Technology of Q 2500 colour TV set

Technology of Q 2500 colour TV setIt can be clearly determined from the linedriver circuit, as from the control of the lineoutput stage, that this is a case of lowresistancecurrent control. The driver stage isequipped with a transistor, which supplies forthe drive transformer T 528 (conversion ratio7:1) output stage the required base controlcurrent of up to 0.9 Ass. To limit inductiveswitching peaks an R/C combination is connected,a.c. to earth, to the collector of Q 526after diode D 527. The driver stage operateswith respect to the output stage in alternatingoperation, i.e. if Q 526 is conducting, thenQ 534 is blocked and vice versa.2.4 Horizontal output stageAs mentioned in the previous section controlof the horizontal output stage, or more preciselythe horizontal switching transistor, alternatesaccording to the switching regulatorprinciple. Essentially the base resistance consistsof R 532 and the series secondary windingof the driver transformer T 528. The parallelarranged resistor R 533 dampens switchingpeaks arising in the inductivity.An additional anti-parallel diode from collectorQ 534 to earth, which was used as a boosterdiode in earlier forms of thyristor deflectionswitching, is not required, as in this switchingconcept the collector-base route of the switchingtransistor Q 534 fulfils the function of thebooster diode by a process of inverse operation.In practice the parallel O/W-modulationdiodes cause a perceptible load reduction ofthe switching transistor. The drawing belowshows that this inverse operation of the transistoroccurs during the first half of the trace,until about the middle of the line. In the secondhalf of the run the transistor operates asnormal with conducting base-emitter diode.Only during the relatively short flyback time isthe switching transistor blocked. The low resistancebase switching already describedenables the base peak current between +0.9A and –0.9 A in both directions to be dissipatedquickly.Document Q 2500 22 © Loewe ProCollege