Circuit Adjustments; Degaussing; Supply Voltage Adjustment; High Voltage Inspection - Changhong Electric 21NF55 Service Manual

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
2
I 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.
Positive of C492
Filament pin of socket
21" CRT anode
Anode of 21" pure flat CRT (including Samsung, BMCC and Toshiba)
Anode of other 21" pure flat CRTs
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.
SERVICE MANUAL
Table 1
Test Point
8
Voltage Value (V)
185V±5V
0.3
6.3V± V
RMS
25.5KV±1.2KV
27.5KV±1.5KV
26.5KV±1.5KV