Service manual

PRODUCT SAFETY SERVICING GUIDELINES FOR AUDIO-VIDEO PRODUCTS
3828VD0171G ii DIGITAL FCS - SAFETY
X-RADIATION
To prevent possible exposure to X-Radiation caused
by excessive CPT anode voltage, The Digital FCS
chassis incorporate a ¨High Voltage Shutdown¨
circuit. This circuit senses the level of a
Shutdown pulse from the ¨Flyback Transformer¨
representative of the actual high voltage on the
CPT anode. When this level exceeds a predeter-
mined voltage, the circuit shuts down the hori-
zontal drive, preventing further generation of
anode voltage. In this condition, the horizon-
tal drive is latched off.
SHUTDOWN CIRCUIT OPERATION
The shutdown pulse voltage from pin 9 of TX3201
(Fly-Back Transformer) is peak detected (recti-
fied) by the action of diode D3205 and capacitor
C3218. This form a DC voltage appearing on C3218
representative of the CPT anode voltage (HV)
produced by TX3201. This DC voltage is divided
down by the voltage divider series circuit of
precision resistors R3001 (with in parallel pro-
vision for R3002) and R3003 (also with in paral-
lel provision for R3004). This divider output
voltage appears on Emitter of Q3000. In the
event that the CPT anode voltage becomes exces-
sive, the voltage in Emitter of Q3000 will be
higher than the voltage in the Base of Q3000
(5.1V Base voltage is fixed by ZD3003) and there-
fore Q3000 enters in conduction mode. Therefore
a DC voltage will appear on R3005 and this volt-
age will put D3000 in conduction. The D3000
cathode voltage level (close to 5V, a ¨logical
high¨) is sent to pin 34 (HPROT input) of video
processor ICM2200 stopping the Horizontal Drive
Output (H-DRV pin 40) pulse train therefore shut-
ting down to Horizontal Output Fly-Back trans-
former. switching off the HV and horizontal de-
flection. If the shutdown condition disappears,
the TV set can be turned-on only if the AC is
interrupted for about 20 Seconds and then the
on/off switch is activated.
CPT ANODE HIGH VOLTAGE MEASUREMENT PROCE-
DURE
Each CPT screen size has it´s own safe operating
anode and shutdown voltage. Critical safety com-
ponent (designated with and ´X´ in the component
designator) are designed to operate the CPT at a
safe operating anode voltage and provide proper
shutdown thresholds. If replacement of any of
these components is deemed necessary, it is im-
portant to use original type LGERS components.
After replacement is made, confirm proper anode
voltage using the following procedure.
Measurement of the CPT anode voltage must be
performed using a high impedance-high voltage
meter, with no raster on the screen, and operat-
ing at nominal horizontal frequency 37.9 kHz.
(This TV internally up-converts the video sig-
nal horizontal frequency from the 15,734 Hz of
NTSC to 37.9 kHz of SVGA for displaying).
After discharging the CPT, connect a high imped-
ance-high voltage meter to the CPT anode. Turn
the television ón´ and confirm a good signal is
being displayed. Reduce Brightness and Contrast
settings until the picture is well extinguished.
Observe the anode voltage meter reading and com-
pare with the table below fro the proper CPT
screen size. If the voltage reading is higher
than the maximum, verify circuit component val-
ues and proper operation.
CRT Anode Voltage
CRT Screen
Size
Nominal Anode Voltage
(KV)
Min. Shutdown Voltage
(KV)
Max. Shutdown Voltage
(KV)
27”
30 ± 1.0
Nominal HV + 2 36
32”
30 ± 1.0
Nominal HV + 2 36