Horizontal Output; Vertical Sweep - Zenith ZVM-1240 Service Manual

The current division of the two transistors of the differ-
ential amplifier is determined by the phase relationship
between the sync and the sawtooth waveform on pin
4 of IC101. This sawtooth voltage is derived from posi-
tive horizontal flyback pulses. When the sync pulse
and sawtooth voltage are in phase, the current division
between the two transistors in the differential amplifier
will be equal. When there is a phase difference, current
will pass into or out
of
pin 5, which is connected by
way of a low-pass filter to pin 7 of the oscillator. This
current controls the oscillator.
Oscillator-The oscillator is an R-C type, with pin 7
being the control point. The timing capacitor, C1 01,
is charged by the external resistor, R104, to a trip volt-
age set in the integrated circuit. When this trip voltage
is reached, the capacitor discharges to a new trip
value. This process is repeated, producing a sawtooth
waveform at pin 7.
The output of the phase detector controls the oscillator
through resistive coupling from pin 5 to pin 7. The hori-
zontal hold control, R101 ,is also connected to pin 7.
The two 100 kO resistors, R123 and R124, in the hori-
zontal hold circuit are used to center the hold control
range.
Regulator-The input to the regulator is at pin 6 of
IC1 01. The regulator is temperature compensated and
consists of two high-current diodes in series with a
zener diode. The zener current is determined by an
external resistor, R108, connected to the
+
16.2-volt
power supply. C102, CX103, and R108 also provide
filtering.
Predriver-The predriver is a 4-transistor circuit,
which takes the sawtooth voltage formed at pin 7 and
produces a variable duty cycle waveform at pin 1 . This
output is fed to the base of 0101 after it is reduced
by resistors R117 and R118. The "on time" of the out-
put waveform is determined by the bias voltage on
pin 8. This voltage is determined by a series of clip
resistors; R106, R107, R109, R127, and R132, that
match the integrated circuit to the monitor.
Page 6
Horizontal Output
The signal from the horizontal output driver, 0101, is
coupled to the base of 0102 through transformer
TX1 01. 0102 controls the current in the primary wind-
ing of horizontal output transformer TX102 to switch
scan current in yoke TX2028 for right-side scan. C118
and the yoke inductance provide a resonant retrace
pulse that resets the beam to the left side of the
screen. Diode CR102 then provides scan for the left
side of the screen. The current through the yoke estab-
lishes the magnetic field necessary to deflect the elec-
tron beam along a horizontal plane. The retrace pulse
is also fed to TX102 where it is stepped up to provide
high voltage for the CRT.
Vertical Sweep
The vertical circuit consists of: 1. A 2-transistor, free-
running oscillator and 2. An IC power amplifier with
retrace pulse generator.
Transistors 0301 and 0302 form a free-running oscil-
lator. Sync pulses injected into the junction of resistors
R302 and R303 lock the oscillator to the proper scan
frequency. A DC reference level, which is determined
by the voltage across resistor R306, is established
across capaCitor C303. The oscillator produces a saw-
tooth voltage which is superimposed on this DC level.
The amplitude of the sawtooth voltage and, in turn,
the vertical size of the CRT screen raster are set by
resistors R309 and R311 and VERT SIZE control
R312. The sawtooth voltage is fed to the noninverting
input (pin 7) of power amplifier IC301. The amplified
signal then drives yoke TX202A. The parabola voltage
across yoke coupling capacitor CX306 is "S" -shaped
by a network consisting of resistors R317 and R316
and capacitors CX309 and CX307. This signal is
summed with the yoke current sample from R319 and
fed back into the inverting input (pin 1) of IC301, pro-
viding linearity correction.
Circuit Description