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Protection Circuit 4

The shutdown latch and general sensing circuitry are located on the G
board. These circuits sense general failures. The Main Micro IC3251
causes the standby light to blink, indicating the circuit at fault.

The remainder of the sensing circuitry is found on the D board monitors
specific power handling circuits on this board. Although the D board sens-
ing circuitry also shuts down the TV when there is a problem, there is no
communications to the Main Micro so there is no blinking light to indicate
the problem area. Therefore, if the TV shuts down without a blinking light
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 4 diagram.
The remaining ones are in the previous diagram.

H. Output OCP Circuit Operation

Resistors R7062/R7022 and transistor Q7003 parts perform current sens-
ing. The current drawn by the Horizontal Output stage passes through
resistors R7062 and R7022. The normal voltage dropped across them is
shown in the chart below.

V o lt a g e A c r o s s P a r a lle l R e s is t o r s R 7 0 6 2 & R 7 0 2 2

G e n e r a t o r p ic t u r e

R 7 0 6 2 & R 7 0 2 2 V o lt a g e

B la c k R a s t e r

0 . 3 4 5 V

W h it e R a s t e r

0 . 5 8 V

If the horizontal output current increases, the voltage dropped across these
resistors rises. If they rise to a dangerous level of 0.6Vdc, Q7003 turns
ON and a positive voltage outputs its collector. This passes through blocking
diode D7102 and trips the shutdown latch on the G board. Excessive
current consumption can be caused by a shorted PWM MOSFET Q4022,
shorted T7002 FBT, leaky Q4018 or just a high screen control adjustment.

H. Output Supply OVP Circuit Operation

The PWM B+ regulating circuit supplies voltage to the Horizontal Output
Transistor Q4018. The following parts monitor this supply voltage:

H . O u t p u t S u p p l y O V P C i r c u it

P a r t s

N o r m a l C o n d i t i o n

P u r p o s e

3 9 V z e n e r d i o d e
D 7 0 2 9

C a t h o d e = 1 0 8 V d c

A n o d e = 3 8 V d c

R e s i s t o r s t r i n g
R 7 0 7 3 - 4 , R 7 0 8 0 - 2

S e r i e s d r o p p i n g
r e s i s t o r s

C 7 0 3 8

2 . 1 7 V

F i l t e r c a p a c i t o r

I C 4 0 0 4

P i n 1 3 o u t p u t = 8 . 3 5 V

H y s t e r e s i s
C o m p a r a t o r

Q 4 0 5 0

O N

I n v e r t e r

At turn on, PWM pulses from Q4022 are rectified by D7024 and produce a
low voltage at IC4004/pin 10. This results in an 8.35Vdc high at the pin 13
output.

If the PWM pulses become excessive or if Q4022 shorts, the higher volt-
age output will break over zener D7029 and be applied to IC4004/pin 10.
When pin 10’s voltage exceeds the 8.35Vdc reference at IC4004/pin 11,
its output at pin 13 will go to 0V. This will forward bias D4047 and turn off
Q4050, producing a high at its collector. The high is applied through D4060
to the latch circuit on the G board, which shuts down the TV.

FBT Output OVP

A flyback transformer T7002 secondary is used to represent the FBT out-
puts. It is monitored for excessive voltage. This voltage is rectified by
D4026 and applied to HV protection IC7001 for comparison to an internal
reference voltage. If the input voltage is exceeded, IC7001/pin 8 will output
a 1-volt high, tripping the Q7001/Q7003 latch. The latch grounds the +12V
from the FBT that forward biased the base of Q4050. This turns the tran-
sistor OFF so voltage can be applied through D4060 to the G board latch
that shuts down the TV.