Zenith ZVM-121 Service Manual page 20

Page 5-6
VERTICAL SWEEP CIRCUIT (Figure
5-7)
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 resistor
networks. Capacitor C606 discharges during the trace
interval toO.6 volts below the emitter voltage ofQ601.
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 conduct
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 C60B. Capacitor C60B 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 produce
linear pictures on the CRT. Across C609 is an inverse
S-correction signal, which is derived through the
shaping network ofR620, 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. Any difference results in a correct-
ing current in the collector of Q602 and in the suc-
ceeding amplifier stages. This returns the yoke cur-
rent to the desired current for producing a linear
picture.