Heathkit IO-4550 - Page 24

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A logic high to the diode switch contro} will turn transistor
Q120 on and reverse bias diodes D109-1 and D110-1. Diodes
D107-1 and D108-1 are then forward biased and the signal
for Channel Y1 is coupled to the vertical deflection
amplifier. With transistor Q120 turned on, Q121 is turned
off. Diodes D109-2 and D110-2 are forward biased, and
D107-2 and D108-2 are reverse biased, which blocks the
Channel Y2 signal. The reverse occurs when a logic low is
coupled to the diode switch control circuit.
Dual-channel operation is enabled when both input channels
are turned on (switch on each Position control). The
TIME/CM switch selects either the chop mode (.2 SEC to
5mS range) or the alternate channel mode (2mS to .2uS
range). When both channels Y1 and Y2 are turned on, logic
highs are coupled to the J and K inputs of display control
1C211. Gate 1C210A is also enabled to put pin 4 of IC210B
high.
In the alternate channel mode, pin 12 of 1C210D is
grounded (low) by the TIME/CM switch — which disables
1C210D and the chopper oscillator (IC208C, IC208B). At
the beginning of a sweep, the high-to-low level transistion at
Q of 1C203B is inverted by !C204B (pin 4 is high) and
coupled through 1C210C (pin 10 is high) to pin 12 of 1C211.
This “sets” the display control. At the end of the sweep, the
low-to-high level transistion from the O output of 1C203B
toggles 1C211 and the Q output changes “state’”’ (low to high
or high to low transistion), Output Q is coupled to the diode
switch control circuit. A logic high will couple the Channel
YI signal to the vertical deflection circuit. A logic low will
couple Channel Y2. Each successive sweep cycle will toggle
1C211 and alternate the channel being coupled through the
diode switch to the vertical deflection circuits.
For chop mode operation, an oscillator provides a switching
signal to the display control. This forces the diode switch to
alternately couple Channels Y1 and Y2 to the vertical
deflection circuit many times during each sweep. Because
the switching rate is fast (approximately 200 kHz) and the
sweep rate is slow, the chopped effect of the display is not
visible. Pin 12 of 1C210D is high, which enables 1C210D.
When a new sweep cycle begins, the Q output of 1C203B
goes high. This high is coupled through 1C210D to pin 9 of
the
chopper
oscillator
(IC208C
and
1C208B)
and
turns
the
oscillator
on.
The
low
from
Q
of
1C203B
is
inverted
by
1C204B
to
enable
1C210C.
Therefore,
each
low-high-low
pulse
from
the
chopper
oscillator
will
be
coupled
through
resistor
R223
and
1C210C
to
the
clock
input
of
1C211.
Each
clock
pulse
is
also
coupled
through
1C210B
to
the
chop
blanking
amplifier.
The
short
time
delay
caused
by
the
RC
network R223 and C215 insures that the CRT will be
blanked when the diode switch switches from one input
channel to the other. This serves two functions: First,
blanking the CRT will remove any trace lines that may
appear when the vertical circuits switch between input
channels. Second, the short delay between blanking and
switching will hide any possible switching noise that may be
generated. At the end of the sweep cycle, the chopper
oscillator is turned off by the QO output of 1C203B. This
removes the possibility of a false trigger after hold-off. A
new sweep cycle will start the chopper oscillator again.
Each chop pulse from 1C210B is coupled to the chop
blanking amplifier through capacitor C417 on the high
voltage circuit board. Capacitor C417 acts as a differentiator
to convert the incoming square wave to dual-polarity spikes
(see Figure 14). Resistor R428 holds transistor Q401 on
until a negative spike turns 0401 off for a very short time,
as determined by the value of C417. The positive portion of
the spike has no effect on the circuit. When Q401 turns off,
the collector voltage rises sharply to +68 volts. This short
pulse is coupled through capacitor C418 to effectively pull
the CRT cathode voltage up to —1632 volts, which will
momentarily blank the CRT. Normal CRT blanking (for
retrace) will be described with the high voltage power
supply.
+#170V
R431 *OBVie =
ov J LJ L
C418
20401
68V 1-—o
-1680V -—
ee aa 3
mais -1700V
+5V-- we Ww
ov
_J
LJ
i
youSf
CHANNELS
SWITCH DURING
THIS TIME
Figure 14