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Theory of Operation
3–64
AM700 Audio Measurement Set Service Manual
The Video Amplifier amplifies the incoming video signal to levels necessary to
drive the crt. A cathode follower input circuit formed by Q1 and its associated
biasing circuit provides impedance matching and current drive to the output
video amplifier. In the output video amplifier (a cascode circuit), the video signal
is inverted by Q2, a common emitter circuit. Its output current directly drives a
common base amplifier, Q3, whose output drives the crt cathode. As the input
video signal is increasing in amplitude, the inverted video signal drives the
cathode negative with respect to its +50 V reference level thereby causing the
beam current to increase.
The incoming +16.5 V is applied to a 3-terminal regulator, U2, where it is
converted to +12 V. A tap on the +12 V source supplies a +5 V Zener diode
voltage regulator composed of VR1 and R1 to supply the +5 V source.
The vertical deflection circuit provides the drive current to the vertical deflection
yoke. A free-running oscillator circuit, U3, produce a ramp signal, and an
amplified vertical deflection signal. The basic frequency of the oscillator is set to
60 Hz by the adjustment of R45, the Vert Hold potentiometer. The length of the
ramp signal, and consequently the vertical synchronization, is set by the V Sync
signal.
When the V Sync is received, the vertical ramp is terminated, and a new one
begins. The RAMP OUT, pin 1, signal is applied to the AMP IN, pin 10, for
amplification to drive the vertical deflection yoke. The output of U3 at pin 10 is
inverted from the original ramp. The ramp retrace level is amplified to produce
large amplitude pulses for retrace of the beam. The ramp between pulses is
nearly flat, and the constant level voltage produces a ramp of current through the
vertical deflection yoke. The vertical height of the display is set by the adjust-
ment of R42, the Vert Size potentiometer.
A portion of the RAMP OUT signal from pin 1 is fed back through a shaping
circuit to the RAMP GEN input on pin 12. The shape of the ramp fed back is
adjustable by R48, the Vert Lin potentiometer, to set the vertical linearity of the
raster. Vertical position is set with R57 which adjust the DC level on the vertical
deflection yoke.
Blanking Amplifier. Blanking between vertical sweeps is produce by the blanking
amplifier, Q6. In the interval between ramps, the level of the vertical deflection
signal from U3, pin 4, rapidly rises toward +30 V to retrace the beam back to the
vertical starting point. That rise is seen by Q6 via C41, and the transistor quickly
saturates. The negative pulse at the collector of Q6 is AC coupled via C42 to the
control grid of the crt to cutoff the beam current between vertical deflection
ramps. In the output of the blanking amplifier, R64, the BLACK LEVEL
adjustment, sets the black level of beam current between blanking pulses.
Video Amplifier
+12 V and +5 V Supply
Vertical Deflection