Zenith C32V23 Service Manual

Also see for C32V23 - 32" Flat-Screen Integrated HDTV: Operating guide & warranty

PV154 - 923-03497 3-9 AB - SERVICINGSERVICINGThe input signal is connected to the inverting input ofthe amplifier through resistor R5121 to avoid Hifrequency oscillations. The non-inverting input isconnected to a voltage bias formed by resistors R5101and R5102. C5122 is for AC decoupling and C5123 isfor hi frequency filter purposes (one for each IC). The ICtakes this voltage bias as a reference voltage for theinput signal. So, by means of moving this referencevoltage the black level could be established. Thisreference voltage is the same for the three-coloramplifiers.The cathode output is from pin 12 of the IC. This signalis passed through transistor Q5121 with the purpose ofsampling the cathode current. This cathode outputtravels through resistors R5127, R5128, and the coilL5121 for peaking improvement.TDA6120Q (221-01404) consists of one monolithicvideo output amplifier for each color. This amplifier canbe split-up in two amplifier stages in series. This givesrise to the term trans-admittance trans-impedanceamplifier. At the second stage voltage, the currentfeedback travels through resistor R5125 connected fromOUT (pin 13) to IN (pin 5). The trans-admittance of thefirst stage is chosen by means of a resistor between pin1 and pin 3, represented on this circuit by R5122.The advantage of this amplifier configuration is that ahigher bandwidth (around 30Mhz) can be realized withrelation to the well know operational amplifier withvoltage feedback. Furthermore, the amplifier has adifferential input which makes the amplifier less sensitiveto interference and flexible regarding DC-DC transfer.Also, this configuration has high input impedance withthe advantage of not loading the drive stage. The signalinput travels through a differential pair of transistors.The signal is connected on pin 2 of the inverting voltageinput. The input configuration consists of a bipolarNPN transistor in a long tailed pair configuration. Theinput current is 30uA.This IC is provided with a black current data pin. Onthis application pin 7 is not used. Instead of this pinthe current is sensed externally by means of transistorQ5121. The frequency response is improved by means ofa capacitor in series with a resistor connected betweenpins 1 and 3 of the IC. These are C5121 and R5123. Thevalue of these elements is a compromise betweenbandwidth, overshoot, and ringing.Block diagram for the video output circuit:SCAN VELOCITY MODULATORThe Scan Velocity Modulator is on a pair of coils installedunder the purity magnets. These coils perform as amagnetic dipole delaying or accelerating the screen spotin the areas of brightness variation.It is installed on the neck of the CRT near the Yokeassembly. This module has access to three signals: video,half tone, and fast blanking. This circuit has severalsections to process the signals and to obtain a currentsignal which will be applied over the coil.It has two inverter amplifiers, two push pull amplifiers,two follower emitter, one differential amp, one switch,and one commuter. There is a coil on the circuit whichreceives the current to produce an effect over the picture.The video passes through an inverter amplifier to increasethe level signal and then it is coupled by a followeremitter to be differentiated. Another inverter amplifierincreases the signal generated by the differentiator andthen two steps of push pull amplifiers drive the signalto feed the Coil.All the time the video is working there is a SVM operationthat improves and enhances the picture in each colortransition. The magnetic field produced by the loadedcoil affects the electron beams to achieve betterdefinition between colors on the screen. The SVMperformance can be adjusted through the factory menuwith item #105, named VMLEVEL, which has three levelsof intensity.