Constant Brightness Function; Crt Compensation Block; Earth Magnetism Cancel Circuit; Rotation Circuit - NEC DPro2070SB Service Manual

(*5): R/G/B GAIN MAX value is the maximum one among GAIN adjustment value mentioned (*3) above.
EEP address (hex)
R
MAX GAIN 89
* Every R/G/B MAX GAIN data applied to the address listed above table are totally same.
In case of repair, after CRT, Pre-AMP (IC211), MAIN-AMP (IC210), etc. are replaced and the
luminance/color coordination is adjusted, the MAX GAIN value mentioned above should be rewritten.

1.3.9 CONSTANT BRIGHTNESS function

The brightness and color coordination of the screen will be deteriorated due to secular deterioration of the
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 Earth magnetism cancel circuit

This model carries IC2S0 (geomagnetism sensor unit) that carries out the voltage conversion of the
magnetic field intensity of a north-south magnetic field and an east-west magnetic field. IC2S0 detects
the detected voltage and controls the various canceling functions described below automatically.
•South-north horizontal magnetic field rotation canceling function
•East-west horizontal magnetic field raster vertical position canceling function
•South-north horizontal magnetic field landing canceling function
•East-west horizontal magnetic field landing canceling function
•South-north horizontal magnetic field convergence canceling function
•East-west horizontal magnetic field convergence canceling function
Here, the output voltage of IC2S0 (earth magnetism sensor unit) operates as follows.
•South-north horizontal magnetic field (IC2S0 pin 5): 1.0V (-0.04mT) to 2.5V (+/-0.00mT) to
•Vertical magnetic field (IC2S0 pin 4): 4.0V (-0.04mT) to 2.5V (+/-0.00mT) to 1.0V (+0.04mT).

1.4.2 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
G B
8a 8b
1- 10
4.0V(+0.04mT).