Changhong Electric 21NF55, Руководство по обслуживанию, Страница: 10 / 83

SERVICE MANUAL
8
CIRCUIT ADJUSTMENT S
Preparation:
Circuit adjustments should be made only after completion of set-up adjustments.
Circuit adjustments can be performed using the adjustable components inside the TV set. For TVs with
I
2
C bus control, first change the bus data.
1. Degaussing
A deg aussing coil is built inside he TV set. Each time the TV is powered on, the degaussing coil will
automatically degauss the TV. If the TV is magnetized by external strong magnetic field, causing
color spot on the screen, use a specific degausser to demagnetize the TV in the following ways.
Otherwise, color distortion will be shown on the screen.
1.1 Power on the TV set and operate it for at least 15 minutes.
1.2 Receive red full-field pattern.
1.3 Power on the specific degausser and face it to the TV screen.
1.4 Turn on the degausser. Slowly move it around the screen and slowly take it away from the TV.
1.5 Repeat the above steps until the TV is degaussed completely.
2. Supply Voltage Adjustment
Caution: +B voltage has close relation to high voltage. To prevent X-ray radiation, set +B voltage to
the rated voltage.
2.1 Make sure that the supply voltage is within the range of the rated value.
2.2 Connect a digital voltmeter to the +B voltage output terminal of the TV set. Power on the TV and set
the brightness and sub -brightness to minimum.
2.3 Regulate voltage adjustment components on the power PCB to make the voltmeter read 115±1V.
3. High Voltage Inspection
Measure voltages of test point s on the main PCB with the digital voltmeter. Measure the CRT high
voltage with the high-voltage testing equipment and heater voltage with the high-frequency
effective voltmeter. The rated values are shown as below.
Table 1
Test Point Voltage Value (V)
Positive of C492 185V
±
5V
Filament pin of socket 6.3V
±
0.3
V
RMS
21” CRT anode 25.5KV
±
1.2KV
Anode of 21” pure flat CRT (including Samsung, BMCC and Toshiba) 27.5KV
±
1.5KV
Anode of other 21” pure flat CRTs 26.5KV
±
1.5KV
Notes:
①
Measure voltage of the related isoelectronic point if it’ s difficult to measure a certain point in Table 1.
②
Anode high voltage differs depending on CRTs used, which should be decided by engineers.