Technology of Q 2500 colour TV set

Technology of Q 2500 colour TV setcurrent now flows via Q 3221 through the coiland then on via R 3223 and R 3224 to earth.The reversed direction of current causes thedeflection to be arrested.The deflection coil is controlled directly fromthe collector connections. Two resistors areconnected in parallel to the deflection windingto stabilise the system and using L 3226 in thecontrol lines a throttling of clock faults isachieved.To prevent overshooting and overloading ofthe output stages a protective circuit is providedon the PCB. It affects the level of theinput signal.For very steep positive or negative flanks diodesD 3318 and D 319 conduct via C 3319.Positive flanks at the output reduce the inputsignal via D 3218, as the signal on the outputis rotated by 180°. Negative flanks on the outputreduce the input signal, as the input signalsare directed to earth via D 3319. The circuitspecification is such that it does not operatefor small signal flanks, thereby ensuringsecure operation of the speed modulator overa wide spectrum.If there is no picture signal, but only noise, theoutput stages have become overheated. Aprotective circuit is therefore provided for thiscase as well.If the current in the output stages increases,there is a larger voltage drop across currentresistor R 3156. The base of Q 3157 goesnegative and the transistor conducts. ElkoC 3158 ensures that transient, but high currentsfor steep flanks are possible. Using transistorQ 3157 in conjunction with transistor Q3216 the input signals of the output phase arereduced and overloading thereby prevented.As there is a time difference of approx. 80 nswith respect to the RGB signals in the speedmodulator circuit up to modulation of the deflectionspeed, the RGB signal circuit must beretarded by 80 ns after decoupling from themodulator if it is to function correctly. For thisreason the 80 ns time lag lines, consisting, forexample, of L 3392/3393, C 3392/3398/399and R 3391/3393 are incorporated into theRGB lines.2.9 Colour stagesIn order to avoid damaging capacitive chargescaused by, for example, long cathode wires,which could lead to the frequency responsebeing cut, the RGB output stages have beenre-incorporated into the c.r.t. circuit board.For each colour channel there is an IC whosesignal bandwidth of >12 MHz guarantees ahigh resolution even for rapid signal changesin both directions.The circuit for the three colour channels isidentical. By using ICs the components in theoutput stages could be reduced to a minimum.The output stages are energised by the speedmodulator with a max. 3 Vss. Via an RC combinationthe respective signals reach pin 3 ofthe ICs and are passed on in the IC to theinverting input of an operational amplifier. Thenon-inverting input and thereby the work pointis set via pin 1 of IC .The resistance between pin 9 and pin 3 providesthe negative feedback for determinationof the amplification factor.Document Q 2500 37 © Loewe ProCollege