Zenith ZVM-121 Service Manual page 22

Page 5-8
HORIZONTAL PROCESSOR (Figure 5-8)
The operation of the horizontal processor (IC501),
221-141,
is the same as the
221-86.
The
221-141
can be
replaced by the
221-86.
However, the
221-86
cannot
be replaced by
221-141.
The
221-141
integrated circuit is divided into four
sections: the phase detector, the oscillator, the reg-
ulator, and the predriver.
The phase detector is comprised of a differential
amplifier and a gated current source. The current
source is strobed on by a negative sync, which is AC
coupled to pin
3.
The current division between the
two transistors of the differential amplifier is deter-
mined by the phase relationship of the sync and saw-
tooth waveform on pin
4.
This sawtooth is derived
from a negative horizontal 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 differ-
ence, 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.
The oscillator is the RC type, with pin
7
being the
control point. The timing capacitor is charged 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 look 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.
The regulator (pin 6) 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.
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. On-time of the output
waveform is determined by the bias voltage on pin
8.
This voltage is determined by a series of clip resistors
that match the integrated circuit to the Monitor.