Sony AV-3420CE Service Manual

FrequencymodulatorQ108 and Q109CouplingX’ formerT101Record amplifierQ201 throughQ204Video headsHead amplifiersand switchersQ221 throughQ227Video amplifierQ301 and Q302LimitersQ303 throughQ310DemodulatorD309, D310Low-pass filterFL301&2This is a symmetrical free-running multivibrator acting asa frequency modulator. Theoscillating frequency varies from3.8 MHz to 5.2 MHz in AV-CESeries Videocorders.T101 couples the modulator out-put to the record amplifiers.These are power amplifiers tofeed record current to the videoheads. R201 adjusts recordcurrent. R209 and R211 adjustthe frequency response charac-teristic.A slip-ring and brush assemblycouples recording current to thevideo heads.The playback video head out-puts are amplified and alter-nately gated on by the 25PGsignal to provide a flat, noise-free output.The PB video head outputs aremixed in the base circuit ofemitter follower Q227. The out-put of Q227 feeds a two stagevideo amplifier that providesmiddle-frequency peaking.The PB signal is amplitude-limited by four diode pairs withtransistors supplying interstageamplification.The input to the demodulatorfrom Q310 is differentiated byR335 and the primary wiringof T301. The push-pull outputof T301 is applied to the pairpulse detectors (demodulator)D309 and D310. These diodesproduce two negative outputpulse per one input pulse. Thus,the carrier frequency is effec-tively doubled and placed out-side the video band.The output of D309 and D310is amplified by Q311 and fed toa low-pass filter to obtain avideo output that is proportionalto the pulse frequency.Video amplifierQ312 and Q313Noise eliminatorQ314Video amplifierQ315Video output from the low-passfilter is amplified by Q312 andQ313. R349 and C328 reducehigh frequency gain to providede-emphasis.Only the high frequencies areseparated at Q314 input, am-plified and sliced by D311/D312diode pair. Noise eliminated highfrequencies are then mixed withoriginal signal at Q315 input.8-2. SERVO CIRCUITThe AV-3420CE has both a head-drum (scanner)servo and a capstan servo system. Section 8-2-1describes the camera-record mode where the headdrum servo free-runs at 13,125 Hz with the capstanservo disabled. Section 8-2-2 describes the TV-recordmode where the capstan servo is locked to theexternal TV signal. Section 8-2-3 describes theplayback mode where the capstan servo is locked tothe playback control-track signal and the head drumservo runs at 13,125 Hz.8-2-1. Camera-Record Mode ServoIn the camera record mode the servo systemacts as a simple speed regulator to make thescanner turn at 25rps. The reference for thesystem is a multivibrator having a constant pulsewidth.Variations in speed, as detected by variationsin the output frequency of the tone (feedback)generator, alter the duty cycle of this multivibratorto produce a dc correction voltage that controlsmotor speed. A belt linkage to the capstan drivepulley provides tape driving power. The capstanservo is not operative in the camera record mode.When using the AVC-3420CE, the Videocorderfree-runs at a constant speed, and supplies verticaland horizontal drive signals to the camera. Thehead drum contains a dc servo motor with a13,125 Hz frequency generator beneath it. Fasterrotation of the head produces a generator outputhigher than 13,125 Hz which lowers the dc motordrive current and thus slows the head rotational speed.The head drum contains a fre-quency generator coil that pro-duces 525 Hz output pulses persingle revolution of the video13,125 HzFrequencyGenerator FG