Changhong 21NF55-21PF93 CRT TV – How to adjust the convergence, setup adjustments, circuit diagram (schematic)

SET-UP ADJUSTMENTS
The following adjustments should be made when a complete realignment is required or a new picture tube is installed.
Perform the adjustments in the following order:
1. Color purity
2. Convergence
3. White balance
The purity/convergence magnet assembly and rubber wedges need mechanical positioning.
For some picture tubes, purity/ convergence adjustments are not required.
Color Purity Adjustment
Preparation:
Before starting this adjustment, adjust the vertical sync, horizontal sync, vertical amplitude and focus.
1 Face the TV set north or south.
2 Connect the power plug into the wall outlet and turn on the main power switch of the TV set.
3 Operate the TV for at least 15 minutes.
4 Degauss the TV set using a specific degaussing coil.
5 Set the brightness and contrast to maximum.
6 Counter clockwise rotate the R/B low brightness potentiometers to the end and rotate the green low brightness potentiometer to center.
7 Receive green raster pattern signals.
8 Loosen the clamp screw holding the deflection yoke assembly and slide it forward or backward to display a vertical green zone on the screen. Rotate and spread the tabs of the purity magnet around the neck of the CRT until the green zone is located vertically at the center of the screen.
9 Slowly move the deflection yoke assembly forward or backward until a uniform green screen is obtained.
10Tighten the clamp screw of the assembly temporarily. Check purity o f the red raster and blue raster until purities of the three rasters meet the requirement.
Convergence Adjustment
Preparation:
Before attempting any convergence adjustment, the TV should be operated for at least 15 minutes.
1 Center convergence adjustment
1.1 Receive dot pattern.
1.2 Adjust the brightness/contrast controls to obtain a sharp picture.
1.3 Adjust two tabs of the 4-pole magnet to change the angle between them and red and blue vertical lines are superimposed each other on the center of the screen.
1.4 Turn both tabs at the same time keeping the angle constant to superimpose red and blue horizontal on the center of the screen.
1.5 Adjust two tabs of the 6-pole magnet to superimpose red/blue line and green line.
1.6 Remember red and blue movement. Repeat steps2.1.3-2..1.5 until optimal convergence is obtained.
2 Circumference convergence adjustment
2.1 Loosen the clamp screw holding the deflection yoke assembly and allow it tilting.
2.2 Temporarily put the first wedge between the picture tube and deflection yoke assembly. Move front of the deflection yoke up or down to obtain better convergence in circumference. Push the mounted wedge in to fix the yoke temporarily.
2.3 Put the second wedge into bottom.
2.4 Move front of the deflection yoke to the left or right to obtain better convergence in circumference.
2.5 Fix the deflection yoke position and put the third wedge in either upper space. Fasten the deflection yoke assembly on the picture tube.
2.6 Detach the temporarily mounted wedge and put it in either upper space. Fasten the deflection yoke assembly on the picture tube.
2.7 After fastening the three wedges, recheck overall convergence and ensure to get optimal convergence. Tighten the lamp screw holding the deflection yoke assembly.
White Balance Adjustment
Generally, white balance adjustment is made with professional equipment. It’s not practical to get good white balance only through manual adjustment. For TVs with I2C bus control, change the bus data to adjust white balance.

Schematic - Full

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 I2C bus control, first change the bus data.
1. Degaussing
A degaussing 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 points 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.

[Anode high voltage differs depending on CRTs used, which should be decided by engineers.]

Focus Adjustment
Caution: Dangerously high voltages are present inside the TV. Extreme caution should be exercised when working on the TV with the back removed.
1 After removing the back cover, look for the FBT on the main PCB. There should be a FCB on the FBT.
2 Power on the TV and preheat it for 15 min.
3 Receive a normal TV signal. Rotate knob of the FCB until you get a sharp picture.
5. Safety Inspection
1 Inspection for insulation and voltage-resistant
Perform safety test for all naked metal of the TV. Supply high voltage of 3000V AC, 50Hz (limit current of 10mA) between all naked metal and cold ground. Test every point for 3 min. and ensure no arcing and sparking.
2 Requirements for insulation resistance
Measure resistance between naked metal of the TV and feed end of the power cord to be infinity with a DC-500 high resistance meter and insulation resistance between the naked metal and degaussing coil to be over 20MΩ .
6. DESIGN/SERVICE mode
1 To enter the USER SERVICE mode
Caution: The user service mode adjustment can be changed only when service personnel adjust the whole set data during servicing. As the control data have dramatic effects on functions and performance of the TV, service personnel should not tell user how to enter the SERVICE mode to avoid improper data settings.
1.1 Set the volume to 0. Then press and hold the MUTE button on the remote control, and press the MENU button on the TV to enter the SERVICE mode. (In this case, the S mode cannot be stored in the EEPROM. To exit from the S mode, turn off the TV set. )
1.2 After entering the S mode, Red“ S”is displayed on the upper center of the screen and MENU1 is default. Use the POS+/- buttons to highlight an adjustment and the VOL+/- buttons to adjust it. The adjusted data are immediately output and stored in the EEPROM

Option set
With remote control system software TDA935X,all options can be set in the SERVICE mode and stored in EEPROM. Data related to picture, sound and geometric adjustment are also stored in EEPROM.