Zenith SM-ZVM-122 Service Manual page 14

Page 4-2
VIDEO OUTPUT
The video is DC-coupled from video driver QB05
and Contrast control RB15 to the base of video out-
put transistor Q201.
The beam limiter current is used to control the
amount of DC coupling. Beam current limiter tran-
sistor QB07 and its associated components sense the
anode current in the tertiary of the horizontal output
transformer. As the beam current increases, so does
the collector current at QB07. This collector current
is fed to the emitter of Q201, which increases the
collector voltage of the video output transistor. In-
creasing the collector voltage decreases the beam
current.
This circuit allows for a maximum of 90% DC cou-
pling. Because this circuit will not limit the
maximum beam current at a sufficiently low value,
diode CRB02 is required. This diode is biased off
until the beam current reaches 210 microamps. At
this point, the diode is allowed to conduct. The
emitter impedance is lowered considerably, which
increases the collector current. This limits the
maximum beam current.
SPOT BURN PROTECTION
When the Monitor is turned off, CRB03 and CB09
keep the collector voltage high. This keeps the CRT
biased off, preventing spot bum.
VERTICAL SWEEP CIRCUIT
The vertical sweep circuit is a self-oscillating DC-
coupled ramp-generating circuit that uses com-
plimentary push-pull class B output transistors, a
driver transistor, a differential amplifier transistor,
and an oscillator transistor.
The emitters of the output stage are fed back to the
input through C613, R61B, R617, and Q601. The dif-
ferentiated, positive-going fly back pulse from the
emitter charges C606 and C60B through Q601. The
capacitors discharge through their respective resis-
tor networks. Capacitor C606 discharges during the
trace interval to 0.6 volts below the emitter voltage
of Q601. At this point, Q601 conducts and turns off
the amplifier stages. This causes the yoke voltage
to fly up and repeat the cycle.
The presence of a sync signal causes Q601 to con-
duct slightly before the voltage on C606 decreases
to 0.6 volts below the Q601 emitter voltage, bringing
the circuit timing into sync with the sync signal.
Capacitor C60B discharges linearly through its resis-
tor network because this network is returned to yoke
current sensing resistor R624, where a ramp voltage
appears of the same amplitude as the ramp voltage
across C60B. A constant voltage appears across the
discharge resistor and maintains the constant dis-
charge current from C608. Capacitor C608 provides
a linear, negative-going ramp voltage of average DC
value, established by R60B and R609, to the base
of Q602. The signal to the emitter of the differential
amplifier comes from the yoke return circuit.
The ramp voltage across R624 has S-correction in
its waveform as required in the yoke current to pro-
duce linear pictures on the CRT. Across C609 is an
inverse S-correction signal, which is derived
through the shaping network of R620, R621, and
C615. This adds to the ramp-plus-S-correction signal
appearing across R624 to produce a linear ramp at
the emitter of Q602. This linear ramp is compared
by Q602 with the linear ramp across C60B. The dif-
ference between the two is coupled to the succeed-
ing amplifiers stages. This returns the yoke current
to the desired current for producing a linear picture.