77
P u r p o s e
S a m p le s th e P W M
d r iv e r s u p p ly v o lta g e
K e e p s Q 4 0 0 5 O N
K e e p s o u tp u t a t 0 V d c
Initially, both Q4001 and Q4005 are turned ON when current flows from
+12V through Q4001/e-b, R4009, R4008 and R4012 to ground. When
Q4005 is ON, its collector voltage is at 0Vdc so the shutdown path ends
here.
A sample of the PWM driver voltage is taken from Q4003/collector via
R4011 and applied to Q4001's base. This voltage is normally not high
enough to turn OFF Q4001, so both Q4001 and Q4005 remain ON.
An excessive level of PWM driver voltage would cut off Q4001, causing
Q4005's collector to rise. This is applied to the latch circuit on the G board
(CN5002/pin 9) which causes the TV to shut off.
H Centering IC4003 OCP Circuit Operation
In this Sony model KW34HD1 TV, the user can center the picture from the
menu. This centering circuit consists of a driver IC4003 that performs this
function by adding DC voltage to the yoke. IC4003's failure could send a
substantial current through the yoke so its current consumption is moni-
tored as it enters IC4003/pin 3. The parts used in this OCP circuit are:
H C e n te rin g IC 4 0 0 3 O C P S e n s in g C irc u it
P a rts
N o rm a l C o n d itio n
R 4 0 0 5 @
0 .1 V a c ro s s
IC 4 0 0 3 /3
R 4 0 0 5
Q 4 0 1 3
O N - 0 .0 2 4 V
a c ro s s R 4 0 2 8
P H 4 0 0 6
O F F
D 4 0 1 1
O F F (0 V o n b o th
s id e s )
The yoke receives signals from both the + and – ends and therefore floats
above ground. The horizontal centering signal is applied to the yoke via
IC4003. The centering IC4003 is powered by +15V from a FBT winding so
this secondary floats as well.
P u rp o s e
C u rre n t s e n s in g
S e ts th e P H 4 0 0 6 p h o to d io d e
th re s h o ld b y p re b ia s in g it.
C o u p le s th e O C P s ig n a l w ith o u t
in te rfe re n c e fro m IC 4 0 0 3 's flo a tin g
+ 1 5 V s u p p ly .
O u tp u t b lo c k in g d io d e
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