NEC FP1355 Service Manual

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Contents

NEC FP1355 Service Manual Manual pdf 35 page image

1 - 291.5 CRT compensation block1.5.1 Rotation circuitThe rotation circuit is a circuit to compensate the picture inclination caused by the earth mag-netism by letting DC current flow to the rotation coil wound on the front side of DY for adjust-ment. It is controlled to 0 to 5V with the reference of 2.5V by IC103 pin 3 (PWM_DAC), andDC current of +/-90mA (max) is made to flow to the rotation coil by IC804 pin 2.This correction circuit has two functions; (1) User adjustment (OSD display) and (2) Southern/Northern horizontal magnetic field rotation cancellation, as follows.(1) User adjustment (OSD display)User provides DC current to the rotation coil according to the value displayed on OSD.(2) Southern/Northern horizontal magnetic field rotation cancellationSouthern/Northern horizontal magnetic field rotation cancellation is to automatically adjustthe variation of raster rotation by earth magnetism.Detection voltage and direction of the southern/northern horizontal magnetic field (pin 2 ofIC214) is detected by IC214 (Earth magnetism sensor unit), and pin 18 of IC101(CPU_ADC) reads the detected voltage and provides DC current to the rotation coil ac-cording to the prescribed control program.1.5.2 Corner purity circuitThe corner purity circuit is a circuit to compensate for the color shade and color deviation ofthe picture corner. On the rear side of CRT, it is adjusted by DC current flowing to the cornerpurity coils installed in the four corners on the display surface.The compensation circuit is composed of the following four functions of (1) User adjustment(OSD display), (2) Aging variation compensation, (3) High/low temperature drift compensationand (4) Southern / Northern horizontal magnetic field landing cancellation.(1) User adjustment (OSD display)The user causes DC current of +/-60mA (max.) to flow to the purity coil of each corneraccording to the value displayed on OSD.(2) Aging variation compensationAs the electronic beam collides with the aperture grille, it is thermally expanded and con-tracted. The thermal expansion/contraction is varied according to the elapse of the powerON/OFF time of the monitor. The color shade and deviation of the picture corner thusgenerated are automatically adjusted.The voltage of the beam current supply pin (T701 pin 9) is detected with R723/R724, andthe voltage that detects the time elapse of the power ON/OFF of the monitor is read fromthe CR charge (integration) circuit composed of C723 and R736 and CR discharge (inte-gration) circuit, composed of C723 and R737 through IC702 (buffer amplifier) by IC101 pin15 (CPU_ADC), and the DC current of +/-17mA(max) flows to the purity coil on each corneraccording to the specified control program.(3) High/low temperature drift compensationThe front panel (glass) is thermally expanded and contracted as the temperature varies inthe installation environments of the monitor. The color shade and deviation of the picturecorner are automatically adjusted. The voltage that detects the temperature variation ofthe installation environments of the monitor is read from the environment temperaturedetection circuit composed of TH101 (thermistor) arranged near the front panel (glass) byIC101 pin 14 (CPU_ADC), and DC current of +/-13mA (max) is made to flow to the puritycoil on each corner according to the specified control program.