Zenith SM-ZVM-122 Service Manual page 15

HORIZONTAL PROCESSOR
The operation of the horizontal processor (IC501),
221-141, is the same as the 221-86. The 221-141 can
be replaced by 221-86. However, 221-86 cannot be
replaced by 221-141.
Integrated circuit 221-141 is divided into four sec-
tions:
• Phase detector
• Oscillator
• Regulator
• Pre driver
Phase Detector
The phase detector is comprised of a differential
amplifier and a gated current source.
The current source is strobed on by a negative sync
that is AC coupled to pin
3.
The current division between the two transistors of
he differential amplifier is determined by the phase
relationship of the sync and sawtooth waveform on
pin 4. This sawtooth is derived from a negative hori-
zontal flyback pulse. When the sync and sawtooth
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 either
flow 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.
Page 4-3
Oscillator
The oscillator is an RC type, with pin 7 being the
control point. The timing capacitor is charge up by
an external resistor to a trip voltage set in the inte-
grated circuit. When this trip voltage is reached, the
capacitor is discharged to a new trip value. This pro-
cess is repeated, producing a sawtooth waveform.
The output of the phase detector controls the oscil-
lator through resistive coupling from pin 5 to pin
7. The Horizontal Hold control is also connected at
pin 7. The two 100 kfl resistors in the horizontal
hold circuit are used to center the hold control
range. The diode in series with the Hold control is
used to temperature compensate the oscillator.
Regulator
Pin 6 on 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 connected to the 12.7-volt
power supply. The voltage set by the regulator is
between 8 and
9
volts.
Predriver
The predriver is a four-transistor circuit which takes
the sawtooth formed at pin 7 and produces a variable
duty cycle waveform at pin 1. This output is used
to drive the horizontal driver. The "on" time of the
output waveform is determined by the bias voltage
on pin 8. This voltage is oetermined by a series of
clip resistors that match the integrated circuit to the
Monitor.
COMPOSITE
VIDEO
INPUT
SYNC
AM PISE P
HOR IZ
PROCES SOR
POWER
SUP PL Y
Figure 4-1
Block
diagram.
VIDEO
OUTPUT
BEAM
CURRENT
LIM ITER
CRT
HOR IZ
OUTPUT
HOR I Z
DRIVER