Zenith ZCM-1490 Service Manual page 51

The 30V horizontal retrace pulse at pin 1 of connec-
tor 8V6 is coupled to a signal-shaping circuit formed
by transistors 07402 and Q7401. The resulting sig-
nal at the collector of 07401 is a pulse with a fast ris-
ing edge and a delayed, slow falling edge. This
shaped pulse is differentiated by C7406 and R7409
and applied to the set input (pin 8) of IC7401, a dual
D-type flip-flop. This half of IC7401 is configured as a
one-shot multivibrator. (A one-shot multivibrator con-
verts an input pulse of short duration. to an output
pulse of longer duration.) An RC network formed by
R7412 and C7409 delays the clock input signal to pin
11 of IC7401 by approximately 4
~S.
This delay in
effect sets the duration of the output pulse. Thus,
when the shaped pulse from 07401 is applied to the
. set input, output 02 at pin 13 becomes logic high.
The RC network samples this logic high output, de-
lays it by about 4
~S,
and applies it to the clock input
(pin 11). When the clock input goes high, the signal
at the 0 input (pin 9) causes 02 to go low. C7408,
R7463 , and R7441 differentiate the vertical retrace
pulse. CR7410 and R7464 sense this and apply it to
the reset input of flip-flop IC740 1, thereby resetting
the flip-flop at the start of each vertical frame. This
prevents flip-flop "Iatchup" caused by static dis-
charge.
The 02 signal at pin 13 of IC7401 is coupled to tran-
sistor 07405. This transistor, along with C7414,
07404, and 07404, forms a horizontal ramp gener-
ator. C7414 is charged by the current source formed
by 07404, 07404, and associated components. A
pulse at the base of 07405 causes this transistor to
conduct, thereby discharging C7414. The horizontal
ramp is formed in this manner. This ramp is buffered
by transistors 07407 and 07408 and coupled to the
south level (R7434) and north level (R7436) controls.
A parabolic waveform is also generated to correct
north and south phase. Transistor 07407 is a modu-
lated current sink for the parabola generator. This
transistor, along with C7417 and 07409, forms a
parabola generator. C7417 is charged by the current
source formed by 07409 and associated com-
ponents. The modulated signal at the base of 07407
causes this tranSistor to conduct, thereby discharging
C7417. To stabilize the DC bias point of C7417, the
current source (07409) is DC-modulated by feed-
Circuit Descriptions
back from 07410. The parabolic waveform is formed
in this manner. This waveform is buffered by 07410
. and coupled to the south phase (R7433) and north
phase (R7435) controls.
07410 also acts as a current sink for the resonant
circuit formed by C7421 and L7401. A sinewave at
twice the horizontal frequency is present at C7421.
L7401 adjusts the phase ofthis sinewave. 07411
buffers the sinewave for the North "W" (R7454) and
South "M" (R7456) controls.
IC7402 is a CMOS switch activated by the appropri-
ate signals from the 0 outputs of IC7401. The
switches within IC7402 are configured (by externai
connections) as single pole double throw switches. In
this way, the proper set of controls (either north or
south) is connected to the following stages based on
which half of the raster is being scanned (either the
top or bottom). Diodes CR7406 and CR7407 activate
the CMOS switches based on the 01 and not-01 sig-
nals from IC7401, pins 2 and 1 respectively. Diodes
CR7408 and CR7409 open both sets of switches
during horizontal retrace time based on the not-02
Signal from IC7401, pin 12.
The· - processed horizontal rate signals, routed
through LC7402 from previous stages, are applied to
the carrier inputs of a balanced modulator-
. demodulator (IC7501). (A balanced modulator-
demodulator forms an output voltage which is a prod-
uct of an input signal and a carrier.) IC7501 and its
associated circuitry modulate the horizontal rate sig-
nal (the carrier) with a vertical ramp (the input signal)
such that as the beam approaches the center of the
screen (from the top), the amplitude of the signal
decreases to zero. As the beam crosses the center
of the screen, the amplitude again begins to
increase, but the phase is now reversed for scanning
the bottom half of the screen. The vertical ramp is
coupled to IC7501 through C7501 from buffer tran-
sistor 07505. The north-south crossover zero control
(R7450) adjusts the crossover point where the
horizontal signal amplitude is reduced to zero and
the phase reverses. Bias for IC7501 is supplied by
07502 and 07503 as a function of the ho.rizontal
scan voltage. The gain of this stage is set by R7510
and R7545.
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