Shutdown Circuit Operation; Crt Anode - LG RU-27FB30C Series Service Manual

SHUTDOWN CIRCUIT OPERATION

The flyback pulse voltage from pin 10 of T402 (Flyback
Transformer) is peak detected (rectified) by the action
of diode D444 and capacitor C450. This form a DC volt-
age appearing on C450 representative of the CRT anode
voltage (HV) produced by T402. This voltage is divided
down by precision resistors R419, R423, R420 and R424.
This lower voltage appears on the zener diode ZDX447;
when this voltage exceeds by 0.7 Vdc the "zener volt-
age" Q403 enters in saturation mode end then the HV
shutdown occurs (pin 49 of IC100).
CRT ANODE HIGH VOLTAGE MEASUREMENT
PROCEDURE
Each CRT screen size has it's own safe operating an-
ode and shutdown voltage. Critical safety component
(designated with an 'X' in the component designa-
tor) are designed to operate the CRT at a safe oper-
ating anode voltage and provide proper shutdown
thresholds. If replacement of any of these compo-
nents are deemed necessary, it is important to use
original type LG Electronics components. After re-
placement is made, confirm proper anode voltage
using the following procedure.
Measurement of the CRT anode voltage must be per-
formed using a high impedance-high voltage meter, with
no raster on the screen, and operating at nominal hori-
zontal frequency, 15.75 Khz (NTSC signal).
After discharging the CRT, connect a high impedance-
high voltage meter to the CRT anode. Turn the televi-
sion 'on' and confirm a good signal is being displayed.
Reduce Brightness and Contrast settings until the pic-
ture is well extinguished.
3828VD0210A
SERVICING
Observe the anode voltage meter reading and compare
with the table below for the proper CRT screen size. If
the voltage reading is higher than the maximum, verify
circuit component values and proper operation.
CRT Screen Size
27"
COMPONENTS WITH ANY INFLUENCE
HV SHUTDOWN PROCEDURE.
·After discharging the CRT, connect a high impedance-
high voltage meter to the CRT anode
·Remove jumper wire from RX3750.
·Connect a variable Resistor (1 MW) in location RX3750.
·Access Video Menu and adjust Brightness and Con-
trast controls for minimum screen luminance (beam cur-
rent to 0 mA).
·Wait until the Video Menu or display disappear.
·Increase (from zero) slowly the resistance value until
shutdown occurs.
·Measure High Voltage shutdown.
3-2
CRT Anode Voltage
Nominal Anode Voltage Max. Shutdown Voltage
(KV)
30 ± 1.0
IN HV INCREASE
Fly-Back Transformer
Deflection Yoke
CX404
CX406
CX405
ICX3751
FCN - SERVICING
(KV)
36