Technology of Q 2500 colour TV set

Technology of Q 2500 colour TV set2.2.7 Voltage stabilisationFor each of the operating voltages U 3.3, U 5and U 12 a V-MOS transistor is used as ahorizontal controller for stabilisation. Theirgate voltages are kept constant by the connectedIC's I 569 for 3.3 V, I 676 for 5 V and I674 for 12 V. In addition, their control input isconnected to the voltage distributor by therespective output voltage.If the output voltage falls under high load, theIC's become high resistant causing the gatevoltage to increase and the horizontal controllersare controlled upwards further, whereuponthe output voltage increases again. If theload decreases the opposite occurs.In addition, voltages U8 for the video controland U9 for the interface with the fixed voltagecontrollers L 7808 and. L 78S09 are stabilised.The controllers I 663 and I 691 are nowalso located in the power supply. The inputvoltage is U 12.2.2.8 Voltage increaseTo avoid overloading the VF output phases,they are supplied with a load-dependent operatingvoltage. The control range of the circuitfor this lies between +18 V in no-load operationand +13 V at full load.The pulses taken from pin 14 of the transformerare rectified by D 681 and applied tothe emitter connections of transistors Q 682and Q681. For no-load or low-load operationno voltage is felt across current measurementresistor R 681. This means that Q 681 isblocked and Q 682 is switched through to itsbase with L level via R 684. The voltage frompin 14 of the transformer increases the VFoperating voltage to +18 V.If the noise level is increased, more current isdrawn accordingly. There is a correspondingvoltage drop across R 681, which means thebase of Q 681 goes more negative, the transistorswitches and as a consequence withbase Q 682 going positive, a greater or lesserdegree of blocking is achieved. The VF operatingvoltage is reduced accordingly.Harmonic distortion is therefore kept low. thuspreventing overheating of the output ICs. Inthis way a noise-level dependent video pumpingcan be prevented.Voltage increase for VF8T63914F689D68113R686D686L686UNF+C681R685Q682R681R687D687C687R684Q68110SM-Trafo12C689L687UNFÐR683R682Document Q 2500 20 © Loewe ProCollege