Heathkit IO-4550 - Page 23

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Initially,
sweep
control
1C203B
is
in
a
reset
condition
(Q
is
low
and
©
is
high)
and
transistor
Q203
is
turned
on.
A
trigger
pulse
from
slope
selector
gate
IC202
will
toggle
1C203B
and
switch
QO
high
and
@
low.
The
low
from
G
turns
transistor
Q203
off
and
toggles
blanking
control
IC203A.
As
Q203
turns
off,
the
timing
capacitor
begins
to
charge
through
the
bootstrap
current
source.
At
the
same
time,
IC203A
switches
the
CRT
blanking
circuit
and
unblanks
the
CRT.
The
“bootstrap”
current
source
is
part
of
the
sweep
generator.
FET
Q204
and
0205,
and
transistor
Q206
forma
voltage
follower
with
a
gain
of
approximately
1.
It
has
a
very
high
input
impedance
to
prevent
circuit
loading,
which
could
cause
a
non-linear
voltage
ramp
(sweep).
The
junction
of
resistors
R252
and
R253
are
held
to
a
level
10
volts
above
the
output
of
the
follower
by
5-volt
zener
diodes
ZD210
and
ZD211.
Since
the
follower
input
voltage
equals
the
output
voltage,
the
voltage
across
the
selected
timing
resistor
will
always
be
constant.
This
will
produce
a
constant
current
to
charge
the
selected
timing
capacitor.
When
variable
control
R253
is
turned
from
its
Cal
position,
the
voltage
differential
is
lowered.
Thus
the
charging
current
will
be
reduced
and
as
a
result,
reduce
the
sweep
speed.
The
Variable
control
is
used
to
provide
continuous
sweep
speeds
between
calibrated
ranges.
Ramp
Zero
contro!
R248
adjusts
the
follower
for
proper
voltage
offset.
The output of the sweep generator is coupled through
resistor divider R259, R261 to the horizontal deflection
amplifier. It is also coupled through Sweep Length control
R255 to sweep and Schmitt gate IC207A. Sweep Length
control R255 is adjusted so that the output of the Schmitt
trigger-type gate will go low when the ramp voltage exceeds
approximately 1.6 volts. (This represents a horizontal sweep
of approximately 11 centimeters.) The low from IC207A
resets (clears) blanking control 1C203A, which blanks the
CRT. Zener diode ZD209 protects the Schmitt trigger from
misalignment or malfunction of the sweep generator,
1C207B. The function of 1C207B will be described later.
The low from sweep end gate IC207A is also coupled to the
sweep end monostable IC204C and D. This converts the low
level signal to a short duration negative pulse (approximately
100nS, which gives the CRT time to fully blank) and
couples it to hold-off_monostable 1C206. The hold-off
monostable
toggles
(Q
goes
low)
and
remains
in
this
condition until it “times out.” The hold-off time is
determined
by
the
TIME/CM
switch,
and
is
of
sufficient
duration
to
insure
complete
retrace.
With
pin
1
of
IC209A
Page 21
high (from 1C207B), the low from Q of 1C206 will force pin
3 of IC209A low and reset sweep control 1C203B. This
forces the Q output high and turns on Q203, which quickly
discharges the sweep timing capacitor (retrace). The low
from pin 3 of IC209A does not affect gate IC204A, because
pin 2 of 1C204A is already low (auto-baseline monostable
1C205 toggled by trigger signal). The low from hold-off
monostable IC206 “‘locks-up’’ sweep control 1C203B, so
that it cannot toggle on a trigger signal until after hold-off.
After 1C206 ‘‘times out”, 1C203B can toggle on the next
trigger signal and start a new sweep cycle.
If for any reason the sweep control circuitry should
“hang-up”, such as at initial turn-on, the ramp voltage would
continue to increase. A voltage level would be reached where
anti-lockup control tC207B would activate and (through
1C209A) reset (clear) 1C203B, and discharge the sweep
timing capacitor, to initiate a new sweep cycle.
Normally, reoccurring Trigger pulses hold monostable 1C205
on. The low at the O output is coupled through 1C209B and
holds the output of IC204A high for normal sweeps to take
place. However, with no input trigger pulses, 1C205 times
out and its G output goes high. This (through !C209B)
forces the output of IC204A low, which allows IC203B to
“free run’’ and produce sweeps for a base line. In “normal”
mode operation, the automatic base line will never appear as
this feature is overridden by switch SW204.
CONTROL
For single-channel operation, the diode switch is latched by
the diode switch control circuit to couple the selected input
to the vertical deflection circuit. When an input channel is
turned on (switch on Position control), a logic high is placed
on the J or K input (depending on which channel is turned
on) of display control 1C211. The first low-to-high level
transition at the O output of 1C203B (sweep stop) is
coupled to pin 5 of 1C204B. With a high on pin 4 of
1C204B, the signal on pin 5 is inverted and coupled through
1C210C (pin 10 high) to the clock input (pin 12) of 1C211.
The high-to-low level transition toggles 1C211 and couples
the information at the J input to the Q output. Thereafter,
each “clock” pulse will have no effect while the J input level
does not change. The QO output (high for Y1 and low for Y2)
is coupled to the input of the diode switch control.