Service manual

1-
10
CRT. The CONSTANT BRIGHTNESS function, however, will recover the deteriorated brightness close to
the initial level (level ensured at outgoing the factory).
If the CONSTANT BRIGHTNESS function is activated, operation will be performed at 106kHz horizontally
and at 85Hz vertically while ignoring the input signal, and the OSM-IC (IC212) will output the reference
image signal. In this condition, R744 detects the beam current flowing to pin 9 of the flyback transformer
T701. This beam current is inverted and amplified by IC703, and then converted into a voltage value by
the current/voltage conversion circuit. After that, the converted voltage value will be input to the A/D
converter (pin 27 of IC102 (microcomputer)). To individually detect the beam current values of 3 colors (R,
G, and B), the desired color only will be brightened by increasing the cut-off voltages of the other 2 colors.
After obtaining the beam current values of 3 colors in this way, the obtained beam current values will be
compared with the beam current values used for factory adjustment (beam current values stored in the
EEPROM). After that, the cut-off voltage values of 3 colors (R, G, and B) will be adjusted so that the beam
current values close to the factory adjustment values can be obtained. In this way, the cut-off conditions of
the CRT will be recovered close to the factory adjustment level.
In addition, if the CONSTANT BRIGHTNESS function is activated, the C_TIME_SEL signal input to the
base of Q704 will be set to the low level, Q704 is turned OFF, and the bias voltage will be applied to pin 5
of IC703. As a result, voltage proportional to the beam current value will be output from pin 7 of IC703. By
the way, difference in the flyback transformer or the CRT may cause difference in the beam current. To
eliminate such difference in the beam current, the DAC voltage (commonly used for the 6H-DC signal) can
adjust the bias voltage input to pin 5 of IC703 described above. During normal operation, the
C_TIME_SEL signal is set to the high level, Q704 is turned ON, and pin 5 of IC703 is grounded via the
GND line so that the output of IC703 pin 7 can be kept at the low level. The signal output from pin 7 of
IC703 is added to ABL signal with MD717 (Diode). When the CONSTANT BRIGHTNESS function is
activated, the ABL signal is input to pin 27 of IC102 as the beam current signal.
1.4 CRT compensation block
1.4.1 Rotation circuit
The rotation circuit is a circuit to compensate the picture inclination caused by the earth magnetism by
letting DC current flow to the rotation coil wound on the front side of DY for adjustment. It is controlled to
0 to 5V with the reference of 2.5V by IC102 pin 45 (PWM_DAC), and DC current of +/-100mA (max) is
made to flow to the rotation coil by IC804 pin 2.
1.4.2 Corner purity circuit
The corner purity circuit is a circuit to compensate for the color shade and color deviation of the picture
corner. On the rear side of CRT, it is adjusted by DC current flowing to the corner purity coils installed in
the four corners on the display surface.
The compensation circuit is composed of the following three functions of (1) User adjustment (OSM
display), (2) Aging variation compensation and (3) High/low temperature drift compensation.
(1) User adjustment (OSM display)
The user causes DC current of +/-120mA (max) to flow to the purity coil of each corner according to
the value displayed on OSM.
(2) Aging variation compensation
As the electronic beam collides with the aperture grille, it is thermally expanded and contracted. The
thermal expansion/contraction is varied according to the elapse of the power ON/OFF time of the
monitor. The color shade and deviation of the picture corner thus generated are automatically
adjusted.
The voltage of the beam current supply pin (T701 pin 9) is detected with R723/R724, and the voltage
that detects the time elapse of the power ON/OFF of the monitor is read from the CR charge
(integration) circuit composed of C723 and R738, and CR discharge (integration) circuit composed of
C723 and R737 through IC702 (buffer amplifier) by IC102 pin 26 (CPU_ADC), then, the DC current of
+/-19mA (max) flows to the purity coil on each corner according to the specified control program.
(3) High/low temperature drift compensation
The front panel (glass) is thermally expanded and contracted as the temperature varies in the
installation environments of the monitor. The color shade and deviation of the picture corner are
automatically adjusted. The voltage that detects the temperature variation of the installation
environments of the monitor is read from the environment temperature detection circuit composed of
TH100 (thermistor) arranged near the front panel (glass) by IC102 pin 25 (CPU_ADC), and DC current
of +/-13mA (max) is made to flow to the purity coil on each corner according to the specified control