Technology of Q 2500 colour TV set

Technology of Q 2500 colour TV setThe switch to limit the contrast and brightnessis controlled with a d.c value via pin 43.The switch is designed in such a way that fora voltage of >3.3 V there is no limitation oncontrast and brightness.For a voltage between 3.3 and 2.2V the contrastis reduced in proportion to the voltage. Ifthe voltage drops below 1.8 V the brightnessas well as the contrast is reduced. At approx.1 V on pin 15 the brightness and the contrastare reduced by 100%, whereupon the screenis black.To achieve beam current limitation the base ofthe diode split transformer influences the d.c.value on pin 43 of TDA 9332. Inversely proportionalvoltage information on the beam currentcan be obtained at the base.In order for the TDA pin to be at >3.3 V with abeam current of zero, the base of the transformeris connected to U 200 via R 551/552.The positive voltage in this operational state ischarged via resistors R 2608, R 2609 andR 2611 of Elko C 2603 and on pin 43 of theTDA there is a voltage of >3.3 V; there is nobeam current limitation.As the high voltage winding in the diode splittransformer operates with a virtual earth, thebase becomes negative with increasing beamcurrent, whereby C 2603 via R 2608, R 2609and R 2611 can be correspondingly dischargedto a greater or lesser degree and thebeam current controlled.Diodes D 2607 and D 2611 are provided sothat the circuit can also react to jumps inbeam current. They ensure the rapid dischargeof C 2603.In addition, for a 4:3 display in a 16:9 TV set,from pin 113 of I 2311 via Q 2612 and R 2601,the voltage on C 2603 drops. This corrects thebeam current limitation for the smaller screenarea. (see also signal board deflection).CCU Pin 94Overbeam current protective circuitU3.3D2596LL103CR258622kC2618100nD2618LL4148Q2616BC847BWR261810kC26161uU5R2617220kR2616220kC2608100nR260822kR260618k100RR1043100RC10431n111213FPW15112.7.1 Overbeam current fuseIn correct operation the beam current measurementinput on pin 43 of TDA is at a d.c.level of 2 to 4 V. According to the level, theRGB output amplifier in this IC is controlled toa greater or lesser extent, which ensures thatthe beam current limitation is implemented.If the c.r.t. is controlled upwards to the fullextent, even though the gain of the amplifierhas been fully reduced , which can occur ifthere is a fault in the RGB output stage or itsDocument Q 2500 33 © Loewe ProCollege