Technology of Q 2500 colour TV set

Technology of Q 2500 colour TV setThe alternating voltage from the mains is rectifiedby the D 613 bridge rectifier. When inresonance, C 620 is loaded to approx. 310 V.Without PFC this means that mains currentwill only be drawn when the sinusoidal halfwaveexceeds 310 V. Sinusoidal current extractionis therefore not guaranteed. With thePFC circuit, C 620 is not directly charged fromthe mains. Charging of C 620 is via the C 628current pump. When the Q 624 switchingtransistor is conducting, C 628 is connected toearth. C 628 is charged via D 617 and L 619.If Q 624 is now switched off, a voltage ofapprox. 400 V is set on the drain.The energy stored both in C 628 and in thecoil can now charge up C 620 via D 623. C628 will naturally also be charged when thesinusoidal mains alternating voltage justcrosses zero and has a low value.Q 624 is switched with a frequency between20 kHz and 40 kHz, depending on load conditions.This also means that C 620 is chargedwith a current at the same frequency. Thecurrent drawn from the mains would also correspondto this frequency, but will be harmonizedby the mains input filter.2.2.6 Secondary sideAlthough the secondary side voltages arerelatively stable, and fast and transientchanges in load can be compensated for bythe field effect transistor in the primary side,stabilisation of voltages in the digital componentis still required.The two secondary voltages U 3.3, U 5, U 8,U 9 and U 12 are stabilisedFor the other secondary voltages:- UB for the horizontal stage- U 25 for the horizontal driver stage- UNF+ and UNF- for the VF output stageStabilisation is not required, as no current isdrawn from these non-controlled levels.Document Q 2500 18 © Loewe ProCollege