Zenith HW20H52DT Servisní příručka stáhnout pdf (Strana 3)

PRODUCT SAFETY SERVICING GUIDELINES FOR AUDIO-VIDEO PRODUCTS

3828VD0205B CH-FLAT - SAFETY

X-Radiation

To prevent possible exposure to x-radiation caused by excessive CRT

anode voltage, the CH-Flat chassis incorporate a “High Voltage

Shutdown” circuit. This circuit senses the level of a flyback pulse

from the “Flyback Transformer” representative of the actual high

voltage on the CRT anode. When this level exceeds a predetermined

voltage, the circuit shuts down the TV set, preventing further gen-

eration of anode voltage.

Shutdown Circuit Operation

(Refer to Figure below)

The flyback pulse voltage from pin 6 of TX3201 (Flyback Trans-

former) is peak detected (rectified) by the action of diode DX3001

and capacitor C3003. This form a DC voltage appearing on C3003

representative of the CRT anode voltage (HV) produced by TX3201.

This voltage is divided down by precision resistors RX3004, RX3005,

RX3006 and RX3007. This lower voltage appears on the zener diode

ZD3000; when this voltage exceeds by 3.5 Vdc the “zener voltage”

the HV shutdown occurs (pin 29 of ICX2200).

CRT Anode High Voltage Measurement Procedure

Each CRT screen size has it’s own safe operating anode and shut-

down voltage. Critical safety component (designated with an ‘X’ in

the component designator) are designed to operate the CRT at a

safe operating anode voltage and provide proper shutdown thresh-

olds. If replacement of any of these components are deemed neces-

sary, it is important to use original type Zenith components. After

replacement is made, confirm proper anode voltage using the fol-

lowing procedure.

Measurement of the CRT anode voltage must be performed using a

high impedance-high voltage meter, with no raster on the screen,

and operating at nominal horizontal frequency, 15.75 Khz (NTSC

signal).

After discharging the CRT, connect a high impedance-high voltage

meter to the CRT anode. Turn the television ‘on’ 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 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 Nominal Anode Voltage (KV) Max. Shutdown Voltage (KV)

20" 26+/-1.0 32

CRT Anode Volta

HV SHUTDOWN PROCEDURE.

·After discharging the CRT, connect a high impedance-high volt-

age meter to the CRT anode

·Access Video Menu and adjust Brightness and Contrast controls

for minimum screen luminance (beam current to 0 mA).

·Wait until the Video Menu or display disappear.

·Connect a variable Resistor (1 MW) between ground and junc-

tion of RX3754 and RX3755, and decrease slowly the resistance

value until shutdown occurs.

·Measure High Voltage shutdown.

Fly-Back Transformer CX3215

Deflection Yoke CX3216

CX3203 RX3753

CX3204 RX3754

CX3205 RX3755

CX3206 RX3756

CX3207

COMPONENTS WITH ANY INFLUENCE IN HV INCREAS

Flyback Pulse

RX3209

C3003

DX3001

RX3007

RX3006

RX3005

C3002

ZD3000

R3003

D3002

RM3002

C3001

RM3001

SHUTDOWN

TO PIN 29

ICX2200

Of TX3201

From Pin 6

OUT

RX3004

1 2 3 4 5 6 7 8 ... 47 48

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